First, rule out pseudothrombocytopenia by asking the following question:
Is the blood sample clotted?
Check for EDTA-dependent platelet antibodies by collecting the sample in
an anticoagulant (e.g., citrate)
If pseudothrombocytopenia has been ruled out, ask the following:
Is the patient taking drugs that could lower the platelet count?
Check for receipt of:
Heparin, which may be associated with heparin-induced thrombocytopenia
IIb/IIIa inhibitors (e.g., abciximab, eptifibatide, tirofiban)
Adenosine diphosphate (ADP)–receptor antagonists (e.g., clopidogrel)
Acute alcohol toxicity
Does the patient have a hematinic deficiency (particularly, acute folate deficiency)?
Does the patient have any of the following:
Sepsis (especially consider)
Human immunodeficiency virus (HIV) infection
Disseminated intravascular coagulation
Major blood loss and hemodilution
Mechanical fragmentation
Post-cardiopulmonary bypass
Intraaortic balloon pump
Renal dialysis
Extracorporeal membrane oxygenation
Immune-mediated disorder
Immune thrombocytopenic purpura
Antiphospholipid syndrome
Post-transfusion purpura
Microangiopathic hemolytic anemia
Disseminated intravascular coagulation
Thrombotic thrombocytopenic purpura
Hemolytic–uremic syndrome
Hypersplenism
Other disorder
Myelodysplastic syndrome
Cancer
Hereditary thrombocytopenia