An increased proportion of PD-1-positive/CD3-positive T cells distinguishes maculopapular immune-related adverse events from conventional drug eruptions

Distinguishing maculopapular immune-related adverse events (irAE) from conventional drug eruptions (DE) can be difficult because of overlapping clinicopathological features. In this single-centre retrospective cohort, the proportion of PD-1-positive cells among CD3-positive T cells (PD-1/CD3%) showed complete separation between groups and may provide an objective adjunct to routine histopathological assessment. This simple metric may support clinicopathological attribution and inform management decisions in patients receiving immunotherapy.