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!


  • Mikaelsson M.E. (1991) The Role of Calcium in Coagulation and Anticoagulation. In: Sibinga C.T.S., Das P.C., Mannucci P.M. (eds) Coagulation and Blood Transfusion. Developments in Hematology and Immunology, vol 26. Springer, Boston, MA. Link.
  • Shabnam I, Chuphal DS, and Joshi BC. Ethylenediaminetetraacetic Acid (EDTA) – dependent pseudothrombocytopenia: a case report. J Clin Diagn Res. 2014 Oct;8(10):FL03-4. PMID 25478363.
  • Tan GC, Stalling M, Dennis G, Nunez M, Kahwash SB. Pseudothrombocytopenia due to Platelet Clumping: A Case Report and Brief Review of the Literature. Case Rep Hematol. 2016;2016:3036476. Epub 2016 Dec 4. PMID 28044112.

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