The protein EGFR, ERCC1, RRM1, thymidylate synthase, or beta-tubulin from cancer cells is detected in a blood sample by enriching the cancer cells from the blood sample followed by performing on the enriched cancer cells an immunoassay capable of detecting the proteins mentioned above. Cancer patients overexpressing EGFR are treated with an anti-EGFR agent, for example cetuximab, panitumumab, erlotinib or gefitinib.