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Placental growth factor predicts time to delivery in women with signs or symptoms of early preterm preeclampsia: a prospective multicenter study

Published:September 10, 2019DOI:https://doi.org/10.1016/j.ajog.2019.09.003

      Background

      There is a robust association between altered angiogenic factor concentrations, which includes placental growth factor and clinically recognized preeclampsia. Alterations in concentrations of angiogenic factors precede the clinical onset of preeclampsia by several weeks. The temporal relationship between the measured angiogenic factors and the time to delivery in women with suspected preeclampsia at <35 weeks gestation, however, remains to be clarified.

      Objective

      The purposes of this study were to examine the relationship between placental growth factor and time to delivery in women at <35 weeks gestation with signs or symptoms of preeclampsia and to compare the performance of placental growth factor to other clinical markers for prediction of time to delivery in preeclampsia.

      Study Design

      Women with signs or symptoms of preeclampsia between 20.0 and 35.0 weeks gestation were enrolled in a prospective, observational study at 24 centers. Blood was collected at presentation for placental growth factor, and subjects were evaluated and treated according to local protocols. Clinical outcomes were obtained, and all final diagnoses were adjudicated by an independent expert panel according to 2013 American College of Obstetricians and Gynecologists’ Hypertension in Pregnancy criteria. Placental growth factor was measured retrospectively on the Alere, Inc, triage platform. A normal placental growth factor was defined as >100 pg/mL; the assay’s limit of detection is 12 pg/mL. Two-by-2 tables were constructed for comparison of test outcomes that included negative predictive value; time-to-delivery was analyzed by survival curves and Cox regression.

      Results

      Seven hundred fifty-three subjects were enrolled; 538 (71%) had a final diagnosis of preeclampsia; 542 (72%) delivered at <37 weeks gestation, and 358 (47%) delivered at <34 weeks gestation. Among the 279 women (37%) with a normal placental growth factor at presentation, the negative predictive value for preeclampsia delivered within 14 days or within 7 days was 90% and 93%, respectively. Compared with women with normal placental growth factor, women with placental growth factor ≤100 pg/mL have a hazard ratio of 7.17 (confidence interval, 5.08–10.13) in Cox regression for time to delivery after adjustment for both gestational age at enrollment and the final diagnosis of preeclampsia. The placental growth factor levels of normal (>100 pg/mL), low (12–100 pg/mL), and very low (<12 pg/mL) have well-separated distributions of time to delivery, with median values of 45, 10, and 2 days, respectively. Subjects with placental growth factor ≤100 pg/mL have a perinatal death rate of 5.7% and a small-for-gestational-age rate of 51.7%; subjects with placental growth factor >100 pg/mL have a perinatal death rate of 0% (no observations in this cohort) and an a small-for-gestational-age rate of 16.8%.

      Conclusion

      In women with suspected preeclampsia at <35.0 weeks gestation, a low placental growth factor was correlated strongly with preterm delivery independent of a diagnosis of preeclampsia or gestational age at presentation, whereas a normal placental growth factor was associated with pregnancy prolongation, even in patients who ultimately had a final diagnosis of preeclampsia. This suggests that placental growth factor levels are superior to clinical markers in the prediction of adverse pregnancy in women with suspected preeclampsia.

      Key words

      Preeclampsia is a heterogeneous syndrome of pregnancy that is recognized by traditional clinical criteria of hypertension plus proteinuria or clinical symptoms such as cerebral, hematologic, or gastrointestinal evidence.
      • Roberts J.M.
      • August P.A.
      • Bakris G.
      • et al.
      Hypertension in pregnancy: report of the American College of Obstetricians and Gynecologists’ task force on hypertension in pregnancy.
      Severe adverse outcomes that are related to preeclampsia (such as eclampsia, stroke, and perinatal death) are tragic, but relatively uncommon. In fact, most perinatal morbidity and medical costs because of preeclampsia are related to issues of prematurity that are incurred from preterm delivery.
      • Stevens W.
      • Shih T.
      • Incerti D.
      • et al.
      Short-term costs of preeclampsia to the United States health care system.

      Why was this study conducted?

      The temporal relationship between the measured angiogenic factors and the time to delivery in women with suspected preeclampsia at <35 weeks gestation remains to be clarified.

      Key findings

      Compared with women with normal placental growth factor, women with a low placental growth factor level (≤ 100 pg/mL) have a hazard ratio of 7.17 (confidence interval, 5.08–10.13) in Cox regression for time to delivery after adjustment for both gestational age at enrollment and the final diagnosis of preeclampsia.

      What does this add to what is known?

