Mitotic Index Calculator
Calculates the percentage of cells actively dividing in a sample. Used by biologists and pathologists to assess tissue growth rates or evaluate tumor aggressiveness.
About this calculator
The mitotic index (MI) measures the proportion of cells in a population that are currently undergoing mitosis — the process of cell division. It is expressed as a percentage and serves as a key indicator of proliferative activity in tissues. The formula is: MI = (mitotic cells / total cells) × 100. A high mitotic index suggests rapid cell division, which can indicate healthy growth in developing tissue or, in clinical contexts, malignancy in tumor samples. Pathologists routinely count cells in stained histological slides and apply this formula to grade cancers. In research, the mitotic index helps compare the effects of drugs or environmental factors on cell cycle progression.
How to use
Suppose you are examining a stained tissue slide and count 200 cells in your field of view, of which 18 are in mitosis (showing condensed chromosomes or a mitotic spindle). Plug the values into the formula: MI = (18 / 200) × 100 = 9%. This means 9% of cells in the sample are actively dividing. A value this high in a tumor biopsy would typically indicate a high-grade, aggressively proliferating neoplasm and would influence the pathologist's staging report.
Frequently asked questions
What is a normal mitotic index in healthy tissue?
Normal mitotic index values vary widely by tissue type. In slowly renewing tissues such as muscle or nerve, the MI is close to 0%, while rapidly renewing tissues like intestinal epithelium may show MIs of 1–3%. Values above 5–10% in solid tumors are generally considered high and correlate with poorer prognosis. Clinical grading systems such as the Elston-Ellis system for breast cancer use mitotic count directly as one of three grading criteria.
How does the mitotic index differ from the proliferation index?
The mitotic index counts only cells visibly in mitosis at the moment of fixation, capturing a snapshot of active division. The proliferation index, often measured using markers like Ki-67 immunostaining, labels all cells that have entered the cell cycle — including those in G1, S, and G2 phases — giving a broader picture of growth fraction. MI tends to be lower numerically than the proliferation index for the same tissue. Both metrics are complementary; pathologists often report both to fully characterize tumor biology.
Why is the mitotic index important in cancer diagnosis?
Cancer cells divide uncontrollably, so a high mitotic index is a reliable proxy for tumor aggressiveness and growth speed. Oncologists use it to assign tumor grade, predict patient prognosis, and guide treatment intensity. For example, in breast cancer grading, more than 10 mitotic figures per 10 high-power fields places a tumor in the highest grade category. Some chemotherapy drugs, such as taxanes and vinca alkaloids, specifically target mitosis, making MI a useful biomarker for predicting drug response.