Background: The predictive value of PD-L1 score to select patients for adjuvant immunotherapy among epidermal growth factor receptor (EGFR)-mutated non-small cell lung cancer (NSCLC) remains controversial, confounded by the use of different assays across trials. We sought to compare the concordance between 3 PD-L1 antibodies among resected EGFR-mutated NSCLC, evaluate intra-tumoral heterogeneity (ITH) and implications on survival outcomes. Methods: Tissue microarrays were constructed from patients with AJCC7 Stage IA-IIIA EGFR-mutated NSCLC who underwent surgery 1/1/2010 – 31/12/2019 at National Cancer Centre Singapore. Patients who received neoadjuvant therapy were excluded. Three 1mm tumor cores per patient were obtained to study ITH, defined as discordant tumor proportion score (TPS) in ≥2/3 cores. TPS was evaluated by 3 independent pathologists for 3 PD-L1 antibodies (SP263, SP142, 22C3) and analysed in tertiles of <1%, 1-49% and ≥50%. Median TPS score across 3 cores for each pathologist per assay was used to assess concordance between assays with Cohen’s kappa test, which was also used to assess agreement between pathologists. Disease-free survival (DFS) and overall survival (OS) were tabulated using Kaplan-Meier method. Results: A total of 218 patients were included. Stage IA comprised 45%(99/218), IB 24%(53/218), II 17%(37/218) and IIIA 13%(29/218). Females made up 65%(142/218), never smokers 84%(183/218) and 71%(155/218) were acinar subtype adenocarcinomas. Ex19del was detected in 47%(103/218) and L858R in 39%(86/218). Distribution by PD-L1 TPS was 85%(185/218) vs 91%(198/218) vs 97%(212/281) for <1%, 13%(28/218) vs 8%(17/218) vs 3%(6/218) for 1-49%, and 2%(5/218) vs 1%(3/218) vs 0% for ≥50%, using SP263, 22C3 and SP142 respectively (p<0.001). Pairwise comparison of TPS scores revealed highest concordance between SP263 and 22C3 (Kappa 0.631), followed by 22C3 and SP142 (Kappa 0.361), and SP263 and SP142 (Kappa 0.192). Agreement between pathologists was excellent across all 3 assays, with median Cohen’s kappa of 1 (range 0.972 – 1). ITH was observed in 13%(29/218), with 5 patients having differing TPS scores in all 3 cores. Discordance was highest among TPS 1-49% at 96%(27/28), followed by ≥50% at 40%(2/5) and least among <1% at 0% (p<0.01). Compared to those with concordant TPS scores across all 3 cores, 5-year DFS (60% vs 47%, p=0.15) and 5-year OS (81%% vs 71%, p=0.043) were inferior for patients with ITH. Conclusions: Concordance of PD-L1 TPS was highest among SP263 and 22C3 and lowest for SP263 and SP142, with low inter-observer variability between pathologists. Heterogeneous PD-L1 expression within tumors was seen in 13% of patients and significantly correlated with inferior OS. Our findings support the relative interchangeability of SP263 and 22C3 among resected EGFR-mutated NSCLC but not SP142, while ITH in PD-L1 expression may represent a novel negative prognostic biomarker.