      In women with suspected preeclampsia at <35 weeks gestation, a low placental growth factor level was strongly correlated with preterm delivery, independent of a diagnosis of preeclampsia or gestational age at presentation. This suggests that placental growth factor levels are superior to clinical markers in the prediction of adverse pregnancy in women with suspected preeclampsia.
      The pathogenesis of preeclampsia is complex and heterogeneous. The traditional diagnosis and staging of preeclampsia are difficult and, despite recent changes in diagnostic criteria, the current clinical definitions do not reliably predict adverse pregnancy outcomes. Therefore, there is a great need for adjunctive and objective quantitative biomarker tests to assist clinicians in treating patients with signs and symptoms suggestive of preeclampsia.
      • Cerdeira A.S.
      • Agrawal S.
      • Staff A.C.
      • Redman C.W.
      • Vatish M.
      Angiogenic factors: potential to change clinical practice in pre-eclampsia.
      There is a robust association between altered angiogenic factor concentrations that include placental growth factor (PlGF) and clinically recognized preeclampsia.
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      Because alterations in concentrations of angiogenic factors precede the clinical onset of preeclampsia by several weeks, a predictive potential of these factors has been reported.
      • Cerdeira A.S.
      • Agrawal S.
      • Staff A.C.
      • Redman C.W.
      • Vatish M.
      Angiogenic factors: potential to change clinical practice in pre-eclampsia.
      Indeed, Levine et al
      • Levine R.J.
      • Maynard S.E.
      • Qian C.
      • et al.
      Circulating angiogenic factors and the risk of preeclampsia.
      demonstrated lower mean PlGF concentrations in subjects who later experienced preeclampsia and in subjects with established preeclampsia, compared with those who never experienced the disease.
      With the use of separate assays, 2 recent studies from European cohorts
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      • Zeisler H.
      • Llurba E.
      • Chantraine F.
      • et al.
      Predictive value of the sFlt-1:PlGF ratio in women with suspected preeclampsia.
      with rates of confirmed disease within 1 and 2 weeks of screening of 18% and 55%, respectively, reported high sensitivity and high negative predictive values using the ratio of FMS-like tyrosine kinase receptor-1 (sFlt)-1:PlGF or PlGF alone for the prediction of preeclampsia. Moreover, some studies reported that the measurement of PlGF levels could be useful clinically to predict of adverse pregnancy outcomes in women with suspected preeclampsia
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      • Griffin M.
      • Seed P.T.
      • Duckworth S.
      • North R.
      • Myers J.
      • Mackillop L.
      Predicting delivery of a small-for-gestational-age infant and adverse perinatal outcome in women with suspected pre-eclampsia.
      or to predict small for gestational age (SGA) in the absence of clinically recognized preeclmpsia
      • Asvold B.O.
      • Vatten L.J.
      • Romundstad P.R.
      • Jenum P.A.
      • Karumanchi S.A.
      • Eskild A.
      Angiogenic factors in maternal circulation and the risk of severe fetal growth restriction.
      These data, although encouraging, do not clarify the temporal association between the measured angiogenic factors and the time to delivery (TTD) in women with suspected preeclampsia at <35 weeks gestation. The objectives of this study were to examine the relationship between PlGF and TTD in women at <35 weeks gestation with signs or symptoms of preeclampsia and to compare the performance of PlGF to other clinical markers for the prediction of TTD.

