Circulating C-peptide and diabetic retinopathy.

Basal serum C-peptide concentrations were determined in a series of adult males referred to a diabetic retinopathy clinic. Degree of retinopathy of the most affected eye of each subject was classified using stereoscopic fundus photographs and fluorescein angiograms. A positive correlation was found between low C-peptide concentration (3.0 +/- 2.1 ng/ml) and proliferative or pre-proliferative diabetic retinopathy. A significantly higher (p less than 0.01) C-peptide level was found in subjects with non-proliferative diabetic retinopathy (6.0 +/- 3.7 ng/ml). Determination of fasting C-peptide levels after the first five-year period of insulin-dependent diabetes appears to be useful as a predictor of risk for proliferative disease.