Antenatal screening for fetal alloimmune thrombocytopenia: the results of a pilot study

Feto-maternal incompatibility for the human platelet antigen HPA-1a is an important cause of severe fetal thrombocytopenia. The incidence is 1 in 1000-2000 pregnancies, which is more common than other conditions for which screening is presently carried out. Antenatal diagnosis and management are now available, but only for subsequent siblings following diagnosis of a previously affected infant. This study describes a pilot prospective screening programme for the antenatal detection of fetomaternal alloimmune thrombocytopenia (FMAIT) due to HPA-1a incompatibility. 3473 women were typed for HPA-1a using a method designed for large-scale typing. 71 women found to be HPA-1a negative were further tested for HLA-DR52a as a risk factor for alloimmunization. All women were monitored for the development of anti-HPA-1a throughout pregnancy and a cord full blood count was taken at delivery. Two affected pregnancies were found and treated: a singleton pregnancy was treated antenatally and a twin pregnancy after delivery. The study showed that screening for FMAIT could be established within the pre-existing antenatal red cell serology programme. It was concluded that screening should be based on platelet typing and offered regardless of parity. Further stratification, combining DR52a typing and HPA-1a antibody screening, although focusing on the group of women at greater risk, may not identify all affected pregnancies. Confirmation of the diagnosis and severity of FMAIT continues to depend on fetal blood sampling during pregnancy or cord blood samples after birth.