      Materials and Methods

      The Preeclampsia Triage by Rapid Assay Trial was a prospective, observational, institutional review board–approved assessment of women 18–45 years old with any signs or symptoms of preeclampsia between 20 and 41 weeks gestation at 24 North American centers. Principal investigators and their study sites are provided in Appendix 1. Subjects were enrolled by gestational age blocks; venous blood samples for this study were collected at enrollment before a final diagnosis was made. The qualifying signs or symptoms of preeclampsia were intentionally broad, intending to mimic the diverse findings encountered in clinical practice: hypertension, proteinuria, laboratory abnormalities, excessive maternal weight gain, fetal growth restriction (estimated fetal weight <10th percentile for gestational age), or clinical symptoms (Table 1). All diagnoses were adjudicated by an independent panel with the final diagnosis of preeclampsia according to the 2013 American College of Obstetricians and Gynecologists Task Force on Hypertension in Pregnancy definitions.
      • Roberts J.M.
      • August P.A.
      • Bakris G.
      • et al.
      Hypertension in pregnancy: report of the American College of Obstetricians and Gynecologists’ task force on hypertension in pregnancy.
      Table 1Signs and symptoms at enrollment
      Enrollment assessmentN (%)
      Some women had >1 sign or symptom.
      New onset hypertension371 (49.3)
      Worsening underlying hypertension230 (30.5)
      New onset proteinuria390 (51.8)
      Headaches262 (34.8)
      Epigastric or right upper quadrant abdominal pain61 (8.1)
      Estimated fetal weight <10th percentile129 (17.1)
      Nausea and vomiting74 (9.8)
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      a Some women had >1 sign or symptom.
      EDTA-anticoagulated plasma samples (K2 EDTA tube; Becton Dickinson Labware, Franklin Lakes, NJ) were labeled and transported to the laboratory where they were immediately centrifuged at 1300g for 10 minutes at 2–8°C; the supernatants were kept frozen at –80°C until assayed. Plasma was analyzed for PlGF retrospectively in batches with the use of the Triage PlGF Test (Alere San Diego, Inc, San Diego, CA) in a central laboratory facility with the use of the Triage PlGF assay according to the manufacturer’s instructions. The test uses fluorescently labeled monoclonal antibodies against PlGF for PlGF quantification and contains chemistries for on-board positive and negative control systems to ensure that the quantitative PlGF result is valid. Briefly, 230 μL of thawed plasma (room temperature) is pipetted into the sample port of a new test cartridge. The cartridge is inserted into the meter, and the results are displayed and printed in 15–20 minutes in picograms per milliliters. Full details of assay methods are available in Appendix 2. The dynamic range is 12–3000 pg/mL; the coefficient of variation is 13%.
      The reference range of PlGF was studied previously in healthy pregnancy by Saffer et al
      • Saffer C.
      • Olson G.
      • Boggess K.A.
      • et al.
      Determination of placental growth factor (PlGF) levels in healthy pregnant woman without signs or symptoms of preeclampsia.
      who established the gestational age dependent 5th percentile of PlGF. In a clinical study of preeclampsia by Chappell et al,
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      in subjects enrolled at <35 weeks gestation, a single cutoff of 100 pg/mL was shown to be as effective as a gestational age dependent cutoff (the 5th percentile); whereas a cutoff value of 12 pg/mL represented the lower detection limit of the assay.
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      • Saffer C.
      • Olson G.
      • Boggess K.A.
      • et al.
      Determination of placental growth factor (PlGF) levels in healthy pregnant woman without signs or symptoms of preeclampsia.
      Chappell et al
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      also showed that PlGF did not perform well for subjects who were enrolled after 35 weeks gestation, even when the gestational age–dependent cutoff was used. Therefore, our study was conducted to validate a prospectively selected single cutoff (100 pg/mL) in the cohort of women who were seen at <35 weeks gestation.
      The PlGF concentration was dichotomized at 100 pg/mL, and the dichotomous PlGF together with gestational age and established clinical markers (eg, blood pressures, proteinuria) to predict the TTD in Cox proportional hazards regression. The most convincing way of combining the standard of care clinical biomarkers was via the final adjudicated diagnosis of preeclampsia. The final diagnosis represents a composite that includes the entire sequence of blood pressure measurements, proteinuria measurements, and other clinical information that was collected during enrollment and throughout the clinical course of the subject. Adjustment for final diagnosis demonstrates that a single PlGF value that was collected at enrollment is an independent predictor of TTD.
      The minimum total sample size was calculated based on the width of the 95% confidence interval of the sensitivity and the specificity, with goal of achieving a width of 10% at a point estimate of 90%. This consideration resulted in a minimum total sample size of 150 women with preeclampsia and 150 women who did not have preeclampsia. This minimum total sample size was exceeded to achieve a minimum of at least 20 preeclamptic and 20 subjects who did not have preeclampsia in each of 4 gestational age intervals spanning the enrollment range from 20–35 weeks gestation, noting that the prevalence of preeclampsia was not controlled by enrollment, resulting in a total of 753 subjects comprised of 538 subjects with preeclampsia and 215 subjects who did not have preeclampsia. All laboratory staff were masked to clinical outcomes. All participants had delivered and pregnancy outcomes recorded before biomarker concentrations were analyzed and revealed. Patients were treated, and delivery timing and mode were determined per local protocols.
      Nonparametric statistical comparisons were made by chi square or Kruskal Wallis test. Cox Proportional Hazards assessment was performed with MATLAB coxphfit (Release 2017b, The MathWorks, Inc., Natick, MA) for the prediction of TTD censored at 14 days with the use of univariate models. The models incorporated either PlGF alone (positive ≤100 mg/mL vs negative >100 mg/mL), final clinical diagnosis (preeclampsia vs not preeclampsia), or adjusted models that incorporated PlGF with gestational age at delivery, presence or absence of proteinuria, or final diagnosis of preeclampsia. For analysis, perinatal death was defined as stillbirth or neonatal death. SGA was defined as birthweight <10th percentile according to Alexander et al.
      • Alexander G.R.
      • Himes J.H.
      • Kaufman R.B.
      • Mor J.
      • Kogan M.
      A United States national reference for fetal growth.

