MSI-High and Mismatch Repair Deficiency: — Testing and Treatment

They describe the same underlying problem from two directions. dMMR — deficient mismatch repair — means the proteins that correct DNA copying errors are not functioning. MSI-H — microsatellite instability-high — is what you see in the DNA when those proteins have failed: an accumulation of errors at short repeated sequences. Laboratories test for both, often at the same time, and NCCN and ASCO treat them as clinically equivalent. If your report shows either finding, the treatment implication is the same.

Not exactly, but they are closely connected. Lynch syndrome is an inherited condition caused by a fault in one of the MMR genes — MLH1, MSH2, MSH6, or PMS2 — that a person is born with. If your tumour shows dMMR, your oncologist may recommend germline testing to find out whether Lynch syndrome is the underlying cause. Not all dMMR tumours are caused by Lynch syndrome — many acquire the fault during the cancer's development. The distinction matters for your family members, who may benefit from genetic counselling and screening.

MSI-H occurs most often in colorectal, endometrial, gastric, and small bowel cancers, and is also found in some ovarian, biliary tract, and other solid tumours. NCCN recommends universal MSI testing for all colorectal and endometrial cancers. For other tumour types, testing is recommended when Lynch syndrome is suspected or when the cancer lacks another clear targetable alteration. Your oncologist will tell you whether your specific cancer type warrants testing if it has not been done.

Usually one to two weeks from when the laboratory receives the sample. Immunohistochemistry results often come back faster than PCR or next-generation sequencing. If your original biopsy tissue is insufficient, a repeat biopsy may be needed before the test can be completed, which adds time. Ask your team when the sample was dispatched and when the result is expected, so you are not waiting without a clear timeline.

MSI-H status is one of the stronger predictors of response to checkpoint inhibitors across cancer types, and NCCN, ASCO, and ESMO guidance supports their use at advanced stages. That said, response varies between individuals, and your oncologist will weigh your overall health, the extent of the disease, and any prior treatments before recommending a specific regimen. MSI-H is meaningful information — but your oncologist's assessment of your complete clinical picture is what shapes the treatment decision.

Yes. Checkpoint inhibitors are given as day-care infusions at CION centres — you do not need an overnight admission for the treatment itself. PET-CT and other response-assessment scans are coordinated with partner imaging centres. CION does not provide CAR-T or cell therapy; if that is being considered in your case, you would be referred to a centre that offers it. Ask at your next appointment which CION centre nearest you administers immunotherapy day care.