The Natural History of Fetomaternal Alloimmunization to the Platelet specific Antigen HPA-1a as Determined by Antenatal Screening.
LM Williamson, et al (Univ of Cambridge, England; East Anglia and Birmingham Centres, England; Addenbrooke’s Hosp, Cambridge, England; et al). Blood 92:2280-2287,1 998.
The most common cause of severe fetal and neonatal thrombocytopenia is immunization against the human platelet antigen (HPA)-1 alloantigen. Fetal therapy is associated with significant risks, and this study clarified the indications for treatment.
Conclusion: 1 in 350 unselected pregnancies is complicated by HPA-1a alloimmunizaiton. This complication results in severe thrombocytopenia in 1 in 1200 through HPA-1a and HLA-DRB3 0101 typing combined with antiHPA-1A, titration. Clinicians can identify most pregnancies at risk of severe thrombocytopenia.
Editorial comments: Alloimmune thrombopenia based on maternal antibody production, usually stimulated by human platelet antigen 1A(HPA-1a) born on fetal platelets and absent in maternal blood, usually appears in the form of fetal haemorrhage in an earlier pregnancy. Genotyping for HLA-DRB3 0101, a gene thought to be predictive of alloimmune sensitization was carried out along with serial assays of HLA-1A antibody.
The only predictor of severe newborn thrombocytopenia of interest was a maternal antibody titer greater than 1 to 32 in a third-trimester pregnancy. Although HLA genotyping for prediction of severity had a specificity of 99%, the prospective value of a positive finding was 35% with a 65% false positives.
This data confirms the lack of cost effectiveness of routine HPA testing in pregnant women but point to some area where future refinements could increase efficacy of therapy using maternal immunoglobulinG or fetal platelet transfusion