Perfusion index association with diabetic retinopathy severity groups across ETDRS zones. Macular retinal mean perfusion indices with SE bars by diabetic retinopathy severity groups: no to mild non-proliferative diabetic retinopathy (NPDR), moderate to severe non-proliferative diabetic retinopathy, proliferative diabetic retinopathy (PDR), calculated for each individual ETDRS zone. CF, central foveal; II, inner inferior; IN, inner nasal; IS, inner superior; IT, inner temporal; OI, outer inferior; ON, outer nasal, OS, outer superior; OT, outer temporal.