Fetomaternal alloimmune thrombocytopenia.

Thrombocytopenia is the second commonest haematological abnormality in the neonatal period after anaemia due to iatrogenic blood letting. One to four percent of all newborn babies have a platelet count < 150 x 10(9)/l at birth and approximately 20-40% of neonates in intensive care units are affected by neonatal thrombocytopenia. The most common cause of severe neonatal thrombocytopenia is fetomaternal platelet incompatibility and subsequent alloimmunisation. During the last decade recent advances in molecular techniques have led to rapid and efficient methods for diagnosis. Progress in fetal medicine has enabled accurate determination of fetal status, allowing improvements in fetal diagnosis and therapy. Human platelet antigen (HPA)-1a is by far the most frequently involved platelet antigen system in Caucasians accounting for 90% of cases, followed at a much lower frequency by HPA-5b (5-15%) and HPA-3a. The incidence is estimated to be 1 per 2000 to 1 per 5000 live births, but this is low in comparison to the incidence of fetomaternal platelet antigen incompatibility especially for the HPA-1 alloantigen system in the Caucasian population in whom the estimated frequency of HPA-1b1b individuals is 2%. Retrospective and prospective studies have reported that the immunogenetic background is important, and the chance of HPA-1a alloimmunisation is strongly associated with maternal HLA class II DRB3*0101 (DR52a) type. A significant association (p = 0.004) between severe thrombocytopenia and a third trimester antiHPA titre >1:32 has been observed. It is now possible to genotype the fetus or neonate and the parents, which provides confirmation as to which HPA systems are incompatible between the mother and father. Simultaneous genotyping of HPA-1, 2, 3 and 5 can be carried out using the polymerase chain reaction-sequence specific primers (PCR-SSP) protocol, which has been widely used for HLA class II determination. The platelet count may continue to fall during the first 48 h after birth and the risk of intracranial bleeding is highest during this period. The best option is transfusion of specially selected antigen negative compatible donor platelets or if unavailable, maternal washed platelets. Antenatal screening for the most common form of fetomaternal alloimmune thrombocytopenia (FMAIT), due to antiHPA-1a is under consideration, but there is no established method at present. The Scottish National Blood Transfusion Service started a study in August 1999 on 25,000 pregnancies to carry out a cost benefit analysis of routine antenatal screening. The aims of the study are to determine the frequency of HPA-1b homozygosity; monitor antibody titres during pregnancy and confirm correlation of antibody emergence with HLA-DRB3*0101, and finally to access cost effectiveness of routine screening across Scotland. Of 26,509 women screened in three Scottish regions 501 (1.9%) are HPA-1b homozygous and about 9%, of the consented women are antibody positive.