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To verify the platelet count, the entire blood smear, including the feather edge, lateral edges, readable area, and thick area, should be examined under low magnification for the presence of clumps of platelets. The blood smear should then be examined under higher magnification for the presence of red cell fragments, bacterial or fungal organisms, debris, and giant platelets. If any of these interferences are present, the automated platelet count is unreliable, and a platelet scan comment should be reported in qualitative terms as normal, increased, or decreased. The comment should also mention the type of interference—for example, “normal platelet count with giant platelets present” or “normal platelet count with red cell fragments present.”

If platelets are clumped after collection in an EDTA-anticoagulated tube that was well mixed at the time of collection, this may represent in vitro EDTA-induced changes. Platelets must be quantified from blood collected directly into a counting diluent using the anticoagulant recommended by the manufacturer of the counting diluent or by estimating the count from a non-anticoagulated blood film.

D’Souza C, Briggs C, Machin SJ. Platelets: the few, the young, and the active. Clin Lab Med. 2015;35(1):123–131.

Gulati G, Song J, Florea AD, Gong J. Purpose and criteria for blood smear scan, blood smear examination, and blood smear review. Ann Lab Med. 2013;33(1):1–7.

Segal HC, Briggs C, Kunka S, et al. Accuracy of platelet counting haematology analysers in severe thrombocytopenia and potential impact on platelet transfusion. Br J Haematol. 2005;128(4):520–525.

Tantanate C, Khowawisetsut L, Pattanapanyasat K. Performance evaluation of automated impedance and optical fluorescence platelet counts compared with international reference method in patients with thalassemia. Arch Pathol Lab Med. 2017;141(6):830–836.

Olga Pozdnyakova, MD, PhD
Associate Professor of Pathology
Harvard Medical School
Medical Director, Hematology Laboratory
Brigham and Women’s Hospital
Boston, Mass.
Member, CAP Hematology/Clinical Microscopy Committee