A “visual platelet estimate” is done by the laboratory when the device performing the blood count analysis detects abnormalities that may interfere with the platelet count. Elements that can interfere with the platelet count are platelet clumps and megathrombocytes. The analysis is then performed under a microscope (blood smear), and the number of platelets viewed replaces the count performed by the device.
Platelet clumps are formed when the blood platelets responsible for coagulation stick together in clusters. The presence of platelet clumps has no clinical consequences other than preventing instruments from properly counting blood platelets. Platelet clumps tend to form in samples collected with difficulty and in a small number of individuals (about 1/2000) whose platelets react with the EDTA anticoagulant in lavender-top tubes. Megathrombocytes are very large platelets that are excluded from the platelet count by automatons. It is normal to find large platelets in response to bleeding. Bone marrow responds by producing young platelets, which are typically larger than old platelets. However, they can also be found in different bone marrow diseases.