Anti-EGFR antibodies benefit only patients whose tumours are wild-type across extended RAS — KRAS and NRAS exons 2, 3 and 4. BRAF V600E status should be known at the same time, because it signals poor prognosis and opens a separate targeted treatment pathway. Mismatch repair or microsatellite instability status should also be tested in every metastatic colorectal cancer, as it identifies candidates for immunotherapy.
Primary tumour side matters too: left-sided RAS wild-type tumours benefit most from first-line anti-EGFR therapy.
- Test extended RAS (KRAS and NRAS exons 2–4) before any anti-EGFR antibody
- Test BRAF V600E at the same time
- Test mismatch repair or microsatellite status in every metastatic colorectal cancer
- Record primary tumour side, as left-sided tumours benefit most from first-line anti-EGFR therapy
Why it matters
The wrong first-line choice in metastatic colorectal cancer is hard to undo.
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