EVIDENCE OF A COLD IMMUNOGLOBULIN-M AUTOANTIBODY AGAINST 78-KD PLATELET GLYCOPROTEIN IN A CASE OF EDTA-DEPENDENT PSEUDOTHROMBOCYTOPENIA

Pseudothrombocytopenia is a phenomenon in which the electronic count shows spuriously low platelet counts in subjects with normal platelet levels. The mechanism of anticoagulant-dependent pseudothrombocytopenia appears to involve cold reactive agglutinins against platelet antigens. The authors report a case of EDTA-dependent pseudothrombocytopenia with evidence of a cold immunoglobulin M antibody against 78-kD platelet membrane glycoprotein (GP). Cell counts were performed by Coulter Counter S-Plus VI (Coulter, Hialeah, FL) in the following anticoagulants: EDTA, Na-citrate, and citrate-theophylline-adenosine-dipyridamol. Anti-platelet antibodies and platelet membrane GP antigens were assayed by an immunofluorescence technique as described by Van dem Borne in 1978. An immunoglobulin M/lambda anti-platelet antibody was found to react in serum as well as in plasma EDTA at room temperature, but not at 37-degrees-C. This antibody appeared to be directed against GP78 membrane antigen because this antigen was not detectable by immunofluorescence in platelets collected in EDTA and Na-citrate anticoagulant, whereas a fluorescence signal was revealed in platelets collected in citrate-theophylline-adenosine-dipyridamol. This evidence was confirmed by platelet clumping inhibition tests in which target platelets were pretreated with anti-GP monoclonal antibodies. Clumping in the presence of pseudothrombocytopenia serum was inhibited by anti-GP78kD and anti-GPIIb/IIIa but not by anti-Ib. In this case, GP78 appears to be involved in platelet clumping, together with IIb/IIIa complex. The partial inhibition of the phenomenon observed in citrate-theophylline-adenosine-dipyridamol is probably related to a lower expression of the membrane antigens in platelets collected in this anticoagulant.