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Microsatellite Instability (MSI) by PCR
- Molecular
PCR and fragment analysis of paired normal and tumor tissue to determine microsatellite instability (MSI) at the standard five NCI-recommended loci. Positive results are reported as MSI-high (at least two markers are unstable) or MSI-low (one marker is unstable). Testing requires paired normal tissue or blood analysis and at least 20% tumor content in tumor samples submitted (after macro-dissection).
Specimen Requirements
Note: An additional patient sample from normal, non-tumor tissue is required for comparison testing in MSI Analysis. Please submit all specimens with one test requisition form. Specimen requirements for normal tissue in order of preference are: 5 mL peripheral blood in EDTA tube OR FFPE tissue slides or block containing only non-tumor tissue. Please label these as "normal tissue". OR In cases where no alternative tissue is available, we can attempt to isolate non-tumor tissue from the tumor specimen submitted. Note "Use tumor sample for normal tissue" on requisition. See requirements below. Specimen requirements for tumor tissue: FFPE tissue: Paraffin block is preferred. Alternatively, send 1 H&E slide plus 5-10 unstained slides cut at 5 or more microns. Please use positively charged slides and 10% NBF fixative. Do not use zinc fixatives.
Storage and Transportation
Use cold pack for transporting block, making sure cold pack is not in direct contact with specimen. Slides can be packed at room temperature.
CPT Code(s)*
81301x1
*The CPT codes provided with our test descriptions are based on AMA guidelines and are for informational purposes only. Correct CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed.
Last Updated: April 22, 2025