Reduction in mean glomerular pore size coincides with the development of large shunt pores in patients with diabetic nephropathy.

The glomerular size selectivity was determined by neutral dextran clearance sieving technique and plasma cystatin C levels in two groups of patients with long-standing type I diabetes mellitus and different stages of albuminuria but normal glomerular filtration rate and in a group of healthy controls. The sieving characteristics of the glomerular filter were determined using a mathematical model of log normal pore size distribution. Patients with albuminuria above 200 microg/min exhibited a significant increase of cystatin c plasma concentrations and a significant reduction in mean glomerular filtration slit size. Only these patients exhibited large, unrestrictive pores in the glomerular filter (shunt). The plasma cystatin c levels correlated significantly with 26-angstrom neutral dextran plasma levels in microalbuminuric patients and in patients with albuminuria above 200 microg/min. We conclude that a reduction in average pore size of the glomerular filter does not occur earlier than the development of large shunt pores. The renal clearance of cystatin c in patients with proteinuric diabetic nephropathy but a normal glomerular filtration rate is reduced due to its molecular size.