False-negative solid-phase platelet crossmatch results due to prozone phenomenon.

Prozone is a known phenomenon affecting immunoassays causing falsely low or negative results when excess target is present in the test system. For assays used to evaluate immune-mediated platelet (PLT) transfusion refractoriness, prozone-like phenomenon has been described in solid-phase human leukocyte antigen (HLA) antibody testing and can be mitigated by diluting samples or pretreating samples with ethylenediaminetetraacetic acid (EDTA) or dithiothreitol. Prozone phenomenon has not yet been described in solid-phase red blood cell (RBC) adherence PLT crossmatch assays. CASE REPORT: A 40-year-old female with myeloid sarcoma and PLT transfusion refractoriness underwent repeated solid-phase PLT crossmatches; however, crossmatch-compatible PLTs units did not yield adequate PLT count responses. Class I HLA antibody testing with neat, diluted, and EDTA-pretreated serum demonstrated significant prozone-like effect and the presence of numerous high strength HLA antibodies. Based on this HLA antibody profile, HLA antigen-negative PLTs gave an adequate PLT count response. It was noted that the HLA types of her crossmatch-compatible PLTs were incompatible with her HLA antibody profile (eg, HLA-A2). With ABO-identical, HLA-A2-positive PLT units, a solid-phase PLT crossmatch was repeated using undiluted and diluted EDTA plasma. Undiluted EDTA plasma demonstrated negative or weakly positive PLT crossmatches while the diluted EDTA plasma demonstrated strongly positive PLT crossmatches. CONCLUSION: The prozone phenomenon can cause false-negative results in solid-phase RBC adherence PLT crossmatch assays, which can be mitigated with sample dilution. In immune-mediated PLT transfusion-refractory patients with high-strength HLA antibodies, sample dilution should be considered to correctly identify compatible PLT inventory.