Can you count on the mitotic index?

The mitotic index (MI) is the most commonly used quantitative measure in anatomic pathology but has been widely criticized for supposed “irreproducibility.” In this article, the authors show that mitotic figure (MF) counts may be described by a Poisson distribution, give confidence intervals for the MI, and show that the supposed irreproducibility is largely a consequence of the counting techniques. They also show how to obtain the MI to a predetermined level of precision and present an analysis of why mitotic figure counting works, when properly used, despite the inherently low precision of the estimates that are usually obtained.