CPT® Code 81301

Microsatellite instability analysis (eg, hereditary non-polyposis colorectal cancer, Lynch syndrome) of markers for mismatch repair deficiency (eg, BAT25, BAT26), includes comparison of neoplastic and normal tissue, if performed

Microsatellite instability (MSI) analysis is a genetic test that evaluates the stability of microsatellite regions in DNA, which are short, repetitive sequences that can vary in length among individuals. This analysis is particularly relevant in the context of hereditary non-polyposis colorectal cancer (HNPCC), commonly known as Lynch syndrome. In individuals with Lynch syndrome, there is a defect in the DNA mismatch repair (MMR) system, leading to an accumulation of errors in the DNA sequence. When these errors occur, the microsatellites can become unstable, resulting in variations in their length. The presence of MSI is indicative of a potential MMR gene mutation, which is a hallmark of Lynch syndrome. The analysis involves comparing neoplastic (tumor) tissue to normal tissue to assess the degree of instability. Specifically, the test examines several microsatellite markers, including BAT25 and BAT26, to determine whether the tumor exhibits high instability (unstable high) or low instability (unstable low). A tumor is classified as unstable high if two or more microsatellite regions show changes, while unstable low indicates that only one region is affected. Conversely, if there are no changes between the tumor DNA and the normal DNA, the tumor is considered microsatellite stable (MSS). The identification of high MSI can suggest the presence of Lynch syndrome, prompting further genetic testing and evaluation. This analysis is crucial as it not only aids in the diagnosis of hereditary cancer syndromes but also has implications for treatment decisions, as tumors with MSI may respond differently to certain chemotherapeutic agents compared to those that are MSS. Routine MSI testing is recommended for individuals diagnosed with colorectal cancer and certain other malignancies to guide clinical management and familial risk assessment.