Thrombocytopenia

Platelets are derived from megakaryocytes in the bone marrow, but they can also be released from circulating megakaryocytes that regularly inhabit the capillaries of the lung. Platelet turnover is about 2 × 1011 per day; the platelet lifespan is 8–10 days. Platelets distribute between the circulation and the spleen. In normal individuals, the splenic pool, which corresponds to approximately 30–40% of the total, is in dynamic equilibrium with the circulating pool and is proportional to the splenic mass. Splenic enlargement from a variety of causes is associated with a great increase in the proportion of platelets sequestered in the spleen, which is presumed to represent slower transit of platelets through the enlarged or congested splenic pulp. Platelet destruction results from phagocytosis by macrophages of the hepatic and splenic reticuloendothelial system.

Thrombocytopenia can be defined as a persistent fall in the platelet count below 150 × 109/L. It can be “mild,” when the platelet count is between 100 and 150 × 109/L, “moderate” (50–99 × 109/L), “severe” (20–49× 109/L), and “extremely severe” (<20× 109/L). Given that automated particle counters may give spuriously low results, especially in the presence of giant platelets, it is important to confirm low platelet counts by examining a well-prepared peripheral blood smear.