A prospective study of maternal anti-HPA 1a antibody level as a potential predictor of alloimmune thrombocytopenia in the newborn.

BACKGROUND Neonatal alloimmune thrombocytopenia is most commonly due to transplacental passage of maternal anti-HPA 1a antibodies. A prospective study was carried out to evaluate the pattern and quantity of maternal anti-HPA 1a antibodies in order to predict the level of thrombocytopenia in the neonates. DESIGN AND METHODS A monoclonal antibody immobilization of platelet antigen assay was used to detect antibodies in maternal samples from 1,990 HPA 1bb women. HLA DRB3*0101 typing was performed in all immunized women by sequencing the HLA DRB3 gene when present. RESULTS Primary immunization more often took place in connection with delivery than during the first pregnancy. There was a strong correlation between maternal antibody levels and the platelet counts in the newborn (R(2) = 0.49, p < 0.001). A maternal antibody level above 3.0 IU/mL measured in gestational week 22 or 34 had a diagnostic sensitivity and specificity of 93% and 63%, respectively, for predicting the grade of neonatal thrombocytopenia. The women who were negative for HLA DRB3*0101 had significantly lower anti-HPA 1a antibody levels than those who were HLA DRB3*0101 positive (p < 0.007). In contrast to primigravida, in whom anti-HPA 1a antibody levels increased during pregnancy, the antibody level decreased in 92 of 147 women who had been pregnant previously (P(92 or more of 147) = 0.003). The anti-HPA 1a antibody level regularly increased after delivery. CONCLUSIONS Maternal anti-HPA 1a antibody levels in weeks 22 and 34 of pregnancy are good predictors of the degree of thrombocytopenia in the newborn both in the first and subsequent pregnancies. Most mothers became immunized at the time of delivery.