Examination of a blood film is essential, when an automated full blood count shows thrombocytopenia. First, it is essential to confirm that the platelet count is genuinely low, and that the apparent reduction is not attributable to platelet aggregation, satellitism, or phagocytosis; the film may also show fibrin strands, indicating partial clotting of the specimen. Second, the film may provide a clue to the cause of a congenital or acquired thrombocytopenia, when there are small platelets (Wiskott–Aldrich syndrome), large platelets (Bernard–Soulier syndrome, MYH9-related thrombocytopenia and others), abnormal platelet granulation (Paris-Trousseau thrombocytopenia), or hypogranular platelets (gray platelet syndrome). Examination of other lineages may also be informative indicating, for example, features of May–Hegglin anomaly, acute promyelocytic leukemia, or a thrombotic microangiopathy.
The film shown is from a 20-year-old man, who presented with significant hemorrhage into his thigh following a sporting injury. His platelet count was 120 × 109/L. The detection of markedly hypogranular platelets including large platelets led to the diagnosis of the gray platelet syndrome, a condition in which α granules, or α and δ granules, are lacking.