e20013 Background: Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) is an emerging target of antibody-drug conjugate therapy for non-small cell lung cancer (NSCLC). High expression of CEACAM5 has been reported in approximately 25% of patients with lung adenocarcinoma (Lefebvre AM, et al. Lung Cancer. 2023; 184:107356). However, CEACAM5 expression has not been systematically examined in a real-world patient population with NSCLC. In this study, we evaluated our experience in the assessment of CEACAM5 expression by immunohistochemistry (IHC) in routinely diagnosed clinical samples. Methods: We assessed the expression of CEACAM5 by IHC on clinical biopsy/resection samples of consecutively diagnosed NSCLC from September 2022 to June 2023 at our institution, using the anti-CEACAM5 clone 769 antibody assay protocol developed for tusamitamab ravtansine clinical trials. Only cases adequate for PD-L1 22C3 and Oncomine Comprehensive Assay v3 NGS assay were included. CEACAM5 expression was scored independently by three pathologists based on the method proposed by Lefebvre et al. Expression levels were categorized as high (≥ 50% tumor cells at ≥ 2+ intensity), moderate (1–49% tumor cells at ≥ 2+ intensity), and negative (0 or 1+ intensity). Interrater reliability was assessed by Kendall’s coefficient of concordance (KCC) and Fleiss’ Kappa. Discordant classification was reviewed with the final classification achieved by consensus. Association with PD-L1 TPS and NGS results was determined by Chi-squared test. Results: This study cohort consisted of 150 consecutively diagnosed NSCLC, including 133 adenocarcinomas and 17 squamous cell carcinomas. The samples were derived from both primary (n = 138) and metastatic sites (n = 12). Both membranous and cytoplasmic CEACAM5 expression was observed in neoplastic cells, but was consistently absent in nonneoplastic cells. The prevalence of consensus high CEACAM5 membranous expression was 21% (n = 32) overall, 20% in primary (n = 28) and 33% (n = 4) in metastatic sites. The level of interrater agreement across all categories was moderate (KCC = 0.77). Interrater agreement between high CEACAM5 expression versus moderate/negative categories combined was also moderate (Kappa = 0.60). Challenging features included the determination of staining intensity and mixed membranous and cytoplasmic staining pattern, and cases near the cut-offs of the defined categories. CEACAM5 expression was not associated with PD-L1 TPS (p = 0.42) or EGFR mutations (p = 0.44). Conclusions: Our data showed that approximately 20% of routinely diagnosed clinical NSCLC samples had high CEACAM5 expression by IHC. The interrater reliability on the determination of high CEACAM5 expression was moderate among the three pathologists. We highlighted the challenging aspects in the evaluation of this biomarker.