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Pathology report

Ki-67 Proliferation Index: — What Your Percentage Means

The Ki-67 percentage on your pathology report tells your oncologist how fast tumour cells are dividing. Many people read it as a survival score. It is not. It is one piece of information that helps your team decide how intensive your treatment needs to be.

Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026

  • A measure of cell division speed — Ki-67 shows what proportion of tumour cells were actively dividing at the time of biopsy.
  • Given as a percentage — The number reflects how many cells tested positive for the Ki-67 protein — not a danger score out of 100.
  • One input into treatment planning — Your oncologist reads Ki-67 alongside tumour grade, hormone receptor status and HER2 result — not in isolation.
  • Not a survival predictor on its own — Ki-67 is a tool for deciding how to treat, not a verdict on what will happen.
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Ki-67 is a protein made only by cells that are dividing. Your pathology report gives it as a percentage of tumour cells that tested positive. Oncologists use that figure, alongside receptor status and tumour grade, to plan treatment intensity. It is a planning tool, not a survival score.

What does Ki-67 measure?

Ki-67 is a protein your cells produce only when they are getting ready to divide or are actively dividing. A pathologist stains your biopsy sample and counts the proportion of tumour cells that carry this protein. That proportion becomes the percentage on your report.

A higher percentage means a larger share of cells were dividing at the time of the biopsy. A lower percentage means fewer were. This tells your oncologist how quickly the tumour appears to be growing — it is a measure of activity, not of spread.

In breast cancer, Ki-67 contributes to classifying a tumour as luminal A-like or luminal B-like. Those categories often carry different treatment approaches. Ki-67 also appears in other cancer types, where its interpretation depends on the specific cancer.

What do the terms on your Ki-67 report mean?

Ki-67 proliferation index
The percentage of tumour cells that tested positive for the Ki-67 protein. It reflects how many cells were in an active phase of division when the tissue was examined.
Low Ki-67
A lower percentage suggests a slower-dividing tumour. In breast cancer, this is one factor that may point toward a less intensive treatment approach — but only when read alongside receptor status, grade and other findings, never on its own.
High Ki-67
A higher percentage suggests cells are dividing more quickly. This often leads oncologists toward treatments that target rapidly dividing cells, such as certain chemotherapy regimens, again as part of a broader assessment of your full pathology.
Cut-off or threshold
The value used to separate low from high on a report. There is no single agreed cut-off across all guidelines — different centres draw this line at different points, which is why the number must always be interpreted by your own team in the context of your full results.
Luminal A-like and luminal B-like
Classification terms for breast cancer subtypes. Ki-67 is one of the criteria that helps distinguish between them. Your oncologist assigns these categories from the full pathology result — not from Ki-67 alone.

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Questions to ask your oncologist about your Ki-67 result

  • What is my Ki-67 percentage, and how does it compare to the threshold your team uses?
  • What are my hormone receptor results — both ER and PR?
  • What is my HER2 status?
  • What tumour grade has been assigned, and what does that mean for my plan?
  • Based on all these results together, what treatment are you recommending?
  • Would a genomic test add any useful information for my case?

Did you know?

Ki-67 scoring is known to vary between laboratories — and sometimes between pathologists reading the same slide — by a margin that can affect how a result is categorised.

This is one reason ASCO and the College of American Pathologists have published standardisation guidelines, and why your team interprets your Ki-67 in the context of your whole pathology report rather than as a standalone figure.

Source: ASCO/College of American Pathologists Ki-67 immunohistochemistry standardisation guidelines

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Common questions

Frequently asked questions

Is a high Ki-67 always a bad sign?

High Ki-67 means more cells were dividing at the time of the biopsy — it is a measure of activity, not a verdict. A higher number often leads oncologists toward more intensive treatment, because some treatments work precisely on rapidly dividing cells. It is one input into a plan that your team builds from receptor status, HER2 result, grade, nodal involvement and your overall fitness. Your oncologist is best placed to explain what your specific result means for your treatment.

What counts as a high Ki-67 percentage?

There is no single universally agreed threshold. Different guidelines and different centres draw the line at different points, and some experts argue that a single cut-off oversimplifies what is a continuous measurement. What matters is how your team interprets your result against the threshold they use, alongside your other pathology findings. Ask your oncologist what threshold they apply and how your number relates to it.

Will my Ki-67 result change my treatment?

Yes, it often contributes to treatment decisions, particularly in breast cancer. A result in the high range may support adding chemotherapy to the plan. A result in the low range may support a less intensive approach. It is almost never the only deciding factor — your team uses Ki-67 alongside receptor status, grade, nodal involvement and tumour size before making a recommendation.

Can Ki-67 change after treatment?

Yes. Ki-67 is sometimes measured again on surgical tissue after treatment given before surgery — neoadjuvant chemotherapy or hormone therapy — to assess how the tumour responded. A fall in Ki-67 is one sign of response. Whether your team uses repeat Ki-67 testing depends on your treatment plan and cancer type. Ask your oncologist whether this applies to your situation.

My report shows a specific Ki-67 number. Can you tell me what it means?

No online source can interpret a specific Ki-67 percentage for you, because the number only makes sense alongside the rest of your report. The same percentage can lead to different plans in different clinical situations — depending on receptor status, grade, nodal involvement and your overall fitness. Bring your full pathology report to your appointment and ask your oncologist to walk through each result with you.

Why do different labs give different Ki-67 numbers?

Ki-67 is measured by staining tissue and counting positive cells, and both the staining technique and the counting method can vary between laboratories. Two labs using different antibody preparations or different scoring rules can produce different results from the same sample. ASCO and the College of American Pathologists have issued guidance to improve standardisation, but variability remains a known limitation. If results from two laboratories differ, ask your oncologist which one your treatment plan is based on.

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