OBJECTIVE To investigate the long-term course of glomerular filtration rate (GFR) in IDDM patients with microalbuminuria in order to identify patients with stable or declining kidney function over a 5-year study.

RESEARCH DESIGN AND METHODS Forty normotensive (129 ± 11/80 ± 8 mmHg) IDDM patients with persistent microalbuminuria (mean urinary albumin excretion [UAE] 84 mg/24 h [range 30–300]) were followed prospectively for 5 years of clinical examinations that included the measurement of GFR (⁵¹Cr-labeled EDTA clearance) at least once a year. The mean GFR at baseline was 120 ± 18 ml · min⁻¹ · 1.73 m⁻².

RESULTS Using multiple regression analysis, the rate of decline in GFR was independently correlated to onset of diabetic nephropathy (P < 0.001) and systolic blood pressure (sBP) at baseline (P < 0.05). Increase in UAE was correlated to the mean HbA_1c during the observation period. Out of 40 patients, 14 progressed to diabetic nephropathy (UAE > 300 mg/24 h). These patients had a significant reduction in GFR (mean −2.2 ± 3.8 ml · min⁻¹ · year⁻¹; P = 0.05), while GFR remained stable in the remaining 26 patients with nonprogressive microalbuminuria (change in GFR 0.5 ± 2.1 ml · min⁻¹ · year⁻¹; NS). The difference in the rate of decline of GFR was significant (mean 2.7 ml · min⁻¹ · year⁻¹; P < 0.05).

CONCLUSIONS Normotensive IDDM patients with nonprogressive microalbuminuria have a stable GFR. Progression of UAE to diabetic nephropathy heralds a progressive loss of kidney function. Efforts should be made to prevent the progression from microalbuminuria to diabetic nephropathy in every IDDM patient with microalbuminuria.