Test Catalog

Test ID: TMSI    
Microsatellite Instability, Tumor

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluation of tumor tissue to identify patients at high risk for having Lynch syndrome, also known as hereditary nonpolyposis colorectal cancer


Evaluation of tumor tissue for clinical decision-making purposes given the prognostic and therapeutic implications associated with microsatellite instability phenotypes

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

When this test is ordered, slide review will always be performed at an additional charge.


See Lynch Syndrome Testing Algorithm in Special Instructions.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Somatic (tumor-specific) microsatellite instability (MSI) is assessed by this test. MSI is characterized by numerous alterations in a type of repetitive DNA called microsatellites and occurs as the result of an impaired DNA mismatch repair process. Impaired DNA mismatch repair is a key factor in tumorigenesis and can occur sporadically or as the result of a hereditary cancer predisposition called Lynch syndrome.


Evaluation for MSI may be valuable for clinical decision making. Current data suggest that advanced stage solid tumors with defective DNA mismatch repair (MSI-H) are more likely to respond to treatment with immunotherapies such as anti-PD-1 therapies. Colon cancers that demonstrate defective DNA mismatch repair (MSI-H) have a significantly better prognosis compared to those with intact mismatch repair (MSS/MSI-L). Additionally, current data indicate that stage II and stage III patients with colon cancers characterized by the presence of defective mismatch repair (MSI-H) may not benefit from treatment with fluorouracil alone or in combination with leucovorin. These findings are most likely to impact the management of patients with stage II disease.


MSI analysis, usually in combination with immunohistochemistry staining of the mismatch repair proteins, can also provide helpful diagnostic information in the context of evaluation for Lynch syndrome. See Lynch Syndrome Testing Algorithm in Special Instructions.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

The report will include specimen information, assay information, and interpretation of test results.


Microsatellite stable (MSS) is reported as MSS (0 or 1 of 7 markers demonstrating instability) or microsatellite instability-high (MSI-H) (2 or more of 7 markers demonstrating instability).

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

The finding of tumor microsatellite instability does not distinguish between somatic and germline alterations.


Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given to us is inaccurate or incomplete.

Supportive Data

A total of 100 accuracy samples were run retrospectively during verification; 55/100 samples were colorectal, 41/100 were endometrial, and the remaining 4/100 were other tumor types. The overall concordance between the Idylla and Promega results was 98/100 (98%). Seventy-nine of 100 samples were microsatellite stable (MSS) by Promega and 77 (97%) had concordant MSS results by Idylla. Twenty-one of 100 samples were microsatellite instability-high (MSI-H) by Promega and 21 (100%) had concordant MSI-H results by Idylla.


In addition to the retrospective samples, 100 consecutive samples were prospectively analyzed, of which 58 were colorectal, 31 were endometrial, and 11 were from other tumor types. Seventy-six of 100 samples were MSS by Promega and all (100%) were MSS by the Idylla assay. Twenty-four of 100 samples were MSI-H by Promega. Twenty-three of 24 (96%) of the MSI-H samples were concordant by Idylla. One patient had an uncommon reason for referral (RFR). The discordant sample DNA was rerun on the Promega platform, but there was not sufficient tissue remaining to rerun this specimen on the Idylla assay. After reviewing the results from the 2 runs on Promega, a consensus decision amongst 5 pathologists was reached and the sample was reclassified as equivocal by the Promega assay.


Precision and reproducibility was evaluated by running 3 MSI-H samples and 3 MSS samples in triplicate on the same instrument. Each of these samples had a 4th cartridge run on a separate instrument. There was 100% concordance between replicates from the 3 MSI-H samples and 3 MSS samples.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Baudhuin LM, Burgart LJ, Lentovich O, Thibodeau SN: Use of microsatellite instability and immunohistochemistry testing for the identification of individuals at risk for Lynch syndrome. Fam Cancer. 2005;4 (3):255-265 2. Terdiman JP, Gum JR Jr, Conrad PG, et al: Efficient detection of hereditary nonpolyposis colorectal cancer gene carriers by screening for tumor microsatellite instability before germline genetic testing. Gastroenterology. 2001 January;120(1):21-30 3. Popat S, Hubner R, Houlston RS: Systematic review of microsatellite instability and colorectal cancer prognosis. J Clin Oncol. 2005 Jan; 23(3):609-618 4. Ribic CM, Sargent DJ, Moore MJ, et al: Tumor microsatellite-instability status as a predictor of benefit from fluorouracil-based adjuvant chemotherapy for colon cancer. N Engl J Med. 2003 Jul 17;349:247-257 5. Kohlmann W, Gruber SB. Lynch syndrome. In: Adam MP, Ardinger HH, Pagon RA, et al. eds. GeneReviews [Internet]. University of Washington, Seattle; 2004. Updated April 12, 2018. Accessed June 22, 2020. Available at www.ncbi.nlm.nih.gov/books/NBK1211/ 6. Kawakami H, Zaanan A, Sinicrope FA: Microsatellite instability testing and its role in the management of colorectal cancer. Curr Treat Options Oncol. 2015 Jul;16(7):30 7. Sargent DJ, Marsoni S, Monges G, et al: Defective mismatch repair as a predictive marker for lack of efficacy of fluorouracil-based adjuvant therapy in colon cancer. J Clin Oncol. 2010 Jul 10;28(20):3219-3226 8. Le DT, Durham JN, Smith KN, et al: Mismatch repair deficiency predicts response of solid tumors to PD-1 blockade. Science. 2017 Jul 28;357(6349):409-413 9. Overman MJ, Lonardi S, Wong KYM, et al: Durable clinical benefit with nivolumab plus ipilimumab in DNA mismatch repair-deficient/microsatellite instability-high metastatic colorectal cancer. J Clin Oncol. 2018 Mar 10;36 (8):773-779

Special Instructions Library of PDFs including pertinent information and forms related to the test