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Universal MMR and MSI testing: why every colon cancer is checked | CION Cancer Clinics
Every colon cancer should be tested for mismatch repair, written MMR or MSI on reports, whatever your age or family history. The result does two jobs at once. It shapes treatment, including whether chemotherapy after surgery or immunotherapy is likely to help, and it flags the small group who may have inherited Lynch syndrome. This page explains why the test is universal and what each result leads to. At CION Cancer Clinics, our team goes through your reports with you and plans the next test or treatment step.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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The short answer
Why is every colon cancer tested for MMR?
Because the result matters for almost everyone, and nobody can guess it in advance. MMR stands for mismatch repair, the cell's system for fixing copying mistakes in its DNA. When that system fails, the tumour behaves differently and responds differently to some treatments. Guidance such as NICE in the UK recommends testing everyone with colorectal cancer, not only selected patients.
One test, two questions
The first question is about you: does this result change your treatment? The second is about your family: could this cancer come from an inherited fault called Lynch syndrome? The same tissue answers both, which is why the test is done early, on the diagnostic biopsy or on the tissue removed at surgery.
Why not test only people with a family history?
Family history misses many people with Lynch syndrome. Families are small, relatives die of other causes, and cancer is often not talked about. An aunt's bowel cancer may be remembered only as a stomach illness. Bowel symptoms are frequently blamed on piles and never investigated, so a pattern never appears on paper. Testing every tumour avoids depending on a family tree that may be incomplete.
What it costs you in time and tissue
Very little. The test uses tissue that has already been removed, so there is no extra procedure and no extra discomfort. The result usually comes back with the other tumour tests, and the tumour board reads them together before a plan is confirmed. For most people the answer is ordinary, and nothing else changes.
A deficient result is not a diagnosis of Lynch syndrome. It is a reason to look further.Two methods, two results
What do MMR and MSI results actually mean?
Laboratories use one of two methods. They answer the same question from different angles.
MMR by staining
A stain checks whether four repair proteins, MLH1, MSH2, MSH6 and PMS2, are present in the tumour cells. A missing protein points to which gene may be at fault. The MMR and MSI testing page covers the laboratory side.
Report wording
- Retained or intact: the protein is present
- Lost or absent: the protein is missing
MSI by DNA test
This test looks for repeating stretches of DNA code that have changed length because repair has failed. It does not show which gene is involved, but it confirms that the repair system is broken.
Report wording
- MSI-high: repair has failed
- MSS or MSI-low: repair is working
Proficient, the usual result
Proficient MMR, written pMMR or MSS, means repair is working. Most colon cancers fall into this group. Treatment follows the stage and the other tumour tests, and Lynch syndrome is unlikely unless the family history says otherwise.
What it means for you
- Treatment is planned on the stage
- RAS and BRAF guide later choices
- Family advice comes from the history
Deficient, the less common result
Deficient MMR, written dMMR or MSI-high, means repair has failed. It changes some treatment choices, and it starts a short series of checks to find out whether the cause was inherited.
What it can change
- Whether chemotherapy after surgery helps
- Whether immunotherapy becomes an option
- Whether relatives are offered testing
Not sure whether this applies to you?
Ask an oncologistAfter a deficient result
What happens next if the result is deficient?
See which protein is missing
Loss of MLH1 together with PMS2 is the most common pattern. In older patients it is often caused by a switch turned off inside the tumour, not by an inherited fault. Loss of MSH2 or MSH6 points more strongly towards Lynch syndrome.
A second tumour test, if needed
If MLH1 is missing, the laboratory checks the tumour for a BRAF change or for that switched-off MLH1 gene. A positive result usually means the cancer arose by chance, and inherited testing is often not needed.
Counselling and a blood test
If the cause is still unexplained, a genetic counsellor talks you through a blood test for inherited faults in the repair genes. This test looks at every cell in your body, not just the tumour.
Testing relatives
If an inherited fault is found, brothers, sisters and adult children can be tested for that exact fault. Those who carry it start earlier and more frequent colonoscopy. Read what a deficient result means for your family.
Side by side
How does the result change treatment?
