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Microsatellite Instability by PCR (MSI)

Microsatellite instability, colon cancer, KEYTRUDA^® (pembrolizumab), PCC Screen, MSI Testing

Test Codes

Beaker: MMSIG

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. Average tissue size 5.0 mm².

Normal Tissue: 10% Formalin-fixed, paraffin-embedded normal tissue block with corresponding H&E slides. Tissue should be well fixed and well processed. Average tissue size 5.0 mm². 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

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.

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.

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 are analyzed by capillary electrophoresis.

Interpretation

Microsatellite stable, unstable-low (1 unstable locus), unstable-high (2 or more unstable loci).

Clinical Utility

Detects instability of microsatellite loci associated with a loss or suppression of DNA mismatch repair complex. In colorectal and endometrial adenocarcinoma, 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. 

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

11/2/2023