3.16—Mitotic index calculations

Mitotic index measures the proportion of cells in a sample that are undergoing mitosis: number of cells with visible chromosomes divided by the total number of cells counted. Multiply by 100 for a percentage.

Count cells systematically, include prophase, metaphase, anaphase and telophase in the numerator, and include every counted cell in the denominator. Keep the unit as a proportion or state clearly that it has been converted to a percentage.

If 32 of 42 cells show visible chromosomes, the mitotic index is 32/42 = 0.76, or 76% when expressed as a percentage.

A high index means more cells were observed in mitosis in that sample; it does not automatically prove faster growth. Sampling region, preparation quality and counting decisions affect the estimate.