      Results

      Over the study period from November 2010 to January 2012, 1258 subjects were enrolled initially. Twenty-four subjects were ineligible or did not follow the protocol, and 11 subjects were withdrawn or lost to follow up, which left 1223 subjects. Six enrollment samples could not be evaluated on the Triage device, which left 1217 subjects in the entire cohort. Seven hundred fifty-three women were enrolled at <35 weeks gestation; these women constitute the subjects of the current analysis and are presented in the concert flowchart (Figure 1).
      Figure thumbnail gr1
      Figure 1Flowchart
      The flowchart explains the study participants.
      PlGF, placental growth factor; wks, weeks gestation.
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      Table 1 describes signs and symptoms at the time of enrollment. Most of the women (79.8%) were enrolled with either new onset hypertension or worsening preexisting hypertension.
      Table 2 describes maternal characteristics at the time of study enrollment. Most of the women (58.2%) were obese; 680 pregnancies (90.3%) were singleton gestations. Body mass index was included as a covariate in the Cox proportional hazards regression. The hazard ratio for body mass index is weak, and the adjusted hazard ratio for PlGF is the same regardless of the body mass index level. Therefore, the results are independent of body mass index. The study is underpowered to report results in the subset of nonsingleton pregnancies. Smoking has been reported to influence angiogenic factors. Baseline characteristics of smoking included a history of smoking (184 women; 24.4%), quit smoking before pregnancy (69 women; 9.2%), quit smoking after pregnancy (60 women; 8.0%), and never quit smoking (55 women; 7.3%).
      Table 2Clinical characteristics at enrollment and delivery
      Characteristics
      Median age was 29.9 years; the interquartile range was 24.8–34.3 years
      N (%)
      Obese: body mass index, kg/m2
       ≥30438 (58.2)
       ≥40164 (21.8)
      Nulliparous378 (50.2)
      White403 (53.5)
      Black209 (27.8)
      Hispanic101 (13.4)
      Previous preeclampsia159 (21.1)
      Pregestational diabetes mellitus47 (6.2)
      Gestational diabetes mellitus103 (13.7)
      Antiphospholipid antibody syndrome5 (0.7)
      Lupus9 (1.2)
      Polycystic ovarian syndrome27 (3.6)
      Singleton gestation680 (90.3)
      Spontaneous labor without cesarean delivery52 (6.9)
      Cesarean delivery
      Includes cases of cesarean delivery that were initiated after spontaneous onset of labor.
      516 (68.5)
      Induced labor, excluding cesarean delivery185 (24.6)
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      a Median age was 29.9 years; the interquartile range was 24.8–34.3 years
      b Includes cases of cesarean delivery that were initiated after spontaneous onset of labor.
      Table 3 summarizes the final adjudicated diagnoses according to the 2013 American College of Obstetricians and Gynecologists definitions.
      • Roberts J.M.
      • August P.A.
      • Bakris G.
      • et al.
      Hypertension in pregnancy: report of the American College of Obstetricians and Gynecologists’ task force on hypertension in pregnancy.
      The overall prevalence of preeclampsia was 71.4%; however, 125 subjects (16.6%) ultimately did not meet the diagnostic criteria for any hypertensive disorder. The inclusion of women with pregnancies that were affected by fetal growth restriction with no evidence of hypertensive disorder introduces heterogeneity; however, in subanalysis, the odds ratio for PlGF predicting preterm delivery in this subset of 125 subjects is 33.5 (95% confidence interval, 9.35–120), which is consistent with the odds ratio of 26.0 (95% confidence interval, 16.8–40.3) reported for the overall cohort (Table 4). The median gestational age at delivery was 34.1 weeks, with 72% women delivering at <37 weeks gestation and 47% women delivering at <34 weeks gestation. The median gestational age at delivery varied by the final diagnosis and was significantly earlier for the 538 subjects with a final diagnosis of preeclampsia (33.1 weeks gestation) vs the other 215 subjects (37.6 weeks gestation). The frequency of preeclampsia within the enrollment gestational week categories of <24+0, 24+0–27+6, 28+0–31+6, and 32+0–34+6 was 50%, 66%, 74%, and 74%, respectively.
      Table 3Final adjudicated diagnoses
      Final diagnosisN (%)
      Preeclampsia538 (71.4)
       Preeclampsia with severe features401 (53.3)
       Preeclampsia without severe features32 (4.2)
       Superimposed preeclampsia105 (13.9)
      Gestational hypertension only43 (5.7)
      Chronic hypertension47 (6.2)
      No hypertensive disorder125 (16.6)
      Small for gestational age
      Of the 292 cases that satisfied the definition of small for gestational age, 234 are classified as preeclampsia, and 58 are classified as not preeclampsia.
      292 (38.8)
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      a Of the 292 cases that satisfied the definition of small for gestational age, 234 are classified as preeclampsia, and 58 are classified as not preeclampsia.
      Table 4Predictions based on placental growth factor ≤100 pg/mL, with diagnostic performance for each prediction
      PredictionSensitivitySpecificityPositive predictive valueNegative predictive valueDiagnostic odds ratio (95% confidence interval)
      All preeclampsia75.768.885.953.06.86 (4.84–9.73)
      Preeclampsia and delivery within 14 d92.563.869.890.321.6 (13.9–33.6)
      Preeclampsia and delivery within 7 d93.958.861.693.222.0 (13.3–31.6)
      Any delivery within 14 d90.568.576.686.420.7 (13.9–31.0)
      Any delivery within 7 d92.562.267.590.720.2 (12.9–31.6)
      Preterm delivery81.785.393.564.526.0 (16.8–40.3)
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      Table 5 summarizes the TTD from enrollment according to final adjudicated clinical diagnosis. Women with preeclampsia plus severe features and those with superimposed preeclampsia were more likely to deliver within 2 days, 7 days, and 14 days compared with the other groups (2-tailed Fisher’s exact probability value <.0001 for the contingency table that compared 2 groups of subjects, preeclampsia plus severe features and superimposed preeclampsia vs all other subjects by 2 delivery intervals, 0–14 days vs >14 days).
