Urinary excretion of retinol-binding protein in type 1 (insulin-dependent) diabetic patients with microalbuminuria and clinical diabetic nephropathy

The urinary excretion of retinol-binding protein (RBP) was studied in 101 insulin-dependent diabetic patients allocated to three groups according to 24-h urinary albumin excretion rate (UAE) (median of three urine collections): group 1 (n=45), normal UAE<30 mg/24h; group 2 (n=27), microalbuminuria (UAE 30–300 mg/24 h); and group 3 (n=29), clinical diabetic nephropathy (UAE>300 mg/24 h). We used 23 healthy subjects as controls. Fractional clearance of RBP (FC-RBP) and its 24-h urinary excretion rate (URBP) were higher in each diabetic group than in healthy subjects, the highest values being found in group 3. Groups 1 and 2 did not differ in URBP and FC-RBP. There was a correlation between FC-RBP and haemoglobin A1c in both the total diabetic cohort (P<0.001) and in diabetic patients in groups 1 and 2 with a glomerular filtration rate of more than 90 ml/min (P<0.05). No correlation was found between FC-RBP and UAE and/or duration of diabetes in any of the diabetic groups. We conclude that the increased urinary excretion of RBP, indicating proximal tubular dysfunction, is already present in normoalbuminuric insulindependent diabetic patients and correlates with metabolic control. Further deterioration in proximal tubular function was not observed in microalbuminuric patients, but is a late event in clinical diabetic nephropathy.