Programmed death ligand 1 (PD-L1) immunohistochemistry (IHC) is accepted as a predictive biomarker for the selection of immune checkpoint inhibitors. We evaluated the staining quality and estimation of the tumor proportion score (TPS) in non-small-cell lung cancer during two external quality assessment (EQA) schemes by the European Society of Pathology. Participants received two tissue micro-arrays with three (2017) and four (2018) cases for PD-L1 IHC and a positive tonsil control, for staining by their routine protocol. After the participants returned stained slides to the EQA coordination center, three pathologists assessed each slide and awarded an expert staining score from 1 to 5 points based on the staining concordance. Expert scores significantly (p < 0.01) improved between EQA schemes from 3.8 (n = 67) to 4.3 (n = 74) on 5 points. Participants used 32 different protocols: the majority applied the 22C3 (56.7%) (Dako), SP263 (19.1%) (Ventana), and E1L3N (Cell Signaling) (7.1%) clones. Staining artifacts consisted mainly of very weak or weak antigen demonstration (63.0%) or excessive background staining (19.8%). Participants using CE-IVD kits reached a higher score compared with those using laboratory-developed tests (LDTs) (p < 0.05), mainly attributed to a better concordance of SP263. The TPS was under- and over-estimated in 20/423 (4.7%) and 24/423 (5.7%) cases, respectively, correlating to a lower expert score. Additional research is needed on the concordance of less common protocols, and on reasons for lower LDT concordance. Laboratories should carefully validate all test methods and regularly verify their performance. EQA participation should focus on both staining concordance and interpretation of PD-L1 IHC.

程序性死亡配体1（PD-L1）免疫组织化学（IHC）被接受为选择免疫检查点抑制剂的预测生物标志物。我们在欧洲病理学会的两个外部质量评估（EQA）计划期间，评估了非小细胞肺癌的染色质量和肿瘤比例评分（TPS）的估计。参与者接受了两个组织微阵列，分别具有三例（2017年）和四例（2018年）的PD-L1 IHC和阳性扁桃体对照，用于通过其常规方案进行染色。在参与者将染色的载玻片退回EQA协调中心后，三名病理学家对每个载玻片进行了评估，并根据染色的一致性从1到5分授予专家染色分数。EQA方案之间的专家评分从3.8（显着提高（p <0.01）n = 67）至4.3（n = 74）5分。参与者使用了32种不同的协议：大多数应用了22C3（56.7％）（Dako），SP263（19.1％）（Ventana）和E1L3N（Cell Signaling）（7.1％）克隆。染色伪影主要由非常弱或较弱的抗原展示（63.0％）或过度的背景染色（19.8％）组成。与使用实验室开发的测试（LDT）的参与者相比，使用CE-IVD试剂盒的参与者得分更高（p<0.05），主要归因于SP263更好的一致性。在20/423（4.7％）和24/423（5.7％）的情况下，TPS分别被低估和高估，这与较低的专家评分有关。还需要针对不太常见的协议的一致性以及LDT一致性较低的原因进行进一步的研究。实验室应仔细验证所有测试方法，并定期验证其性能。EQA的参与应集中在PD-L1 IHC的染色一致性和解释上。