Slide 1: While working in clinic, your female patient’s CBC shows a new thrombocytopenia. She reports no history of bruising or excessive bleeding. You send off a workup for her low platelets, which comes back negative. What common lab additive might be the source of the thrombocytopenia?

Slide 2: EDTA! EDTA is found in lavender-top hematology tubes. It helps to anti-coagulate blood by chelating calcium. In its ionized 2+ form, calcium functions as a procoagulant cofactor for several factors within the clotting cascade (including factors V and VIII). EDTA is considered a particularly useful agent for anticoagulation because it does not distort the shape or size of red blood cells.

Slide 3: However…in up to 2% of hospitalized patients, EDTA can precipitate conformational changes to platelet glycoproteins and phospholipids. These changes cause the platelets to agglutinate excessively. These “hyper-clumped” platelet aggregates are not sensed by automated hematology analyzers, resulting in falsely depressed platelet counts. This phenomenon has been described as EDTA-dependent pseudothrombocytopenia, or EDTA-PTCP.

Slide 4: EDTA-PTCP: suggested workup. Manual blood smear to assess for the presence of large platelet clumps previously undetected by computer analysis. Repeat CBC using citrated or heparinized blood samples. However, up to 17% of EDTA-PTCP patients will still demonstrate pseudothrombocytopenia!