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CION Cancer Clinics
Genetic testing after a colon cancer diagnosis: who needs it and why | CION Cancer Clinics
After a colon cancer diagnosis, almost everyone should have the tumour tested for a DNA repair fault. Only some people then need a blood test for an inherited gene fault, usually because of the tumour result, a young age, many polyps or a family pattern. This page explains the two tests, who is offered which, what the results mean and what they change for your family. At CION Cancer Clinics in Hyderabad, our oncologists explain colon cancer in plain words and plan care around you and your family.
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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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On this page
- Should you have genetic testing after being diagnosed with colon cancer?
- Tumour test or blood test: what is the difference?
- Who is usually offered the blood test after a colon cancer diagnosis?
- What happens when genetic testing is arranged
- The words on a genetic report, in plain language
- What this page cannot tell you
- Common questions about genetic testing after colon cancer
The short answer
Should you have genetic testing after being diagnosed with colon cancer?
Almost everyone with colon cancer should have one genetic test on the tumour itself. Only some people need a second test, on blood, to look for an inherited fault. The tumour test usually decides whether the blood test is worth doing.
Two tests that answer two different questions
The tumour test is done on the biopsy or the bowel removed at surgery. It checks how the cancer behaves, and whether its mismatch repair system, the cell's spell-checker for DNA, is working. You may see this written as MMR or MSI on your report. The blood test, called germline testing, looks at the genes you were born with. Germline means present in every cell from birth, and therefore something that can be passed on.
Why the answer matters
For you, a result can change the size of the operation, the medicines offered if the cancer spreads, and checks for other cancers later. For your brothers, sisters and children, it can decide whether colonoscopy should start years earlier than usual or not at all. Most people tested turn out not to carry an inherited fault, and that answer is useful too.
Most colon cancer is not inherited. Testing is how you find the minority that is.Side by side
Tumour test or blood test: what is the difference?
Tumour test
Blood (germline) test
Done on the biopsy or surgical specimen
Done on a blood or saliva sample
Looks at the cancer's own genes
Looks at the genes you were born with
Offered to almost everyone with colon cancer
Offered when the tumour or family history suggests it
Guides treatment choices for the cancer
Guides screening for you and your relatives
Says nothing certain about the family
Can be repeated in relatives for the same fault
Not sure whether this applies to you?
Ask an oncologistWho should ask
Who is usually offered the blood test after a colon cancer diagnosis?
Doctors look for a handful of clues. Any one of them is enough to raise the question with a genetic counsellor.
The tumour shows a repair fault
A mismatch repair deficient tumour is the commonest route into Lynch syndrome, the main inherited cause of colon cancer. Extra tumour tests first check whether the fault arose inside the tumour alone, which is common in older patients.
If those extra tests explain it, a blood test is usually not needed.You were diagnosed young
Colon cancer before the age of fifty is more likely to have an inherited cause, even with a clear tumour test and no family history. Many specialists now offer a blood test to younger patients as a matter of routine.
There were many polyps
A colonoscopy that found many adenomas alongside the cancer points towards a polyposis condition rather than Lynch syndrome.
Genes usually checked
- APC, behind familial adenomatous polyposis
- MUTYH, behind MUTYH-associated polyposis
- Rarer polyposis genes on a wider panel
The family history looks unusual
Several relatives on one side with bowel, womb, ovarian or stomach cancer, especially at young ages. A second cancer in you, such as womb cancer before or after the colon cancer, counts too.
Step by step
What happens when genetic testing is arranged
The tumour result comes back
The pathology laboratory reports mismatch repair status, usually by staining the tissue for four repair proteins. The tumour board reads it alongside the stage and your family history before anything else is decided.
A conversation before the test
A genetic counsellor draws your family tree and explains what each possible result would mean. This is the time to ask about cost, who will see the report and how relatives would be told.
A blood sample goes to the laboratory
One sample, sent to a partner laboratory, is usually checked against a panel of colon cancer genes at once. It does not delay surgery or chemotherapy, though the tumour board may wait for it if the answer could change the operation.
The result is explained in person
A report is read with you, not handed over cold. If a fault is found, you leave with a plan for your own checks and a letter you can give to relatives explaining the test they can have.