Deficient (dMMR, MSI-high)
Proficient (pMMR, MSS)
Stage 2: chemotherapy after surgery is often not needed
Stage 2: the decision rests on other high-risk features
Stage 3: chemotherapy after surgery is still usually advised
Stage 3: chemotherapy after surgery is usually advised
Spread disease: immunotherapy is often an early option
Spread disease: chemotherapy, guided by RAS and BRAF results
Family: further checks for Lynch syndrome
Family: guided by the family history alone
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Did you know
Most deficient results in colon cancer are not inherited. In older people the usual cause is a switch turned off inside the tumour itself, which cannot be passed on. That is why a deficient result leads to more checks, not straight to alarm for the whole family.
Being straight with you
What this page cannot tell you
It cannot tell you whether you have Lynch syndrome. Only the sequence above, ending in counselling and a blood test where needed, can answer that. It also cannot tell you whether immunotherapy suits your cancer. That depends on the stage, your general health and the tumour board's view, and the MSI-high immunotherapy page explains the treatment itself.
Who this does not apply to
If you have never had cancer and are worried about your family history, a tumour test cannot help, because there is no tumour to test. You need a genetic counsellor who starts from your relatives' diagnoses. Benign polyps are not routinely tested either. Rectal cancers are tested too, but their treatment path differs, so read the rectal cancer section.
If your report has no MMR result
Ask for it. The test can be done on the stored tissue block from the biopsy or the operation, without a new procedure. CION arranges it through partner laboratories, and the tumour board reads it alongside your other results.
Not sure whether your tumour was tested? Call the helpline with your report and someone will check it for you.Questions we are asked
Common questions about MMR and MSI testing
Does a deficient MMR result mean I have Lynch syndrome?
No. Most deficient results come from a change inside the tumour that cannot be passed on. Further tumour tests and, where needed, a blood test after genetic counselling settle the question. Until then, treat the result as a reason to look further, not as an answer.
Do I need a new biopsy for MMR testing?
Usually not. The laboratory uses tissue already taken at colonoscopy or surgery, which is stored in a small wax block. If the biopsy was done at another hospital, ask them to release the block or slides so the test can be arranged.
I am older with no family history. Why test me?
Because the result can still change your treatment, especially at stage 2 or if the cancer has spread. Older age makes an inherited cause less likely, but it does not rule one out. The test answers the question instead of guessing at it.
Is MMR testing the same as MSI testing?
They measure the same problem in different ways. Staining looks for the repair proteins themselves, while MSI testing looks for the DNA damage left behind when repair fails. The two usually agree, and your report will say which method was used.
Does an MSI-high result mean I can have immunotherapy?
It makes immunotherapy a strong option for colon cancer that has spread or cannot be removed. For earlier stages, surgery remains the main treatment, and immunotherapy before surgery is still being studied. Your oncologist will explain where it fits for your stage.
Is the test expensive?
MMR staining is one of the simpler tumour tests, and the price varies by laboratory. It is often done as part of a wider molecular panel that also checks RAS and BRAF, which costs more. Ask for a written estimate before the sample is sent.
Should my children be tested if my tumour is deficient?
Not straight away. First the cause has to be found. If an inherited fault is confirmed in you, adult children and siblings can be offered a test for that exact fault. If the change is in the tumour only, relatives follow screening advice based on the family history.
When should the result be ready?
Ideally before the treatment plan is final, because it can change that plan. It is often requested on the biopsy at diagnosis, or on the specimen after surgery. If chemotherapy is being planned and the result is missing, it is fair to ask about it.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- NICE — Molecular testing strategies for Lynch syndrome in people with colorectal cancer
- American Cancer Society — Immunotherapy for Colorectal Cancer
- American Cancer Society — Testing for Colorectal Cancer
- National Cancer Institute — Genetic Testing Fact Sheet
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
What an MMR-deficient result means for your familyWho in the family needs testing, and whenLynch syndrome and colon cancerThe inherited condition the test is looking forWhat MSI-high status means for your outlookHow a deficient tumour tends to behaveWhich biomarkers are tested in colon cancerMMR alongside RAS, BRAF and HER2
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