PD-L1 expression in gastric cancer assessed with antibodies 28-8 and 22C3.

Programmed cell death ligand 1 (PD-L1) has emerged as a key biomarker in determining both the progression and prognosis of gastric cancer (GC). Consequently, its detection through immunohistochemical (IHC) analysis is essential for guiding appropriate treatment selection. We conducted an analytical, observational study based on a five-year retrospective cohort of patients diagnosed with gastric cancer at a comprehensive oncology center in Colombia. All patients underwent immunohistochemistry (IHC) to assess PD-L1 expression and calculate the Combined Positive Score (CPS). The objective of this study was to compare the positivity detection rates between the 28-8 and 22C3 assays and to evaluate the concordance between them. A descriptive analysis of clinical and pathological variables was performed. Univariate analysis was used to determine the frequency of PD-L1 expression and CPS distribution. Kaplan-Meier survival analysis was applied and stratified by PD-L1 status. Concordance between the two assays was assessed using the kappa (κ) index. A total of 175 patients diagnosed with gastric cancer (GC) and tested for PD-L1 expression were included in the study. Complete pathological and IHC data were available for 155 patients. Among them, 39.4% were tested with the 28-8 assay, and 60.6% with the 22C3 assay. PD-L1 positivity was observed in 34.4% of cases using the 28-8 assay and in 28.7% using the 22C3 assay. Among patients with available follow-up data (n = 34), PD-L1 testing was performed using the 22C3 clone, with a positivity rate of 35.3%. In the subgroup analysis of patients tested with both antibodies (n = 20), 60% were PD-L1 positive. Within this subset, 25% had a CPS of 1-4, 16.7% had a CPS of 5-9, and 58.3% had a CPS ≥ 10. For the 22C3 assay specifically, PD-L1 positivity was observed in 40% of cases, with 37.5% having a CPS of 1-4 and 62.5% a CPS ≥ 10; notably, no cases in this group had a CPS of 5-9. The concordance rate between the 28-8 and 22C3 assays was 61%, as measured by the kappa index. The 28-8 clone identified a higher proportion of patients with PD-L1 expression compared to the 22C3 antibody. However, both assays demonstrated a concordance rate of 61%. In the study population, the subgroup with a Combined Positive Score (CPS) ≥ 10 was the most prevalent, suggesting that high PD-L1 expression is relatively common and potentially clinically relevant in this cohort.