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IHC Markers Explained

Ki-67 IHC Staining — What That Percentage Actually Measures

Ki-67 is a number that appears on many cancer pathology reports. It shows what proportion of tumour cells were actively dividing when the biopsy was taken. This page explains how that number is produced and why it can differ between laboratories.

Medically reviewed by Dr. Mohammed Imaduddin, Surgical Oncologist, MBBS · MS (General Surgery) · M.Ch (Surgical Oncology) · Last reviewed September 2026

  • Marks dividing cells only — Ki-67 stains cells that are actively dividing. Resting cells contain none of the protein and do not stain.
  • Reported as a percentage — The number on your report — such as 15% or 40% — is the proportion of counted tumour nuclei that stained positive.
  • Variable between labs — The same tumour can produce different numbers in different laboratories. Your oncologist accounts for this.
  • Interpretation is clinical — What your specific number means for your cancer is a question your oncologist answers, not this page.
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Ki-67 is a protein found only in actively dividing cells. Your pathologist counts what proportion of tumour cell nuclei stain positive for it and reports that as a percentage. It is a measure of how fast the tumour was growing at the time of your biopsy. What that percentage means for your situation is a question for your oncologist.

How is Ki-67 detected in a biopsy sample?

Ki-67 is a protein cells make only when they are actively dividing. Resting cells contain none of it. This makes it useful as a marker: wherever the antibody stains, a cell was dividing at the moment the biopsy was taken.

The laboratory cuts a thin slice of biopsy tissue onto a glass slide and applies an antibody — most commonly one called MIB-1 — that binds to the Ki-67 protein inside dividing cell nuclei. A colour reaction then makes those bound nuclei turn brown.

Resting cell nuclei remain unstained. This lets the pathologist count dividing and non-dividing tumour cells under a microscope and report the proportion of positives as a percentage.

How is Ki-67 scored?

FeatureManual countAutomated image analysis
Who countsA pathologist, counting by eye under a microscopeSoftware scanning a digital image of the slide
Area selectedPathologist chooses the hot spot — the area with the most stainingA defined region or the whole slide, set by the lab
Cells countedA selection of cells in chosen areasThousands of cells across the slide
Main variableThe pathologist's choice of counting areaSoftware thresholds set by the individual laboratory
Where usedMost centres in IndiaSpecialist and academic laboratories

What factors can shift the Ki-67 number between labs?

  • Fixation timeToo short or too long in fixative changes how well the antibody binds to the protein in the nucleus.
  • Antibody cloneMIB-1 and SP6 are the two most common clones; they can give different readings on the same tissue.
  • Counting areaCounting a high-activity hot spot gives a higher number than averaging across the whole tumour.
  • Number of cells countedCounting more cells gives a more stable result; counting too few raises the chance of a misleading number.
  • Manual versus automated countingEach method introduces its own sources of variation, independent of the tumour's actual biology.
  • Tumour heterogeneityOne tumour can have high-activity and low-activity areas. Where the biopsy needle landed affects the result.

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Terms you may see on your report

Ki-67 index
The percentage of counted tumour cell nuclei that stained positive for Ki-67 protein. This is the core number on your report.
Proliferation index
Another name for the Ki-67 result. Some reports use this term; it means the same thing.
MIB-1
The name of the antibody clone most commonly used to detect Ki-67. Some reports say 'Ki-67 by MIB-1' to specify the method.
Hot spot
The region of the slide with the highest density of Ki-67-positive nuclei, where the pathologist focuses the count.
Nuclear staining
Staining that appears inside the cell nucleus. Ki-67 is always nuclear — only the nucleus turns brown, never the cytoplasm.
IHC
Immunohistochemistry — the laboratory method that uses antibodies to detect proteins in tissue slices mounted on glass slides.

Why can two labs report a different number from the same tumour?

Two laboratories examining material from the same biopsy can produce different Ki-67 numbers. This is not a sign that one result is wrong.