      Table 5Time to delivery according to final diagnosis
      Percentage of N with column totals summing to 100.
      Time to delivery, dPreeclampsia with severe features (n=401)Superimposed preeclampsia (n=105)Preeclampsia without severe features (n=32)Gestational hypertension (n=43)Chronic hypertension (n=47)No hypertensive disorder (n=125)
      0–247.129.518.89.34.313.6
      3–715.220.09.49.32.15.6
      8–148.710.53.12.32.14.8
      >1428.940.068.879.191.576.0
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      a Percentage of N with column totals summing to 100.
      Figure 2 compares TTD according to PlGF levels of normal (>100 pg/mL), low (12–100 pg/mL), or very low (<12 pg/mL). Within the group of 280 subjects who had a very low PlGF level (<12 pg/mL), delivery within 3 days was 5.6-fold more likely than delivery after 14 days after enrollment (174 subjects vs 31 subjects; P<.0001). Figure 3 is a survival curve that summarizes the relationship between PlGF concentration and TTD in the presence or absence of the final diagnosis of preeclampsia. The PlGF levels of normal, low, and very low have well-separated distributions of TTD with median values of 45, 10, and 2 days, respectively.
      Figure thumbnail gr2
      Figure 2Time to delivery
      Time to delivery is presented by placental growth factor concentration. The placental growth factor levels were normal (>100 pg/mL), low (12–100 pg/mL), very low (<12 pg/mL).
      PlGF, placental growth factor.
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      Figure thumbnail gr3
      Figure 3Relationship between placental growth factor concentration and time to delivery according to final diagnosis of preeclampsia
      The proportion of subjects undelivered versus time to delivery is presented as a set of survival curves for three levels of placental growth factor and two categories of final diagnosis. The placental growth factor levels were normal (>100 pg/mL), low (12–100 pg/mL), very low (<12 pg/mL).
      GA, gestational age; PE, preeclampsia, TTD, time to delivery.
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      The strength of PlGF as a predictor of TTD can also be demonstrated by calculation of the hazard ratio in Cox regression. The test positive group (PlGF ≤100 pg/mL) compared with the test negative group (PlGF >100 pg/mL) has a hazard ratio of 9.37 (95% confidence interval, 6.69–13.12) in univariate Cox regression. The hazard ratio is 9.21 (95% confidence interval, 6.58–12.91) when adjusted for the gestational age at enrollment. The hazard ratio is 7.55 (95% confidence interval, 5.35–10.65) when adjusted for both the gestational age at enrollment and the presence of proteinuria at enrollment. The hazard ratio is 7.17 (95% confidence interval, 5.08–10.13) when adjusted for both gestational age at enrollment and the final diagnosis of preeclampsia. The final diagnosis is a relatively weak predictor compared with PlGF in the same regression, where it has a hazard ratio of 2.43 (95% confidence interval, 1.75–3.36). These hazard ratios demonstrate that PlGF is a strong and independent predictor of TTD.
      Table 6 illustrates the relationship between TTD, final diagnosis, PlGF dichotomous category (Test+ or Test–), and 2 selected perinatal outcomes of importance: perinatal death and SGA (birthweight <10th percentile) infants. Regardless of the presence or absence of a final diagnosis of preeclampsia or delivery within or after 2 weeks from enrollment, a PlGF value of <100 pg/mL is associated with the higher risk of subsequent perinatal death (1.3–11.4%) and/or delivery of a SGA infant (48.9–63.2%). To simplify, subjects with PlGF ≤100 pg/mL have a perinatal death rate of 5.7% (95% confidence interval, 3.8–8.2%) and an SGA rate of 51.7% (95% confidence interval, 47.1–56.3%); subjects with PlGF >100 pg/mL have a perinatal death rate of 0% (95% confidence interval, 0–1.3%) and an SGA rate of 16.8% (95% confidence interval, 12.6–21.8%).
      Table 6Time to delivery and diagnosis by placental growth factor (dichotomized at approximately 100 pg/mL) with perinatal outcomes in each category
      Time to delivery, dN (%)Diagnosis, n (%)Placental growth factor category, n (%)Death, n (%)Small for gestational age, n (%)
      Less than 10th percentile. Test+, placental growth factor, ≤100 pg/mL; Test–, placental growth factor, >100 pg/mL.
      ≤14401 (53.3)Not preeclampsia, 43 (10.7)Test–, 11 (25.6)04 (36.4)
      Test+, 32 (74.4)3 (9.4)16 (50)
      Preeclampsia, 358 (89.3)Test–, 27 (7.5)05 (18.5)
      Test+, 331 (92.5)19 (5.7)162 (48.9)
      >14352 (46.7)Not preeclampsia, 172 (48.9)Test–, 137 (79.7)019 (13.9)
      Test+, 35 (20.3)4 (11.4)19 (54.3)
      Preeclampsia, 180 (51.1)Test–, 104 (57.8)019 (18.3)
      Test+, 76 (42.2)1 (1.3)48 (63.2)
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.
      a Less than 10th percentile. Test+, placental growth factor, ≤100 pg/mL; Test–, placental growth factor, >100 pg/mL.
      Finally, the potential clinical utility of a single PlGF measurement above or below 100 pg/mL in the prediction of preeclampsia and/or delivery within 7 or 14 days is illustrated in Table 4. The sensitivity and specificity for a final diagnosis of preeclampsia, within either 7 or 14 days, is only 76% and 68%, respectively. On the other hand, for ruling out deliveries within 7 or 14 days of enrollment, with or without preeclampsia, the sensitivities of a single PlGF measurement are ≥90%, and the negative predictive values are high (approximately 90%). In addition, for the prediction of preterm delivery at <37 weeks gestation, a PlGF ≤100 pg/mL shows good specificity and sensitivity (85% and 82%, respectively) and a high positive predictive value of 93.5%. In the 280 patients with a very low PlGF level (<12 pg/mL) compared with the rest of the cohort, the prediction of preterm delivery at <37 weeks gestation is very strong, with a positive predictive value of 98.9% and an odds ratio of 72.5 (95% confidence interval, 22.9–229.4).

