The short answer
Ki-67 is a marker of how quickly cancer cells are dividing. A higher percentage means more cells are actively growing, which can suggest a faster-growing tumor.
Ki-67 measures the share of cancer cells that are actively dividing.
It is reported as a percentage — higher means more cells are growing.
A higher score can suggest a faster-growing tumor.
It is used alongside other findings, not on its own.
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The full explanation.
A measure of growth speed
Ki-67 is a protein that appears in cells while they are actively dividing. A pathology test measures what percentage of the cancer cells contain it. That percentage — the Ki-67 score — gives a rough sense of how quickly a tumor is growing.
Reading the number
The score is written as a percentage. A low number means relatively few cells are dividing; a high number means many are. Because a higher score can point to a faster-growing tumor, it is one clue doctors use when judging how a cancer may behave.
Why there is no single cutoff
What counts as "high" depends on the type of cancer. A score that is concerning for one cancer may be typical for another, so there is no universal threshold. This is why the number is best interpreted by a doctor familiar with your specific diagnosis.
Used with other findings
Ki-67 is never the whole story. It is read alongside the tumor's grade, stage, and other markers. Together these help build a fuller picture of the cancer and guide treatment choices.
Words to know
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Common questions
▸What is Ki-67?
Ki-67 is a protein found in cells that are actively dividing. A lab test measures the percentage of cancer cells that carry it, giving a sense of how fast the tumor is growing.
▸Is a high Ki-67 bad?
A higher percentage suggests more cells are dividing, which can mean a faster-growing tumor. But its meaning varies by cancer type and is weighed with other factors.
▸What is a normal Ki-67?
There is no single cutoff that applies to all cancers. The interpretation depends on the cancer type, so it is best understood with your doctor.
▸How is it used?
Ki-67 is one piece of information used with grade, stage, and other markers to understand a tumor's behavior and help plan treatment.
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