Low-level HLA antibodies do not predict platelet transfusion failure in TRAP study participants.

In the Trial to Reduce Alloimmunization to Platelets (TRAP) study, 101 of 530 participants became refractory to platelet transfusions without evidence of HLA or human platelet antigen (HPA) antibodies. We used a more sensitive bead-based assay to detect and quantify HLA antibodies and a qualitative solid-phase enzyme-linked immunosorbet assay for HPA to determine whether low-level antibodies could predict refractoriness in longitudinal panels from 170 lymphocytotoxicity assay (LCA)(-) and 20 LCA(+) TRAP participants. All TRAP recipients who previously tested LCA(+) were HLA antibody(+), using the bead-based system. Levels of HLA or HPA antibodies did not predict refractoriness among LCA(-) recipients, although higher levels of HLA antibodies were associated with refractoriness among LCA(+) recipients. These data demonstrate that weak to moderate HLA antibody levels detectable by modern binding assays are not associated with platelet refractoriness.