Best Practice PD-L1 Staining and Interpretation in Gastric Cancer Using PD-L1 IHC PharmDx 22C3 and PD-L1 IHC PharmDx 28-8 Assays, with Reference to Common Issues and Solutions.

Programmed death-ligand 1 (PD-L1) biomarker testing in gastric cancer is required to identify patients suitable for immunotherapy. However, the PD-L1 testing landscape is complex, with various PD-L1 tests available and multiple algorithms that combine tumor and immune cell staining. To provide guidance on the best practices for PD-L1 testing in gastric cancer, we reviewed the literature and incorporated our extensive experience using the PD-L1 IHC PharmDx 22C3 and 28-8 assays and scoring with the combined positive score (CPS) algorithm. This review summarizes inter-reader agreement and PD-L1 assay concordance studies in gastric cancer, highlights practical challenges and pitfalls encountered in our own laboratory, and proposes solutions to address them. Accurate and consistent interpretation of PD-L1 CPS in gastric cancer is challenging, but can be improved with training, experience, and close attention to interpretation guidelines. Techniques are available that can optimize the automated staining of PharmDx PD-L1 assays using the Autostainer Link 48 to ensure consistent staining performance. The PD-L1 IHC PharmDx 22C3 and PD-L1 IHC PharmDx 28-8 assays show high concordance when used according to manufacturers' guidelines.