      Comment

      Principal findings

      This prospective multicenter study with blinded adjudication of preeclampsia diagnosis by an independent expert panel shows that, among women with signs or symptoms of preeclampsia at <35 weeks gestation, PlGF levels (dichotomized approximately 100 pg/mL) had a sensitivity of 76% in the prediction of a final diagnosis of preeclampsia. Further, PlGF levels strongly and independently predict the TTD within 7 and 14 days (sensitivity 91–94%), irrespective of a diagnosis of preeclampsia. In addition, PlGF levels identified pregnancies that were at risk of perinatal death, preterm delivery, and an SGA infant. Moreover, PlGF levels may be useful to identify women with confirmed preeclampsia who are not at risk for preterm birth or other adverse perinatal outcomes.

      Results

      Ischemic placental syndrome that results from defective deep placentation is associated with a wide spectrum of obstetric complications that can include ≥1 of the following occurrences: preeclampsia, fetal growth restriction, preterm birth, abruptio placentae, oligohydramnios, or fetal death. Some authors refer to these as the “Great Obstetrical Syndromes.”
      • Brosens I.
      • Pijnenborg R.
      • Vercruysse L.
      • Romero R.
      The “Great Obstetrical Syndromes” are associated with disorders of deep placentation.
      PlGF is produced in the placenta and is considered a marker of placental function and potentially fetal growth restriction and/or fetal death.
      • Benton S.J.
      • McCowan L.M.
      • Heazell A.E.P.
      • et al.
      Placental growth factor as a marker of fetal growth restriction caused by placental dysfunction.
      • Chaiworapongsa T.
      • Romero R.
      • Erez O.
      • et al.
      The prediction of fetal death with a simple maternal blood test at 24-28 weeks: a role for angiogenic index-1 (PlGF/sVEGFR-1 ratio).
      Disease of the placental vascular bed that results in placental ischemic injury is associated with decreased PlGF production. Our findings reveal that extremely low PlGF values can be a marker for the great obstetrical syndromes.
      In a prospective multicenter study from the United Kingdom that had a similar design to our study, Chappell et al
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      examined the diagnostic accuracy of a low plasma PlGF concentration (<5th percentile for gestation; <100 pg/mL) in 287 women with suspected preeclampsia between 20 and <35.0 weeks gestation. The overall prevalence of preeclampsia in their study was 48.9%. In addition, they reported that a PlGF at <5th percentile had a high sensitivity (0.96; 95% confidence interval, 0.89–0.99) and a high negative predictive value (0.98; 95% confidence interval, 0.93–0.995) for the development of preeclampsia within 14 days; however, the specificity was lower (0.55; 95% confidence interval, 0.48–0.61). The authors concluded that, for women who present at <35 weeks gestation with suspected preeclampsia, a low PlGF level has high sensitivity and high negative predictive value for preeclampsia developing within 14 days and that low PlGF levels are superior to other currently used biomarkers to predict preeclampsia.
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      In a subsequent study, the same investigators evaluated the role of PlGF in predicting delivery of a small-for-gestational-age infant and other adverse perinatal outcome in women with suspected preterm preeclampsia.
      • Griffin M.
      • Seed P.T.
      • Duckworth S.
      • et al.
      Predicting delivery of a small-for-gestational-age infant and adverse perinatal outcome in women with suspected pre-eclampsia.
      The primary outcome was the delivery of an SGA infant with a birthweight <3rd percentile for gestational age. As in our study, they found that low PlGF values could be a helpful adjunct for the identification of those women who are at high risk for an SGA infant who could benefit from close antenatal surveillance and timely delivery.
      • Griffin M.
      • Seed P.T.
      • Duckworth S.
      • et al.
      Predicting delivery of a small-for-gestational-age infant and adverse perinatal outcome in women with suspected pre-eclampsia.
      The concentration of free PlGF is related inversely to the circulating amount of its soluble receptor, sFlt-1, which is synthesized and secreted by the placenta.
      • Levine R.J.
      • Maynard S.E.
      • Qian C.
      • et al.
      Circulating angiogenic factors and the risk of preeclampsia.
      The ratio of sFlt-1 to PlGF has been reported to predict preeclampsia and TTD accurately in women with suspected preeclampsia.
      • Zeisler H.
      • Llurba E.
      • Chantraine F.
      • et al.
      Predictive value of the sFlt-1:PlGF ratio in women with suspected preeclampsia.
      In a European cohort (n=550) between 24+0–36+6 weeks that used an sFlt-1:PlGF ratio cutoff of 38, Zeisler et al
      • Zeisler H.
      • Llurba E.
      • Chantraine F.
      • et al.
      Predictive value of the sFlt-1:PlGF ratio in women with suspected preeclampsia.
      reported a negative predictive value of 99.3% for preeclampsia and delivery within 1 week of presentation, compared with our negative predictive value of 93.2%. Notably, the prevalence of preeclampsia in this cohort was only 17.8%, much less than the rate of 71.4% in our cohort, which demonstrated that PlGF retains a clinically useful sensitivity and negative predictive value in a high prevalence and high-risk early preterm population.

