Pseudothrombocytopenia: a laboratory artifact with potentially serious consequences.

When a low platelet count is unexpected or is unaccompanied by signs or symptoms of hemorrhage, pseudothrombocytopenia should be suspected. The platelet number and morphologic features should be examined on a smear prepared from blood anticoagulated with EDTA. Platelet counts should be repeated either by obtaining a finger-stock specimen with an ammonium oxalate Unopette and counting by phase microscopy or by collecting both an EDTA- and a sodium citrate-anticoagulated venous sample, performing a platelet count on both test tubes, and examining a blood smear from each test tube. If a Coulter Model S Plus IV or V or the Technicon H6000 is used for performance of platelet counts, examination of the histogram display of the Coulter instrument or the peroxidase X-Y display of the Technicon H6000 should alert the instrument operator to the presence of EDTA-induced platelet clumping and prevent the reporting of a spuriously low platelet count.