Pseudothrombocytopenia and other conditions associated with spuriously low platelet counts.

Abstract

Accurate measurements of the platelet count are necessary to diagnose thrombocytosis or thrombocytopenia correctly, gauge the severity of the clinical risk and identify the most appropriate therapeutic intervention. Despite increased diagnostic accuracy with the electronic counters, it is still unsatisfactory in rare situations. Conditions causing spurious thrombocytopenia include the following. 1) Pre-analytical errors, such as difficult venipunctures, over-/under-filling of blood collection tubes, insufficient mixing of blood with the anticoagulant (EDTA), which may cause fibrin formation; 2) EDTA-induced, temperature- and time-dependent, antibody-mediated in vitro platelet agglutination, with consequent reduction in the number of single platelets in the sample; the condition, referred to as Pseudothrombocytopenia, is benign and does not need follow-up or medical interventions; the use of alternative in vitro anticoagulants does not prevent agglutinates formation in all samples; accurate platelet counts could be obtained by testing EDTAsamples immediately after blood collection. 3) EDTA-induced in vitro platelet adherence to leukocytes (Platelet Satellitism), caused by bridging IgG antibodies binding to GPIIb-IIIa on platelets and the Fc receptor-III on leukocytes; occasionally, leukocytes may phagocytose platelets and/or form platelets/leukocytes clumps. 4) Presence of large/giant platelets (commonly from patients with congenital or acquired thrombocytopenia) that are not recognized as such by electronic counters, which distinguish platelets from other cells based on their smaller size. 5) Blood from patients with type 2B von Willebrand Disease, which may display large/giant platelets and platelet agglutinates. All the above conditions are easily identifiable by microscopic examination of anticoagulated peripheral blood smears, which is an indispensable diagnostic procedure in hematology.