An MSI screening test looks for changes in the DNA sequence by comparing normal tissue and tumor tissue. Defects in the MMR genes (MMR-D) result in an increased accumulation of DNA errors, and stretches of DNA called microsatellites are especially prone to these errors.
Most cancers do not have errors in their microsatellites, this is known as a MSS or microsatellite stable tumor.
77-89% of Lynch-related tumors show MSI (also called MSI-high), but 10-15% of sporadic tumors also show it.
Given that both Lynch and sporadic tumors can show MSI, additional testing is needed to determine if an individual with an MSI-high tumor has Lynch syndrome.
