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Fetal fibronectin (fFN) is used as a biomarker for preterm delivery. Currently, its use is discouraged if there has been vaginal manipulation in the previous 24 hours. Our objective is to determine if there are differences between fFN results before and after vaginal manipulation in the form of sterile vaginal exam (SVE) or transvaginal ultrasound (TVUS).
Prospective observational study of women between 22-33 6/7 weeks at risk for preterm birth (PTB) due to a history of PTB, short cervix, multifetal gestation or symptoms of preterm labor at a single center. We excluded women with vaginal bleeding or infection, placenta previa, ruptured membranes, cervical dilation >3cm, or any form of vaginal manipulation in the previous 24 hours. Specimen 1 was collected prior to planned SVE or TVUS and Specimen 2 was collected within 4 hours from SVE or TVUS. The agreement between Specimens 1 and 2 was assessed using descriptive statistics. Sample size calculations indicated that 290 subjects would be needed to produce a two-sided 95% confidence interval with a width of 0.04 (margin of error of 0.02), assuming a proportion of agreement of 0.975. Proportions for test characteristics with confidence intervals (CI) were calculated.
310 women were enrolled. Specimen 1 was positive in 37 (12%) and negative in 273 (88%). Specimen 2 was positive in 39 (13%) and negative in 271 (87%). (Figure 1) Of the specimens with discordant results, 14 (5%) negative Specimen 1 results subsequently became positive for Specimen 2, and 12 (32%) positive Specimen 1 results became negative for Specimen 2 . Overall, there was a 91.6% agreement between Specimens 1 and 2 (CI 88%-94%). The strongest test characteristic of Specimen 2 for predicting Specimen 1 was specificity. (Figure 2)
There is a high degree of agreement between pre- and post-vaginal manipulation ffN results (92%). The specificity of Specimen 2 for predicting Specimen 1 was high at 95%. Therefore, in cases where fFN tests are collected after TVUS or SVE, a negative result can be interpreted as valid, while a positive result should be repeated in 24 hours due to an unacceptably high cross-over rate. Future directions include a comparison of test characteristics between Specimen 1 and 2 in predicting PTB.