LAUSR

Atezolizumab has emerged as a novel treatment in patients with triple-negative breast cancer showing unprecedented survival gain. This breakthrough benefit is exclusively observed in PD-L1-positive patients, but PD-L1 positivity still lacks a reliable definition. In this review, we discuss the current data on PD-L1 testing and its impact on the treatment of patients. The VENTANA SP142 assay was used in the trial that led to the approval of atezolizumab in the first-line setting. However, recent data show that this assay appears to be less sensitive to detect PD-L1 expression compared with other assays. Also, the intra- and inter-pathologist variability is substantial when performing various assays. PD-L1 testing remains a challenge in clinical practice. The variability and performance of assays may be an issue, and clinicians should avoid the interchangeability between assays. More studies are needed to elucidate the best way to use PD-L1 testing.