Colorectal Cancer: Is Genetic Testing Really Necessary? (2026 Update)

Depending on the stage and clinical situation, colorectal cancer may require marker testing such as MSI/MMR, RAS, BRAF, and HER2 — these play a key role in determining the treatment direction, especially in advanced or metastatic stages or when there is a family history. Not every patient needs the same tests.

Key Takeaways

• In colorectal cancer, MSI/MMR, RAS, BRAF, and HER2 marker tests are performed to help select drugs in advanced or metastatic disease.
• If there is a family history or the cancer was diagnosed at a young age, a separate test to check for hereditary colorectal cancer may also be recommended.
• For wild-type RAS, EGFR-targeted agents are used; for BRAF mutations or HER2 positivity, tailored drug combinations are used; and MSI-High tumors are known to respond particularly well to immunotherapy.
• Korea's 5-year relative survival rate for colorectal cancer is 75.6% (National Cancer Registration Statistics 2019–2023, National Cancer Center Korea).
• Biomarker testing is a different concept from genetic testing that looks for inherited mutations — it examines changes within the cancer cells themselves (U.S. National Cancer Institute, NCI).

What Is Genetic (Marker) Testing and Why Is It Done?

• Marker testing looks for genetic and protein changes in cancer cells to help determine which drugs are likely to be effective (NCI). In colorectal cancer, the main markers are RAS, BRAF, HER2, and MSI/MMR.
• Even with the same diagnosis of colorectal cancer, different marker results can lead to completely different drug choices. If RAS is wild-type, EGFR-targeted therapy is considered; if there is a BRAF mutation or HER2 positivity, matching drug combinations are used. MSI-High (high microsatellite instability) tumors are known to respond especially well to immune checkpoint inhibitors.
• This testing is especially important in advanced or metastatic colorectal cancer, where systemic therapy is used. In early-stage colorectal cancer, where surgery alone often completes treatment, it is not always necessary.

How Is This Different from Hereditary Colorectal Cancer Testing?

• Marker testing and hereditary colorectal cancer testing are different tests. Marker testing looks at changes within the cancer cells themselves, while hereditary colorectal cancer testing looks for germline mutations that can be passed down to family members (NCI).
• If there is a family history or the cancer was diagnosed at a young age, testing for hereditary colorectal cancer may be recommended. Depending on the results, the recommended starting age and interval for screening in family members may change, so it helps to prepare your family history in advance.

What Is the Order of Testing?

• Step 1: Colonoscopy is used to confirm the location and size of the lesion, and a biopsy is performed.
• Step 2: Abdominal and chest CT (and pelvic MRI for rectal cancer) are used to assess metastasis and extent of invasion.
• Step 3: If the cancer is judged to be advanced or metastatic, MSI/MMR, RAS, BRAF, and HER2 marker tests are performed on tissue or blood.
• Step 4: If there are relevant findings such as family history or young age at diagnosis, hereditary colorectal cancer testing is additionally considered.
• Step 5: Based on the combined results, an appropriate combination of surgery, chemotherapy, targeted therapy, and immunotherapy is determined.

FAQ — Can I Use Test Results Obtained Overseas As-Is?

In most cases, yes, they can be used as reference. However, since testing methods and standards vary by institution, markers that are critical for drug selection are sometimes re-confirmed using tissue slides or paraffin blocks. The date the test was performed and whether the specimen came from the primary tumor or a metastatic site are also important details to check.

FAQ — Without Marker Testing, Is Targeted Therapy Impossible?

Targeted therapies are known to be effective only when the relevant mutation is confirmed, so targeted therapy is never selected without test results. If the relevant mutation is not present, standard treatment shifts to cytotoxic chemotherapy, immunotherapy, surgery, or radiation therapy instead of targeted agents.

Sources and Notes

• This article was written based on notes on colorectal cancer, targeted therapy, and pre-treatment checklists (National Cancer Registration Statistics 2019–2023, National Cancer Center Korea; National Cancer Information Center; NCCN patient guidelines; U.S. National Cancer Institute, NCI).
• This content is intended for general medical information purposes only. Diagnosis, treatment methods, and outcomes may vary depending on individual circumstances. Please consult with your medical team to determine an accurate diagnosis and treatment plan.