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Microsatellite Instability

Microsatellite instability, KEYTRUDA^® (pembrolizumab), MSI Testing, Microsatellite Instability by PCR (MSI), Lynch syndrome, MMSIG

Test Codes

EPIC: LAB3001

Department

Molecular Pathology

Specimen Collection Criteria

Collect:

Abnormal Tissue: 10% Formalin-fixed, paraffin-embedded tumor block with corresponding H&E slides. Tissue should be well fixed and well processed. Please submit the best tumor block or multiple blocks if this specimen is a small biopsy.

Normal Tissue: 10% Formalin-fixed, paraffin-embedded normal tissue block with corresponding H&E slides. Tissue should be well fixed and well processed. Please submit the best normal tissue block or multiple blocks if this specimen is a small biopsy. A fresh Peripheral Blood EDTA specimen will be accepted when a normal tissue block is unavailable.

• The specimen must be accompanied by a completed requisition and must contain the patient name, date of birth, collection date, ordering physician, and source of specimen.   

Physician Office/Draw Specimen Preparation

Maintain paraffin-embedded tissue at room temperature (20-26°C or 68-78.8°F) until transport.

Preparation for Courier Transport

Transport: Paraffin-embedded tissue, at room temperature (20-26°C or 68-78.8°F).

Rejection Criteria

• Tissue treated with decalcifying agents other than Mol Decal (EDTA).
• Fixatives other than 10% neutral buffered formalin.
• Specimens fixed/processed in alternative fixatives (e.g., alcohol, zinc formalin).
• Improper labeling or inadequate information.
• Inadequate tumor cellularity, at discretion of medical director.
• Poor quality and/or quantity of extracted genomic DNA.
• Patients for which a suitable source of non-neoplastic DNA (surgical specimen or peripheral blood) cannot be obtained.

Testing will be cancelled on specimens meeting the above criteria with client notification.

Inpatient Specimen Preparation

Specimens at Royal Oak may be sent to the Surgical Pathology tube station, #201. In-house specimens are also picked up by a Surgical Pathology assistant every hour on the hour.

Tissue blocks for in-house specimens are typically available in the department of anatomic pathology. Tissue blocks from outside institutions should be obtained via normal protocols for outside surgical specimens.

In-Lab Processing

Unstained sections of 5-µm thickness are cut from selected tissue blocks. The number of sections cut and the need for macro-dissection are determined by the pathologist, based upon the amount of available tissue and the tumor cellularity.

Maintain specimens at room temperature (20-26°C or 68-78.8°F) until testing.

Storage

Specimen Stability for Testing:

Paraffin-embedded Abnormal and Normal Tissue:
Room Temperature (20-26°C or 68-78.8°F): Indefinitely
Refrigerated (2-8°C or 36-46°F): Unacceptable
Frozen (-20°C/-4°F or below): Unacceptable

Peripheral Blood EDTA:
Room Temperature (20-26°C or 68-78.8°F): 72 hours
Refrigerated (2-8°C or 36-46°F): 72 hours

Specimen Storage in Department Prior to Disposal:

Room Temperature (20-26°C or 68-78.8°F): 7 days

Specimen Storage (DNA post testing):

Frozen (-70 to -80°C): Indefinitely

Laboratory

Royal Oak Molecular Pathology Laboratory

Performed

As received. Test run once per week.
Results available in 7-10 business days. 

Reference Range

Microsatellite stable tumor. Results obtained with tumor DNA are compared with results obtained with the patient's normal DNA. A tumor is considered microsatellite stable if no new peaks are identified in the tumor specimen when compared to the normal DNA results.

Test Methodology

DNA is extracted from the tissue. Primers to six NCI consensus microsatellites are used to amplify both tumor and normal DNA. Fluorescently labeled products undergo fragment analysis by capillary electrophoresis.

Interpretation

Microsatellite stable (no unstable loci), unstable-low (1 unstable locus), unstable-high (2 or more unstable loci).

Clinical Utility

This assay detects instability of microsatellite loci associated with a loss or suppression of DNA mismatch repair complex. Microsatellite unstable-high (MSI-H) status can correspond with both sporadic and inherited (i.e., Lynch Syndrome) microsatellite instability, as well as having prognostic implications. However, this assay cannot distinguish between sporadic and germline causes of MSI-H. For MSI-H cases, further assessment by mismatch repair immunohistochemistry and possible MLH-1 promoter methylation is recommended to further characterize the affected gene(s) and help determine the likelihood of a somatic versus germline alteration.

Pembrolizumab (Keytruda) is FDA-approved and NCCN guideline-recommended for use in any MSI-H solid tumor that is unresectable or metastatic.

CPT Codes

81301

Contacts

Last Updated

10/6/2025