Programmed Death Ligand-1 Testing in Triple-Negative Breast Cancer: National Practice and Interlaboratory Variation in the Netherlands.

[PURPOSE] This nationwide study aimed to evaluate programmed death ligand-1 (PD-L1) test characteristics and test results during the first 2 years after the introduction of PD-L1 testing in triple-negative breast cancer (TNBC) and to analyze interlaboratory variation in PD-L1 positivity rates, assessed by a combined positive score (CPS), using nationwide real-world data from the Netherlands.

[MATERIALS AND METHODS] Pathology reports of TNBC patients mentioning PD-L1 were retrieved from the Dutch nationwide pathology database (Palga). PD-L1 test characteristics and results were evaluated per year between August 2022 and August 2024. To assess consistency between laboratories, interlaboratory variation in PD-L1 positivity rates (CPS, ≥10) was analyzed using funnel plots with 95% CIs around the national mean. Laboratory-specific PD-L1 positivity rates were case-mix adjusted using multivariate logistic regression.

[RESULTS] During the study period, 926 PD-L1 tests were performed (423 in year 1 and 503 in year 2). CPS was applied in 94.5% of cases. The national mean PD-L1 positivity rate was 46.7%. After adjustment for case-mix variables, laboratory-specific positivity rates ranged from 21.3% to 70.7%, with 3 of the 19 laboratories showing significant deviations from the national mean.

[CONCLUSIONS] PD-L1 testing for TNBC is generally well conducted according to the national guidelines in the Netherlands, with consistent use of CPS. However, the observed interlaboratory variation in positivity rates highlights the need for further standardization of testing methods to ensure accuracy of patient selection for immunotherapy.