Pseudothrombocytopenia caused by platelet agglutinins that are reactive in blood anticoagulated with chelating agents.

The phenomenon of in vitro platelet clumping and consequent pseudothrombocytopenia in the presence of EDTA was studied in the plasmas and sera of three patients. A heterogeneity was identified among these platelet agglutinins. lgG agglutinins present in two patients recognized antigens that were present when EDTA was added to platelets. This effect of EDTA did not appear to be mediated by calcium chelation. On the other hand, an IgM antibody from another patient appeared to react with platelet antigens that were expressed when the calcium concentration was less than 50 giM. EDTA. citrate, and oxalate all induced platelet agglutination with this IgM agglutinin. Thus, pseudothrombocytopenia in chelated blood is caused by a group of agglutinins that are heterogeneous with respect to their physicochemical properties and mechanism of action.