Introduction: Carcinoembryonic antigen (CEA) is a main prognostic marker and can detect colorectal cancer (CRC) recurrence, but it has low sensitivity. Carbohydrate antigen 19-9 (CA19-9) can be used as a supplemental tumour marker along with CEA. This study investigated the utility of preoperative and follow-up serum CA19-9 assessment for CRC.~Materials and Methods: We retrospectively assessed 4,972 patients who underwent surgery for primary CRC between January 2004 and December 2015 at Seoul National University Hospital. Data on demographics, preoperative and follow-up CEA and CA19-9 levels, recurrence, and survival were obtained and analysed with respect to tumour marker levels to ascertain their prognostic and diagnostic values.~Results: The 5-year relapse-free survival rates were 72.2±0.8%, 52.5±2.2%, 55.5±3.2%, and 32.1±2.3% in the normal CEA and CA19-9, high CEA, high CA19-9, and high CEA and high CA19-9 groups, respectively (all p<0.001). Patients whose elevated CEA or CA19-9 reduced to normal had better survival outcomes than those with postoperatively elevated levels. Elevated follow-up CA19-9 and CEA levels were related to higher incidences of distant metastasis (CA19-9: 14.0% vs. 23.1%, p=0.004, CEA: 12.6% vs. 30.1%, p<0.001) but not with local recurrence. Combined follow-up CEA and CA19-9 increased the sensitivity for recurrence to 31.4%, with a 5% difference compared with CEA alone. In the subgroup with high preoperative CA19-9 levels, sensitivity increased by 18.2% overall.~Conclusion: CA19-9 is a valuable prognostic and diagnostic marker for CRC when used adjunctively with CEA and can be a supplementary marker with CEA to improve sensitivity, especially with elevated preoperative CA19-9.
4,972 patients who underwent surgery for primary CRC between January 2004 and December 2015 at Seoul National University Hospital were retrospectively reviewed.~CA19-9 is a valuable prognostic and diagnostic marker for CRC when used adjunctively with CEA and can be a supplementary marker with CEA to improve sensitivity, especially with elevated preoperative CA19-9.