Relative incidence of agranulocytosis and aplastic anemia.

Agranulocytosis and aplastic anemia are both rare, life-threatening blood dyscrasias. Agranulocytosis is mainly caused by medicines, whereas the etiology of aplastic anemia is largely unexplained. In two epidemiologic studies using the same methods, we observed a striking inverse relationship between the incidence of the two diseases in different regions, including five countries in Europe, and Israel and Thailand. The annual incidence of agranulocytosis ranged from 1.1 to 4.9 cases per million, and that of aplastic anemia, from 0.7 to 4.1 per million; the inverse correlation was consistent among the regions (R2 = 0.74). There is no clear explanation for this previously unreported pattern, but it seems unlikely to be due to methodology.