MMP9 and non-proliferative diabetic retinopathy: MMP-9 concentrations differed significantly between the mild NPDR group (27 ± 24 pg/mL) and the PDR group (157 ± 90 pg/mL, p < 0.001); and between the moderate or severe NPDR group (56 ± 32 pg/mL) and the PDR group (p < 0.001).