IMUNOHISTOCHEMISTRY: MICROSATELLITE INSTABILITY (MSI) PANEL OR MISMATCH REPAIR (MMR) PROTEIN IHC

What this test checks

Tests tumour tissue for loss of four DNA mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) to identify microsatellite instability — a sign of defective DNA repair that has major implications for immunotherapy and hereditary cancer risk.

Why do people get this test?

MSI-high tumours respond dramatically to immunotherapy (PD-1 inhibitors) regardless of cancer type. MSI testing also identifies Lynch syndrome — a hereditary cancer predisposition requiring family screening and surveillance.

When you should consider this test

• For all colorectal cancers (now recommended routinely)
• When considering immunotherapy (pembrolizumab) for any solid tumour
• When hereditary cancer syndromes (Lynch syndrome) are suspected based on family history or young age of diagnosis

How to prepare

No preparation needed — performed on biopsy or surgical tissue.