The most common sources of difference are the antibody clone used, the fixation time of the tissue, and whether the pathologist counted a hot spot or averaged across the tumour. International bodies including the International Ki-67 in Breast Cancer Working Group have published standardisation guidelines to narrow this variation, but it persists.

Your oncologist interprets your result knowing how your specific laboratory is calibrated and what is typical for your cancer type.

Questions families ask about Ki-67

Does a higher Ki-67 always mean a more dangerous cancer?

A higher Ki-67 index means tumour cells were dividing more rapidly at the time of the biopsy. Faster division is generally associated with more aggressive behaviour. But what it means for your cancer depends on the cancer type and the full picture of your pathology report. In some cancers, a higher Ki-67 is linked to better response to certain treatment approaches. Your oncologist interprets it alongside all the other findings, not as a standalone verdict.

My report says Ki-67 is 30%. Is that high?

Whether 30% is high, borderline or low depends entirely on your cancer type and your laboratory's reference range. The same number carries different meaning in a breast cancer report than in a lymphoma report, and different laboratories calibrate their thresholds differently. This page cannot interpret your specific result — that depends on your cancer type, stage and full pathology panel. Your oncologist will explain what your number means in your situation.

Can Ki-67 change during treatment?

Yes. Treatments that slow or stop cell division can reduce the Ki-67 index measured in a later biopsy or surgical specimen. A lower Ki-67 after treatment, compared with the original result, is sometimes used as an indicator of response. How to interpret any change — and what it means for your ongoing care — is a clinical judgement for your oncologist based on your treatment type and your cancer.

Why does my report use different names — proliferation index, MIB-1 score?

These refer to the same measurement. MIB-1 is the antibody clone most labs use to detect Ki-67; some reports name the clone to be precise about the method. Proliferation index is the broader term for any measure of how fast cells are dividing; Ki-67 measured by MIB-1 is the most common way to produce it. If your report uses any of these names, it is describing the same number: the proportion of tumour nuclei that stained positive.

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Common questions

Frequently asked questions

What does Ki-67 positive mean on a pathology report?

A Ki-67 positive cell was actively dividing at the time the biopsy was taken. The Ki-67 index — the percentage figure on your report — is the proportion of tumour cells in this state. It is a measure of the tumour's growth rate at that moment, not a verdict on the cancer. Your oncologist interprets it alongside your other pathology findings.

What is a normal Ki-67 level?

There is no single normal range that applies across all cancer types or all laboratories. What is considered low, intermediate or high differs by cancer type and by the laboratory's calibration. Ask your oncologist what the reference range is for your specific cancer and where your result falls within it — that is more useful than comparing it to a general figure.

Is Ki-67 the same as tumour grade?

They are related but not the same. Tumour grade is an overall assessment of how abnormal the cells look, taking in nuclear appearance, how many cells are dividing, and tissue architecture. Ki-67 feeds into grade in some cancer types, particularly breast cancer, but grade incorporates other observations. Ki-67 is one of the tools pathologists use to assign grade, not a grade by itself.

Can Ki-67 vary in different parts of the same tumour?

Yes. Most tumours have areas of faster and slower growth. A needle biopsy samples only a small part of the tumour, so Ki-67 from a biopsy can differ from Ki-67 measured on the full surgical specimen. This is one reason your oncologist may weigh the two results differently when both are available.

Why does my Ki-67 result differ from what someone else with the same cancer reported?

Because the number depends on where in the tumour the biopsy was taken, the laboratory's method, and the antibody clone used — all of which can differ between people even with the same diagnosis. Comparing your Ki-67 to someone else's is rarely informative. Your oncologist reads your result in the context of your own full pathology report.

Should I ask for Ki-67 testing if it is not on my report?

Ki-67 is not routinely ordered for every cancer type. Whether it would add useful information for your cancer is a question for your oncologist. If you have read that Ki-67 is relevant for your cancer type and it is absent from your report, it is reasonable to ask your oncologist whether it should be ordered and what difference it would make to your care.

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