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MSI-H / dMMR testing: what it means for you

Waiting on test results can feel like one more hard thing, for you and the people who love you. Here's what the MSI-H / dMMR test looks for and how it can help your care team pick the treatment most likely to work for you.

ℹ️ General information to help you and your loved ones talk with your care team, not medical advice. Read our disclaimer.

Matters most in: Colorectal cancer, Ovarian cancer, Prostate cancer

Why it matters

Tumors with MSI-H/dMMR often respond very well to immunotherapy. About 15% of colorectal cancers are dMMR, and some are linked to Lynch syndrome, an inherited condition, so family testing may be suggested. Testing is recommended for all colorectal cancers.

Treatments it can point to

• Immunotherapy such as Keytruda or Opdivo, alone or in combination (Keytruda (pembrolizumab), Opdivo (nivolumab))
• Keytruda is approved for MSI-H/dMMR solid tumors of any type that have grown after treatment (Keytruda (pembrolizumab))

Only your care team can say which treatment fits you. Drug approvals depend on cancer type, stage and prior treatment.

Tissue vs. blood testing

Tissue (stain or DNA test); some blood tests also report MSI.

Most tests use a piece of tumor tissue from a biopsy or surgery. Some can also use a blood sample (a 'liquid biopsy') that looks for tumor DNA floating in the blood. Blood tests are easier and faster, but they can miss a change that a tissue test would find, so a negative blood test is often followed by a tissue test.

❓ Questions you and your loved ones can ask

• Is my tumor MSI-high or dMMR?
• Could I have Lynch syndrome, and should my family be tested?
• When will my results be back, and will treatment wait for them?
• Will my insurance cover this test?

Written by the Cancer Side Effects Guide team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write