What should you know about microsatellite instability immunohistochemistry?

A laboratory applies antibodies to a biopsy sample to see whether selected mismatch repair proteins are present in tumour cells. Loss of one or more proteins can suggest that the repair system is not working normally. This finding may support additional testing for microsatellite instability, help classify the tumour, and influence treatment discussions. A retained pattern does not answer every question about a tumour, and an unusual or unclear pattern may need review by a specialist laboratory.

The test is performed on tissue already collected when possible, so it may not require another procedure. Results should be interpreted alongside the tumour type, medical history, family history, and other pathology findings.

Ask which proteins were tested, whether the pattern was retained or lost, and whether the laboratory recommends a second test. If loss is reported, your team may discuss genetic counselling or testing, because some mismatch repair findings can be linked with an inherited condition while others arise only in the tumour. Treatment choices depend on the complete diagnosis rather than this result alone.

Keep a copy of the pathology report and tell your clinician about relatives with cancers that may be relevant to inherited risk. The biopsy process and the laboratory method are explained in more detail on the Biopsy and Immunohistochemistry pages.

Contact your cancer team if you do not understand the report, have not received a planned result, or are unsure whether a follow-up appointment is arranged. Seek urgent medical help for heavy bleeding, worsening pain, fever, breathing difficulty, confusion, or new weakness after a biopsy or other procedure. The test itself does not diagnose an emergency; symptoms and recovery problems should be assessed on their own merits.

Which mismatch repair proteins were tested, and what pattern was found? Does this result suggest microsatellite instability or require another test? Could genetic counselling be useful for me or my family? How does the result fit with the biopsy diagnosis and treatment plan?