Pseudothrombocytopenia due to agglutinins.

Pseudothrombocytopenia may have any of a number of causes, one of which is agglutination in vitro. This phenomenon was found in samples of blood from six patients. A serum factor responsible for the agglutination was demonstrated. The factor was dependent upon the presence of EDTA and was more active at room temperature than at 37 C. It could be identified as an IgM immunoglobulin in four cases. In the other two cases definite characterization was not possible, but there was some evidence in favor of an IgM factor. All six patients had elevated serum IgM levels, but they had different and unrelated clinical disorders.