What is the Ki-67 labeling index?


The Ki-67 labeling index is a number that tells doctors what percentage of cells in a tissue sample are actively dividing. It is based on a test for Ki-67, a protein that appears in the nucleus of cells when they prepare to divide. Cells that are resting and not dividing do not show Ki-67.

By calculating the labeling index, pathologists can measure how many cells are dividing compared to the total number of cells in the sample.

How is the Ki-67 labeling index measured?

Pathologists perform a laboratory test called immunohistochemistry (IHC). This test uses antibodies that attach to the Ki-67 protein in dividing cells. Under the microscope, these cells appear stained (positive), while resting cells remain unstained (negative).

The labeling index is then calculated as a percentage. For example:

  • If 10 out of 100 cells are positive for Ki-67, the labeling index is 10%.

  • If 70 out of 100 cells are positive, the labeling index is 70%.

Sometimes the calculation is done by visually estimating the percentage, and other times digital image analysis is used for greater accuracy.

What does the Ki-67 labeling index mean?

The Ki-67 labeling index gives information about how quickly cells are growing:

  • Low Ki-67 labeling index – Only a small number of cells are dividing. The tissue or tumor is growing slowly.

  • High Ki-67 labeling index – A large number of cells are dividing. The tissue or tumor is growing more quickly.

The exact meaning of a “low” or “high” index depends on the type of tumor. For example, in neuroendocrine tumors, the Ki-67 labeling index is an essential part of grading. In breast cancer, it helps separate slow-growing tumors from those that may behave more aggressively.

Ki-67 labeling index

Is a high Ki-67 labeling index always cancer?

No. A high Ki-67 labeling index does not always mean cancer. Some normal tissues naturally contain many dividing cells. For example, in lymph nodes, immune cells multiply quickly when the body is fighting an infection. In these situations, the Ki-67 labeling index will be high but the finding is normal.

This is why the Ki-67 labeling index must always be interpreted in the context of what tissue is being tested and whether there is a tumor present.

Why is the Ki-67 labeling index included in my pathology report?

Your pathology report may include the Ki-67 labeling index if you have a type of cancer where this test provides important information about the tumor’s behavior. A higher index may suggest a more aggressive tumor that grows faster, while a lower index suggests slower growth.

Doctors use this information along with other features of the tumor, such as its type, grade, and stage, to guide treatment decisions and provide information about prognosis.

Questions to ask your doctor

  • What is the Ki-67 labeling index in my tumor?

  • Does this number mean my tumor is slow growing or fast growing?

  • How does the Ki-67 labeling index affect the treatment options you are recommending?

  • Is the labeling index used to grade or stage my tumor type?

  • Could my Ki-67 labeling index reflect normal dividing cells rather than tumor cells?

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