External quality assessment demonstrates that PD‐L1 22C3 and SP263 assays are systematically different

PD‐L1 inhibitors are part of first line treatment options for patients with advanced non‐small cell lung cancer. PD‐L1 immunohistochemistry (IHC) assays act as either a companion or a complementary diagnostic. The purpose of this study is to describe the experience of external quality assurance (EQA) provider UK NEQAS ICC and ISH with the comparison of different PD‐L1 assays used in daily practice. Three EQA rounds (pilot, run A and run B) were carried out using formalin fixed paraffin embedded samples with sample sets covering a range of epitope concentrations, including ‘critical samples’ near to clinical threshold cut‐offs. An expert panel (n = 4) evaluated all returned slides simultaneously and independently on a multi‐header microscope together with the participants own in‐house control material. The tonsil sample was evaluated as ‘acceptable’ or ‘unacceptable’, and for the other samples the percentage of PD‐L1 stained tumour cells were estimated in predetermined categories (