Middle cerebral artery peak systolic velocity in the prediction of fetal anemia

Abstract

The fetal middle cerebral artery peak systolic velocity (MCA PSV) has been suggested as a potential test to predict the fetal hematocrit level. We tested the hypothesis that a low fetal hematocrit is associated with an increase in MCA PSV in a prospective study of normal and alloimmunized pregnancies. Fetal hematocrit and MCA PSV were obtained in 26 alloimmunized fetuses, immediately before their first fetal blood transfusions between 15 and 35 weeks. Results were compared with the MCA PSVs from 170 control fetuses not at risk of alloimmune anemia between 13 and 37 weeks. In control fetuses, PSV varied with gestation (PSV = 0.56 - 0.032 GA + 0.00086 GA2, where GA is gestational age; R2 = 0.41). The correlation between PSV and hematocrit Z scores (Pearson correlation coefficient r = -0.69) was highly significant (P = 0.0001). Using a PSV > 1 SD, the sensitivity of the test in predicting a fetal hematocrit < 2 SD below the mean was only 64% but the specificity was 100%. However, the sensitivity of the test in predicting a fetal hematocrit < 3 SD and < 4 SD rose to 73% and 83%, while the specificity was still good (93% and 80% respectively). MCA PSV and fetal hematocrit are highly significantly correlated. The sensitivity of the test was good and the high positive predictive value indicates that the presence of a raised PSV (defined as > 1 SD) is a strong indicator of fetal anemia. In conclusion, MCA PSV is a useful test in clinical practice for the detection of fetal anemia.