Assessment of programmed cell death ligand 1 (PD-L1) immunohistochemical 
staining is used for decision on treatment with programmed cell death 1 and 
PD-L1 checkpoint inhibitors in lung adenocarcinomas and squamous cell 
carcinomas. This study aimed to compare the staining properties of tumor cells 
between the antibody clones 28-8, 22C3, SP142, and SP263 and investigate 
interrater variation between pathologists to see if these stainings can be 
safely evaluated in the clinical setting. Using consecutive sections from a 
tissue microarray with tumor tissue from 55 resected lung cancer cases, staining 
with five PD-L1 assays (28-8 from two different vendors, 22C3, SP142, and SP263) 
was performed. Seven pathologists individually evaluated the percentage of 
positive tumor cells, scoring each sample applying cutoff levels used in 
clinical studies: <1% positive tumor cells (score 0), 1-4% (score 1), 5-9% 
(score 2), 10-24% (score 3), 25-49% (score 4), and >50% positive tumor cells 
(score 5). Pairwise analysis of antibody clones showed weighted kappa values in 
the range of 0.45-0.91 with the highest values for comparisons with 22C3 and 
28-8 and the lowest involving SP142. Excluding SP142 resulted in kappa 
0.75-0.91. Weighted kappa for interobserver variation between pathologists was 
0.71-0.96. Up to 20% of the cases were differently classified as positive or 
negative by any pathologist compared with consensus score using ≥1% positive 
tumor cells as cutoff. A significantly better agreement between pathologists was 
seen using ≥50% as cutoff (0-5% of cases). In conclusion, the concordance 
between the PD-L1 antibodies 22C3, 28-8 and SP263 is relatively good when 
evaluating lung cancers and suggests that any one of these assays may be 
sufficient as basis for decision on treatment with nivolumab, pembrolizumab, and 
durvalumab. The scoring of the pathologist presents an intrinsic source of error 
that should be considered especially at low PD-L1 scores.