What does elevated fluorescence of PI alone in Annexin V/PI staining indicate?

An Annexin V/PI assay can be used as an indicator for apoptosis or necrosis. Annexin V stains positive in apoptotic cells, while PI (propidium iodide) stains positive in necrotic cells. Live cells stain negative for both Annexin V and PI.

A result of Annexin V-/PI+ thus indicates necrotic cells. It must be noted, however, that necrotic cells can stain Annexin V+/PI+ as well, which is also an indicator of late apoptotic cells.