Giant platelets have a diameter of over 5 µm and are by definition as big as a small erythrocyte. They are usually circular and possess fine basophilic granules. The edges of the cell are not clearly defined.
Giant platelets are often not recognized as platelets by automatic cell counters. This can result in a false reduction of the platelet count. A counting chamber can be used to determine the true values.
Giant platelets are usually an indication of increased megakaryocytopoiesis. They typically occur, therefore with increased platelet consumption (e.g. immune thrombocytopenia) and with increased autonomous production (essential thrombocythemia). Giant platelets are congenitally present in the May-Hegglin anomaly and can be found in Bernard-Soulier syndrome as well.