      Clinical implications

      This study was designed to collect specimens for the validation of the PlGF assay. The statistical analysis plan for the validation was created according to the study by Chappell et al
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      that was the initial study to propose (1) a fixed 100-pg/mL cutoff for women enrolled at <35 weeks gestation and (2) an endpoint of delivery (or delivery for confirmed preeclampsia) within 14 days. Chappell et al proceeded to conduct an randomized controlled trial for the management of suspected preeclampsia based on medical decision logic using PlGF at the 100 pg/mL.
      • Duhig K.E.
      • Myers J.
      • Seed P.T.
      • et al.
      Placental growth factor testing to assess women with suspected pre-eclampsia: a multicentre, pragmatic, stepped-wedge cluster-randomised controlled trial.
      Although the randomized controlled trial reported excellent and important results, the medical decision logic (with the use of PlGF at the 100-pg/mL cutoff) is based on a single observational cohort, the original study by Chappell et al in 2013. Our study verifies the original observations that were used as the basis for a successful randomized controlled trial.
      In contrast to the study of Chappell et al,
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      the rate of preeclampsia in our cohort was 71.4%. This finding is likely the consequence of our stratified enrollment from all gestational ages, rather than consecutively enrolled patients as in their study. Nonetheless, our findings are in agreement with those of Chappell et al regarding the sensitivity (92.5%) of low PlGF levels that predicted preeclampsia with delivery within 14 days, with a negative predictive value of 90.3%
      The overall incidence of SGA in our study participants was 38.9%, whereas the reported incidence by Chappell et al
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      was 47%. In addition, the incidence of SGA was 43.4% in those with a final diagnosis of preeclampsia but was 17.8% in those without preeclampsia. It is important to note that the rate of an SGA infant was substantially higher in those with low PlGF level, irrespective of the presence of preeclampsia. Furthermore, a low PlGF level was more likely to be associated with preterm delivery and perinatal death. Indeed, all of the perinatal losses in this study were associated with a low PlGF level.
      Gestational age of ≥35 weeks was excluded in the cohort, because the cutoffs for effective use of PlGF after 35 weeks gestation are unclear. This limitation was shown in a previous study of preeclampia.
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      The reference range of PlGF was studied in healthy pregnancy, in which it was shown that healthy subjects have rapidly dropping PlGF values as they approach term.
      • Saffer C.
      • Olson G.
      • Boggess K.A.
      • et al.
      Determination of placental growth factor (PlGF) levels in healthy pregnant woman without signs or symptoms of preeclampsia.
      These values are often <100 pg/mL, which makes PlGF difficult to interpret for a gestational age of ≥35 weeks. Furthermore, a test with applicable predictive value for 14 days would allow a window of management up to 37 weeks gestation, when delivery for preeclampsia is specified by American College of Obstetricians and Gynecologist recommendations.
      • Roberts J.M.
      • August P.A.
      • Bakris G.
      • et al.
      Hypertension in pregnancy: report of the American College of Obstetricians and Gynecologists’ task force on hypertension in pregnancy.
      The TTD could be influenced by demographic variables and clinical variables, which include differences in the practice of medicine at the different clinical sites where the study was performed. Statistical control of these variables on the predictive performance of PlGF was considered. However, it was found that the dominant variables that influence TTD were gestational age at enrollment and final diagnosis, which itself is a composite of all the clinical variables collected during and after enrollment.

      Research implications

      Alterations in concentrations of angiogenic factors have been shown to precede the clinical onset of preeclampsia by several weeks. Levine et al
      • Levine R.J.
      • Maynard S.E.
      • Qian C.
      • et al.
      Circulating angiogenic factors and the risk of preeclampsia.
      demonstrated lower mean PlGF concentrations in subjects who later experienced preeclampsia and in subjects with established preeclampsia, compared with those who never experienced the disease. Our results would also suggest that low PlGF levels might identify those who are at high risk for adverse pregnancy outcome, which includes perinatal loss, irrespective of a final diagnosis of preeclampsia.
      Because all previous studies, including our current investigation, were observational in nature, we agree with the conclusions of Chappell et al,
      • Chappell L.C.
      • Duckworth S.
      • Seed P.T.
      • et al.
      Diagnostic accuracy of placental growth factor in women with suspected preeclampsia: a prospective multicenter study.
      Seely et al,
      • Seely E.W.
      • Solomon C.G.
      Improving the prediction of preeclampsia.
      and others that randomized trials are needed to determine the potential clinical value of adding PlGF measurements or sFlt-1:PlGF ratio in women with suspected preeclampsia to assist in triage decisions and treatment of such women.

