William Aird: Pseudothrombocytopenia

William Aird shared a post on X:

“I tweeted a poll asking for the next step in a patient with thrombocytopenia and a platelet clumping on a peripheral smear. 62% of you answered correctly, namely to repeat the CBC in a green top (heparin-containing) tube.

Key points:

1)Pseudothrombocytopenia is mediated by EDTA-dependent antibodies that react with platelets in blood that is anticoagulated with EDTA (chelates calcium, necessary for clotting reactions), causing platelet clumping and falsely low platelet counts.

2)Pseudothrombocytopenia is purely an in vitro artifact.

3)EDTA-containing collection tubes are preferred for CBCs/peripheral smears because, compared with other anticoagulants, EDTA allows the best preservation of cellular components and blood cell morphology

4)EDTA-dependent antibodies function optimally between 0°C and 4°C and can belong to any immunoglobulin idiotype (most commonly IgG and IgM).

5)EDTA-mediated pseudothrombocytopenia occurs in 0.07 to 0.20% of the general population and 0.1 to 2% of hospitalized patients.

6)Diagnostic criteria for pseudothrombocytopenia:

•  Absence of clinical bleeding
• Automated platelet count <150 × 109 /L
• Platelet clumps on smear
• Normal platelet count using a different anticoagulant (typically sodium citrate, but whatever works!)

7)Platelet satellism is a related phenomenon in which there is rosetting of platelets around polymorphonuclear neutrophils. Both pseudothrombocytopenia and platelet satellism are mediated by EDTA-dependent antibodies, but the two generally do not occur in the same patient.”

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Source: William Aird/X

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William Aird is a Professor of Medicine at Harvard Medical School and a Hematologist at Beth Israel Deaconess Medical Center. He is the Founder and Executive Director of The Blood Project and serves as Senior Deputy Editor at EBSCO Information Services.