Ms. J, age 55 years, was referred to hematology for evaluation of isolated thrombocytopenia (platelet count 10 K/μL) found during a routine complete blood count (CBC) performed as part of her annual examination. She reported no history of thrombocytopenia or other hematologic disorder, and there was no history of bleeding. In general, her personal and family histories were unremarkable.

The patient was not taking medications and reported no allergies or history of smoking, alcohol, or illicit drugs.

Physical examination and vital signs were normal. There was no pallor, icterus, palpable nodes, hepatomegaly, splenomegaly, or any signs of bleeding or thrombosis.

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1. Laboratory data

Ms. J’s labaratory data were as follows: WBC 9,600/μL; hemoglobin 14.2 g/dL; hematocrit 42.3%; mean corpuscular volume 80.0 fL; and platelets 10 K/μL. A chemistry panel, prothrombin time, and partial thromboplastin time were normal. A peripheral blood smear showed significant platelet clumping (Figure 1).

2. Diagnosis and discussion

Pseudothrombocytopenia is defined as a falsely low platelet count, with an estimated frequency of between 1 in 1,000 and 1 in 10,000 blood specimens collected in ethylenediaminetetra-acetic acid (EDTA)-containing tubes. The condition has been reported to occur in 2% of hospitalized patients;1-4 most of the time this is due to an in vitro platelet clumping process that leads to a falsely decreased platelet count.

Pseudothrombocytopenia is a diagnostic challenge for health-care providers and should be part of the early differential diagnosis in all cases of thrombocytopenia to avoid unnecessary tests and treatments.

Although seen in fewer than 2% of all cases of decreased platelets counts,3,4 some authors refer to pseudothrombocytopenia as the second most common cause of thrombocytopenia.2

The three different scenarios in which pseudothrombocytopenia can be seen are EDTA-induced platelet clumping, platelet satellitism, and giant platelet syndromes.3-5 In all three, the platelet count is falsely low because giant platelets and aggregates are not counted by the electronic particle counter.6