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Background: Atezolizumab, one of the immune checkpoint inhibitors, has been approved as an adjuvant treatment following resection and platinum-based chemotherapy in patients with stage II-IIIA non-small cell lung cancer with 1% or more programmed death ligand-1 (PD-L1) expression. The Food and Drug Administration (FDA) has approved SP263 as a companion diagnostic assay for adjuvant treatment with atezolizumab; however, in clinical practice, the 22C3 assay is most commonly used for advanced non-small cell lung cancer. Therefore, our study aimed to compare two PD-L1 assays, SP263 and 22C3, to evaluate whether 22C3 could replace SP263 when deciding whether to administer adjuvant atezolizumab., Methods: We retrospectively and prospectively analyzed 98 patients who underwent surgical resection at Kanagawa Cancer Center (Japan). An immunohistochemistry assay was performed for all the cases with both SP263 and 22C3. We statistically analyzed the concordance of PD-L1 expression between SP263 and 22C3 assays., Results: The concordance between the two assays using Cohen's kappa was κ = 0.670 (95% CI: 0.522-0.818) at the 1% cutoff and κ = 0.796 (95% CI: 0.639-0.954) at the 50% cutoff. The Spearman correlation coefficient of 0.874 (p < 0.01) indicated high concordance. PD-L1 expression with 22C3 resulted slightly higher than that with SP263., Conclusions: This study showed a high concordance of PD-L1 expression with the SP263 and 22C3 assays. Further studies examining the therapeutic effects of adjuvant atezolizumab are required., (© 2024 The Authors. Thoracic Cancer published by John Wiley & Sons Australia, Ltd.)