Pseudothrombocytopenia induced by a monoclonal IgM kappa platelet agglutinin.

Pseudothrombocytopenia is an in vitro phenomenon usually associated with anticoagulant (EDTA)-dependent IgG platelet agglutinins. A low Coulter platelet count was investigated in a 63-year-old woman with multiple sclerosis and a monoclonal IgM kappa gammopathy. An unusual type of EDTA- and temperature-independent IgM platelet agglutinin was identified by in vitro agglutination of donor platelets and was removed by immunoabsorption.