MSI testing is typically performed on a tissue sample obtained from a biopsy or surgical resection. The most common methods include:
1. Polymerase Chain Reaction (PCR) - This method amplifies specific microsatellite regions to detect length variations. Five markers are commonly used: BAT-25, BAT-26, D2S123, D5S346, and D17S250. Tumors are classified as MSI-H if two or more markers show instability, and as MSS if none or only one marker is unstable.
2. Immunohistochemistry (IHC) - This method detects the presence or absence of MMR proteins (MLH1, MSH2, MSH6, and PMS2) in tumor tissue. Loss of one or more of these proteins suggests MSI-H status.
3. Next-Generation Sequencing (NGS) - This advanced method sequences large regions of the genome, including microsatellites, to detect instability. NGS can provide comprehensive information about the genetic landscape of the tumor.
