TRANSPLACENTAL ALLOIMMUNIZATION AGAINST P LATELET ANTIGENS - PREVALENCE AND FEATURES IN A CHILEAN POPULATION

Background: Neonatal alloimmune thrombocytopenia (NAIT) is a result of fetomaternal incompatibility. Platelet destruction is caused by a mat ernal antibody directed against a fetal platelet antigen inherited fro m the father adn lacking on the mother's platelets. The incidence and features of transplacental alloimmunization depend on the frequency of expression of platelet specific antigens, which are highly variable a mong different populations. Aim: To determine the prevalence and chara cteristics of transplacental alloimmunization in a large group of preg nant women in Chile. Material and methods: We studied 3,041 samples ob tained during the third trimester of gestation. In all samples, anti p latelet antibodies were screened by ELISA with platelet membranes fixe d to a microtiter plate. Positive samples were further studied for ant igenic specificity with the monoclonal antibody specific immobilizatio n of platelet antigens (MAIPA) test. Results: Anti platelet antibodies were found in 261 samples (8.5%). The MAIPA test identified 6 samples with antibodies directed against major platelet membrane glycoprotein s, 2 anti GPIb, 2 anti GPIIb/IIIa and 2 anti GPIa/IIa. In four cases, anti HLA antibodies coexisted. Two cases corresponded to well defined platelet antigen systems: one anti HPA-1a and one anti HPA-5b. No clin ical evidence of thrombocytopenia of the newborn was detected in all t hese cases with anti GP antibodies. Conclusions: A prevalence of plate let specific antibodies of 0.2% with only one anti HPA-1a was detected . These findings are in contrast with those of other populations but i n accordance with the low frequency of the HPA-1b/b phenotype in the C hilean population. The very low incidence of platelet specific antibod ies and the lack of association with clinical thrombocytopenia in the newborn, do not support the recommendation of routine antenatal screen ing to all women in Chile.