Summary & Overview
CPT 81301: Microsatellite Instability (MSI) Testing in Tumor Tissue
CPT code 81301 represents the technical laboratory assay to detect microsatellite instability (MSI) in tumor tissue, a molecular diagnostic commonly used in colorectal and other cancers to characterize tumor biology. MSI testing has national clinical relevance because results can affect prognosis, guide genetic evaluation for Lynch syndrome, and inform therapy selection in certain oncology pathways. Payers commonly involved in coverage of molecular oncology testing include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. This publication provides a concise overview of CPT code 81301, outlining the clinical context for MSI testing, typical sites of service, and which payers are referenced in coverage discussions. Readers will find benchmark and policy-oriented content including national coverage considerations, common billing practice notes, and clinical context that clarifies when MSI testing is performed and why it matters for patient management. Data not available in the input will be noted where relevant. The goal is to equip billing managers, laboratory directors, and policy analysts with a clear, nationally focused summary of CPT code 81301 and the key issues surrounding its use in molecular oncology diagnostics.
Billing Code Overview
CPT code 81301 describes a laboratory technical procedure to detect microsatellite instability (MSI) in tumor cells using tissue specimens, commonly performed for cancers such as colorectal (colon) cancer. The service involves laboratory processing and analysis of tumor tissue to identify instability in microsatellite regions of DNA, which has implications for tumor characterization and potential downstream clinical decision-making.
Service Type: Laboratory molecular diagnostic test (MSI testing)
Typical Site of Service: Clinical laboratory or hospital pathology lab
Clinical & Coding Specifications
Clinical Context
A 62-year-old patient with a recently resected stage II colon adenocarcinoma undergoes tumor testing to evaluate microsatellite instability (MSI) status. Formalin-fixed, paraffin-embedded tumor tissue from the surgical specimen is submitted to the molecular pathology laboratory. The lab technician performs the 81301 assay to detect MSI in tumor cells, which informs prognosis and eligibility for further germline testing for Lynch syndrome or consideration of immune checkpoint inhibitor therapy. The clinical workflow includes: specimen accessioning and verification, selection of representative tumor block, DNA extraction, PCR-based or next-generation sequencing MSI analysis, analytic interpretation by a molecular pathologist, and reporting of MSI-high, MSI-low, or microsatellite stable results back to the treating surgical oncologist or medical oncologist. Associated documentation in the chart includes pathology accession numbers, tumor percentage, test method, and test report with implications for genetic counseling referral when MSI-high is detected.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
26 | Professional component | Use when billing only the professional interpretation component of the test separate from the technical component. |