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A mathematical formula has been developed to estimate fetal hemoglobin based on the middle cerebral artery peak systolic velocity (MCA-PSV). A preliminary study reported strong correlation between estimated and observed fetal hemoglobin prior to the first intrauterine transfusion. The goal of this study was to assess the accuracy of this model in the prediction of fetal hemoglobin after multiple fetal transfusions.
This was a retrospective study of data from 162 cordocentesis procedures performed for fetal red-cell alloimmunization in patients with a history of 1 (N=60), 2 (N=39), 3 (N=31) or greater than 3 (N=32) intrauterine transfusions. Doppler velocimetry of the MCA-PSV was performed prior to cordocentesis. Expected hemoglobin values for gestational age were calculated using previously described formulas. Predicted hemoglobin multiples of the median (MoM) were calculated as 0.6835 + 1.2794*MCA-PSV MoM – 1.2885*MCA-PSV MoM2 + 0.2861*MCA-PSV MoM3. Median hemoglobin for gestational age was calculated as e2.84-(8.55/GA). The product of the predicted hemoglobin MoM and median hemoglobin was used to calculate expected hemoglobin. Observed fetal hemoglobin was measured at time of cordocentesis. Patients were subdivided based on the number of previous transfusions. Linear regression analyses were used to assess the correlations between observed and calculated pretransfusion fetal hemoglobin levels.
The median gestational age was 27.0 (IQR 23.3, 31.8), 28.0 (24.0, 31.3), 29.0 (27.0, 33.0), and 31.9 (29.2, 33.8) weeks for patients with history of 1, 2, 3, or more than 3 previous transfusions, respectively. The median observed hemoglobin for each respective group was 9.7 (IQR 7.5, 11.2), 8.7 (8.0, 10.0), 10.0 (9.1, 11.4), and 9.0 (7.6, 10.0) g/dL. Scatterplots showing regression lines between calculated and measured fetal hemoglobin are shown in Figure 1. There were significant correlations after 1 (r2 = 0.51; P < .001) and 2 (r2 = 0.41; P < .001) previous transfusions. There was no correlation after 3 (r2 = 0.003; P = .77) or greater than 3 transfusions (r2 = 0.09; P = .10).
Calculated fetal hemoglobin is well correlated with observed fetal hemoglobin after 1 or 2 previous intrauterine transfusions. The ability to calculate fetal hemoglobin with MCA-PSV is diminished after 3 or more intrauterine transfusions.