      Strengths and limitations

      Strengths of this study include enrollment at 24 study sites and a large participant cohort that encompassed a wide demographic and ethnic profile across North America. Plasma testing was performed in a central laboratory that ensured that results were obtained with rigorous quality control. All final diagnoses were adjudicated by an independent expert panel with revisions to accommodate the 2013 American College of Obstetricians and Gynecologists definitions of hypertension in pregnancy.
      • Roberts J.M.
      • August P.A.
      • Bakris G.
      • et al.
      Hypertension in pregnancy: report of the American College of Obstetricians and Gynecologists’ task force on hypertension in pregnancy.
      Study limitations include test results that were not validated in a repeat sample or by comparative testing at a second laboratory. Further, the inclusion of 24 study sites with potentially varying presentations of preeclampsia and indications for delivery represents a potential study limitation. Some explanation is perhaps required on the reason that these data were delayed in their submission for publication. Although study enrollment was conducted between November 2010 and January 2012, the specimens were banked, and the PlGF analysis was not complete until August 2016. The project was suspended for business reasons; in August 2018, the manufacturer allowed the investigators to move forward on the publication.

      Conclusions

      In summary, in women with suspected preeclampsia at <35 weeks gestation, a low PlGF level is correlated strongly with the need for preterm delivery independent of a final diagnosis of preeclampsia or gestational age at presentation, whereas a normal PlGF predicts pregnancy prolongation, even in patients destined to acquire a final diagnosis of preeclampsia. Among women who delivered within 2 weeks of enrollment, 89% had a final diagnosis of preeclampsia, of which, 92% had a low PlGF level with high rates of adverse perinatal outcome. In contrast, only 8% of patients with preeclampsia who delivered within 2 weeks of enrollment had a normal PlGF level, but these subjects had no perinatal deaths and a low rate of SGA infants. This suggests that, in women with suspected preeclampsia, low PlGF levels identify those who are at high risk for adverse pregnancy outcome, irrespective of a final diagnosis of preeclampsia.

      Appendix

      Appendix 1Preeclampsia Triage by Rapid Assay Trial investigators and institutions
      Principal investigatorInstitution
      R.H. BallMaternal Fetal Services of Utah, Salt Lake City, UT
      J.R. BartonBaptist Health Lexington, Lexington, KY
      R. BeyerleinClinical Trials of America, Inc, Eugene, OR
      K.A. BoggessUniversity of North Carolina, Chapel Hill, NC
      C.A. CombsObstetrix Medical Group, San Jose, CA
      L. A. FrielUniversity of Texas Health Sciences Center, Houston, TX
      C. Gyamfi-BannermanColumbia University Medical Center, New York, NY
      H.Y. HowNorton Healthcare, Louisville, KY
      S. JoyCarolinas HealthCare System, Charlotte, NC
      A. LeeCorvallis Clinic, Corvallis, OR
      B.J. LevinePhoenix OB/GYN, Moorestown, NJ
      D.F. Lewis, JrGreater Cincinnati OB/GYN, Inc, Cincinnati, OH
      J.N. Martin, JrUniversity of Mississippi, Jackson, MS
      A.L. NessSaint Peter’s University Hospital, New Brunswick, NJ
      L. PereiraOregon Health & Science University, Portland, OR
      R.R. ReaganWomen’s Healthcare, San Diego, CA
      C.J. RobinsonMedical University of South Carolina, Charleston, SC
      S. SadanandanRegional Obstetrical Consultants, Chattanooga, TN
      C. SafferWest Coast OB/GYN, San Diego, CA
      S. SunderjiProMedica Physician Group,Toledo, OH
      F. TillmanDiscovery Clinical Trials, Dallas, TX
      P. von DadelszenBC Women’s Hospital and Health Centre, Vancouver, BC, Canada
      A. WagnerSaginaw Valley Medical Research Group, LLC, Saginaw, MI
      D.A. WoelkersUniversity of California, San Diego, CA
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.

      Appendix 2 Placental growth factor assay methods

      The Alere Triage PlGF Test (Alere San Diego, Inc, San Diego, CA) is a fluorescence immunoassay used with the Alere Triage Meter for the quantitative determination of placental growth factor in EDTA-anticoagulated plasma specimens. The test procedure involves the addition of several drops (230 μL) of plasma to the sample port of the single-use disposable plastic cartridge. The test uses fluorescently labelled monoclonal antibodies against placental growth factor for quantification and contains chemistries for on-board positive and negative controls systems to ensure that the quantitative placental growth factor result is valid. Calibration information is supplied by the manufacturer in the form of a lot-specific erasable programmable read-only memory chip that is provided with each kit of devices. The measurable range of the assay is 12–3000 pg/mL. The total precision (coefficient of variation) on plasma controls for placental growth factor at concentrations of 85 and 1300 pg/mL is 12.8% and 13.2%, respectively. The concentration of placental growth factor is displayed on the meter screen approximately 15–20 minutes from the addition of specimen.
      Barton et al. Low placental growth factor is predictive of preterm delivery. Am J Obstet Gynecol 2020.

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