Is there a relationship between anti-HPA-1a concentration and severity of neonatal alloimmune thrombocytopenia?

There is uncertainty about the relationship between anti-HPA-1a levels and severity of neonatal alloimmune thrombocytopenia (NAIT). To investigate this relationship further,the concentration of anti-HPA-1a in HPA-1b homozygous women was determined, using a newly developed quantitative ELISA that uses purified anti-HPA-1a to obtain a standard curve. Seventy-eight samples collected from 22 HPA-1b homozygous pregnant women at various stages of pregnancy were tested. These included five women who had delivered babies with severe NAIT. A national HPA-1a antibody standard (NIBSC 93/710), designated as 1 arbitrary unit/mL (AU/mL),was used in each ELISA to calibrate the purified anti-HPA- 1a, enabling the presentation of results as AU/mL. Moreover, selected samples were also assayed by PAK 12 and their reactivity compared with quantity of antibody. The use of the purified HPA- 1a antibody yielded consistent sigmoid curves, enabling the measurement of HPA-1a antibody concentration in the test samples. The antibody concentration was significantly correlated with the antibody titer in the 78 samples studied (R = 0.54, p < 0.001). Furthermore, there was a significant correlation between PAK 12 and the quantitative ELISA in a selected number of cases, with or without NAIT (R = 0.71, n = 10; p < 0.02). On the other hand, there was no correlation of antibody concentration with NAIT incidence (R = -0.046). This study indicates that there is no relationship between anti-HPA-1a concentration and severity of NAIT when ELISA is used, although the correlation between ELISA and other methods, such as monoclonal antibody immobilization of platelet antigens (MAIPA) assay, remains to be determined.