Serum Carcinoembryonic Antigen Levels Across Molecular Subtypes and Their Clinical and Prognostic Implications in Metastatic Non-Small Cell Lung Cancer

Background and Objectives: Serum carcinoembryonic antigen (CEA) is a widely used biomarker in non-small cell lung cancer (NSCLC). However, its association with oncogenic driver alterations and prognostic significance across molecular subtypes in metastatic disease remains insufficiently defined. Materials and Methods: This retrospective multicenter study included 332 patients with metastatic NSCLC harboring oncogenic alterations (EGFR, ALK, ROS1, KRAS, and others) from eight oncology centers in Türkiye. Baseline serum CEA levels measured at metastatic diagnosis were analyzed on the natural logarithmic scale. Associations between CEA levels, molecular subtypes, clinical features, and overall survival (OS) were evaluated using generalized linear models and Cox proportional hazards regression. Results: Baseline CEA levels differed significantly across molecular subtypes (p = 0.001), with EGFR-mutant tumors showing the highest median levels. Multivariable analysis identified driver alteration, histology, and metastatic burden as independent determinants of baseline CEA. Higher baseline CEA and metastatic site count were independently associated with increased mortality risk (HR 1.151 and 1.279 per unit increase, respectively; p < 0.001), while female sex was protective (HR 0.626; p = 0.004). KRAS mutations were associated with poorer survival compared with EGFR (HR 2.370; p < 0.001). Kaplan–Meier analyses showed a consistent trend toward longer OS in patients with CEA < 5 ng/mL, with significance only in the rare alteration subgroup. Conclusions: Baseline CEA may reflect underlying tumor biology across molecular subtypes and are associated with survival outcomes in metastatic NSCLC. However, given the variability across subgroups and modest effect sizes, these findings should be interpreted with caution. Prospective studies evaluating longitudinal CEA dynamics are warranted.