Initial fetal platelet counts predict the response to intravenous gammaglobulin therapy in fetuses that are affected by PLA1 incompatibility.

OBJECTIVE: Fetal alloimmune thrombocytopenia is the result of maternal fetal platelet antigen incompatibility; intracranial hemorrhage is its most serious complication. Our previous studies have demonstrated an inability to accurately predict fetal platelet counts in this disorder. The goal of the present investigation was to identify factors that would predict the response of the fetal platelet count to therapy so that use of fetal blood sampling could be minimized. STUDY
DESIGN: Patients who were eligible for the study were all those who (1) had alloimmune thrombocytopenia secondary to Pl(A1) (HPA-1a, Zw(A)) platelet antigen incompatibility, (2) were treated with maternally administered intravenous immunoglobulin at 1 g/kg of body weight per week, with or without low dose steroids, and (3) had percutaneous fetal blood sampling before the initiation of therapy (first fetal blood sampling) and again 3 to 7 weeks afterwards (second fetal blood sampling).
RESULTS: In this retrospective review, 74 patients who were affected by alloimmune thrombocytopenia had a median platelet count of 21,000 per microliter at the first fetal blood sampling and 47,000 per microliter at the second fetal blood sampling, with a median increase in platelet count of 24,000 per microliter. Response to treatment was defined as either (1) an improvement in platelet count (the second fetal blood sampling greater than the first fetal blood sampling, and second fetal blood sampling > 20,000 per microliter) or (2) a minimal decline in platelet count (the first fetal blood sampling > or = 40,000 per microliter and the difference between the first and second fetal blood sampling < or = 10,000 per microliter). The first fetal blood sampling had prognostic value for the second fetal blood sampling (P = .0001), although the previous sibling birth platelet count and history of sibling intracranial hemorrhage did not predict the platelet count at the first or second fetal blood sampling or the change in platelet count between the samplings. When the patients were segregated to first fetal blood sampling of > 20,000 per microliter versus < or = 20,000 per microliter, the response rates for the 2 groups were 89% (33/37 patients) versus 51% (19/37 patients; P = .001).
CONCLUSION: In fetal alloimmune thrombocytopenia secondary to Pl(A1) platelet antigen incompatibility, fetuses with platelet counts > 20,000 per microliter at the initiation of therapy were predicted to maintain their platelet count at the second fetal blood sampling at > 20,000 per microliter. The characteristics of the previous sibling, as previously reported, did not predict the initial fetal blood sampling, the second fetal blood sampling, or the response to treatment.