UNBOUND IMMUNOGLOBULINS ARE A MAJOR SOURCE OF ERROR IN THE QUANTITATION OF PLATELET SURFACE-BOUND IMMUNOGLOBULIN LEVELS

Analysis of assays for the determination of platelet-associated immunoglobulins (PAIgs) has led to disagreement over the amount of surface-bound Igs in both normal controls and patients with elevated platelet Ig levels. By the use of radiolabeled platelets and platelet counts, it was demonstrated that more than 10(8) platelets are disrupted after each centrifugation during platelet isolation procedures, releasing intraplatelet contents into the fluid phase of resuspended platelets in buffer. It was shown that suspensions of whole washed platelets contain a significant, but generally overlooked, amount of unbound Igs that have been liberated from platelets disrupted during processing. When platelet suspensions are evaluated for Igs with assays that fail to incorporate a final separation of whole platelets from the suspension fluid, unbound Igs as well as those bound to the platelet surface are measured, which yields a logarithmic overestimation of surface-bound Igs. It has been further demonstrated that patients infected with human immunodeficiency virus type 1 can have elevations of PAIgG, PAIgA, and PAIgM in both the thrombocytopenic and nonthrombocytopenic states. These elevations are due to increased internal platelet pools of Igs and not to increased surface-bound Igs.