Neonatal Alloimmune Thrombocytopenia due to Anti-HPA and the Incidence of HPA Antibodies in Pregnant Japanese Women.

We screened 5, 213 pregnant Japanese women for HPA antibodies in each trimester using MPHA. HPA antibodies or anti-Naka (three cases) were detected in 47 women (0.9%), and five neonates (0.1%) developed thrombocytopenia (NAIT). Antibody specificities were 36 cases of anti-HPA-5b, five of anti-HPA-4b, three of anti-Naka, and one each of anti-HPA-2b, -3a and -4a. None had anti-HPA-la.In 12 of 16 cases in which the time of antibody production could be ascertained, antibody was first detected in the third trimester. More significantly, nine of 16 women were primi-gravidas and without a history of blood transfusion.In some cases, HPA antibody seemed to be naturally occuring, since two anti-HPA-4b positive women were primi-gravidas, without transfusions and thier husbands were HPA-4 (a+b-).To date, we have experienced 18 NAITP neonates out of 15 couples (including the above five cases). Among these HPA-incompatible couples, 12 had anti-HPA-4b, and one each had anti-HPA-3a, -4a and anti-Naka. Severe NAITP was caused by anti-HPA-3a, and by anti-HPA-4b with high antibody titer.