HPA-1a antibody potency and bioactivity do not predict severity of fetomaternal alloimmune thrombocytopenia.
Ghevaert, Cedric; Campbell, Kate; Stafford, Prachi; Metcalfe, Paul; Casbard, Angela; Smith, Graham A.; Allen, Dave; Ranasinghe, Edmund; Williamson, Lorna M.; Ouwehand, Willem H.
[Miscellaneous Article] Transfusion. 47(7):1296-1305, July 2007.

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BACKGROUND: The antenatal management of fetomaternal alloimmune thrombocytopenia (FMAIT) due to HPA-1a antibodies remains controversial, and a test identifying pregnancies that do not require therapy would be of clinical value.

STUDY DESIGN AND METHODS: The statistical correlation was analyzed between clinical outcome and 1) anti-HPA-1a potency in maternal serum samples determined by a monoclonal antibody immobilization of platelet (PLT) antigen assay with an international anti-HPA-1a potency standard and 2) anti-HPA-1a biological activity measured by a monocyte chemiluminescence (CL) assay.

RESULTS: A total of 133 pregnancies with FMAIT due to anti-HPA-1a were analyzed. In 97 newly diagnosed cases, there was no difference in antibody potency or CL signal between cases with intracranial hemorrhage (ICH; n = 15), those with no ICH but a PLT count of less than 20 x 109 per L (n = 52), and those with a PLT count of at least 20 x 109 per L (n = 30). In 22 previously known pregnancies, the positive predictive value of maternal anti-HPA-1a of greater than 30 IU per mL for a PLT count of less than 20 x 109 per L was 90 percent, but the negative predictive value was only 66 percent. Antibody potency tended to stay stable throughout pregnancy (n = 16) and from one pregnancy to the next (n = 16).

CONCLUSION: Neither severe thrombocytopenia nor ICH in HPA-1a-alloimmunized pregnancies can be predicted with sufficient sensitivity and specificity for clinical application from maternal anti-HPA-1a potency or bioactivity.

Copyright (C) 2007 Blackwell Publishing Ltd.