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On your report
The words on a genetic report, in plain language
- Pathogenic variant
- A gene change known to cause raised cancer risk. This is a positive result, and relatives can be tested for that exact change.
- No variant found
- No inherited fault was detected in the genes tested. Your family's screening is then based on the family history instead.
- Variant of uncertain significance
- A change the laboratory cannot yet classify. It should not alter treatment or testing of relatives until it is reclassified.
- Lynch-like
- The tumour has a repair fault but no inherited cause is found. Your counsellor will explain how family screening is handled in that situation.
- Cascade testing
- Testing relatives one step at a time, starting with the closest, for the fault already found in you.
Being straight with you
What this page cannot tell you
It cannot tell you whether your cancer is inherited. That needs your tumour report, your family tree and, where it is warranted, a blood test read by someone trained to interpret it. If your tumour shows a repair fault, our page on what an MMR-deficient result means for your family is the next step.
It cannot interpret your result
The same gene name can carry very different risks depending on the exact change. A result that says "uncertain" is not the same as a positive one. Bring the report to your oncologist or a genetic counsellor rather than searching for the variant name online. The detail of the tumour test itself is explained on our page on MMR and MSI testing.
Who this does not apply to
If you were diagnosed later in life, your tumour shows normal repair proteins, there were few or no polyps and nobody else in the family has had a related cancer, a blood test is unlikely to change anything. A good counsellor will tell you that as readily as they would recommend a test. Rectal cancer follows a similar logic but is covered on our rectal cancer pages.
If you are unsure whether your situation counts, call the helpline and describe it. Someone will give you an honest answer.Questions we are asked
Common questions about genetic testing after colon cancer
Does every colon cancer patient need a genetic blood test?
No. Almost everyone should have the tumour tested for mismatch repair status, but the blood test is offered when that result, your age, your polyps or your family history point towards an inherited cause. Most people with colon cancer do not need it.
Will genetic testing delay my surgery or chemotherapy?
Usually not. Treatment goes ahead while the test runs. Occasionally, when a result could change how much bowel is removed, the tumour board may suggest waiting for it before the operation. That decision is made case by case and explained to you.
My tumour is MMR deficient. Does that mean I have Lynch syndrome?
Not necessarily. Many MMR deficient tumours, especially in older patients, arise from a change inside the tumour alone. Extra tumour tests help sort this out, and only then is a blood test needed. Read more on our Lynch syndrome page.
Does a tumour result affect my treatment?
It can. A mismatch repair deficient cancer behaves differently, which matters for some chemotherapy decisions after surgery. If the cancer spreads, it can make immunotherapy an option. Your oncologist will explain which of these applies to your stage.
Who in my family should be tested if I carry a fault?
Usually your brothers, sisters, parents and adult children first, since each has a chance of sharing the fault. They are tested only for your exact change, which is simpler than your first test. Those who do not carry it can usually follow ordinary screening.
Can the test be done if the relative with cancer has died?
Sometimes. A stored tissue block from an earlier operation can occasionally be tested. If not, a living relative may be offered testing on the strength of the family history. A genetic counsellor will tell you which route is possible in your family.
Will a positive result affect my insurance?
India has no specific law on genetic results and insurance, and the question is not fully settled. It is reasonable to raise it with your counsellor before testing rather than afterwards. Some relatives choose to arrange cover before they are tested.
How do I arrange genetic testing in Hyderabad?
Start with your oncologist and your tumour report. CION's medical oncologists can review it and arrange genetic counselling, with testing at a partner laboratory. Our page on genetic counselling in Hyderabad explains the process.
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
- National Cancer Institute — Genetic Testing Fact Sheet
- American Cancer Society — Colorectal Cancer Risk Factors
- National Cancer Institute — Colon Cancer Treatment
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
Is my colon cancer hereditary?The first question most families askUniversal MMR and MSI testingWhy the tumour test is done for everyoneWhat an MMR-deficient result means for your familyThe next step if your tumour shows a repair faultGenetic counselling for colon cancer in HyderabadHow to get a referral and what the visit involves
Talk to us
Not sure whether you need a genetic test?
Share your tumour report and tell us who in the family has had cancer. We will tell you honestly whether a genetic referral is worth it, and arrange it if it is. One helpline serves every CION centre.