Background: Serum carcinoembryonic antigen (CEA) is a widely used tumor marker in colorectal cancer (CRC), but within normal range of preoperative CEA levels the clinical significance of CEA is unknown. Objective: The aim of this study was to evaluate the usefulness of CEA within the normal range as a prognosticator of non-metastatic CRC. Methods: This retrospective cohort study included 2021 CRC patients with normal preoperative CEA who underwent elective curative surgery (discovery group). We determined the optimal cut-off value for disease-free survival (DFS) discrimination using the Contal and O’Quigley method. We also assessed the prognostic significance of the cut-off value in a prospective cohort of 171 stage III colon cancer patients treated with oxaliplatin-based adjuvant chemotherapy (validation group). Results: The optimal cut-off CEA value was 2.1 ng/mL in the discovery group. The DFS rates were significantly poorer in patients with high-normal preoperative CEA levels (2.1–5.0 ng/mL) than in those with low-normal CEA levels (< 2.1 ng/mL) in both groups. A high-normal CEA level was an independent risk factor for DFS in both groups, and was associated with inferior DFS in patients with stage II and III disease and in never or former smokers. The correlation between DFS and CEA levels was more distinct in left-sided colon and rectal cancer. Conclusions: A high-normal preoperative CEA level (≥ 2.1 ng/mL), even within the normal range, was an independent prognosticator for poor DFS in CRC. The usefulness of CEA was influenced by smoking status and tumor location in addition to tumor stage.
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