Features of pseudothrombocytopenia due to a cold agglutinin:
(1) The patient has thrombocytopenia on an automated analyzer.
(2) Examination of the blood smear shows platelet clumps.
(3) The platelet count estimated from the number of platelets seen on the smear is much higher than the result given by the analyzer.
(4) The clumps persist in blood collected in sodium citrate or other non-EDTA anticoagulants.
(5) The platelet count is normal if the blood is kept warm from collection to analysis. Some cold agglutinins may be very temperature dependent and any drop in temperature is enough to cause agglutinins to form.
The cryoglobulins are usually IgM but can be mixed IgM-IgG.
Some patients will have a history of a recent infection (CMV, hepatitis C, etc). In these patients the cold agglutinins may be transient.
Differential diagnosis:
(1) antiplatelet antibodies
(2) EDTA-associated thrombocytopenia (platelet clumps in EDTA anticoagulated blood but not in citrate anticoagulated blood)
