Understanding PD-L1 Testing in Lung Cancer

PD-L1 testing measures the percentage of tumor cells expressing the PD-L1 protein, and is one of the most widely used biomarkers for predicting whether a lung cancer might respond well to checkpoint inhibitor immunotherapy.

PD-L1 is a protein some cancer cells display on their surface that can suppress nearby immune cells, helping the tumor evade immune attack. A pathology test on tumor tissue measures what percentage of cells stain positive for PD-L1, typically reported as a percentage such as less than 1%, 1-49%, or 50% or higher.

Higher PD-L1 expression is generally associated with a greater likelihood of responding to checkpoint inhibitor immunotherapy, and PD-L1 level is used in treatment planning discussions. However, it is an imperfect predictor: some tumors with low PD-L1 expression still respond to immunotherapy, and some with high expression do not.

Because PD-L1 testing alone doesn't fully predict response, it is typically considered alongside other factors, including tumor mutational burden and the presence of specific co-mutations such as STK11 or KEAP1, which research has linked to weaker immunotherapy response even in PD-L1-positive tumors.