Tubular function and tubulointerstitial disease

Abstract Tubular transport determined by the fractional excretion (FE) of filtered solutes was studied in 129 nephrotic patients; 72 patients with mesangial proliferation (MesP-NS) and intact tubulointerstitium (group 1), 13 patients with MesP-NS and superimposed tubulointerstitial fibrosis (TIF; group 2), 27 patients with mild focal segmental glomerulosclerosis (FSGS; group 3), and 17 patients with severe FSGS (group 4). In the 72 nephrotic patients with MesP-NS and normal tubulointerstitium (no TIF), tubular transport was intact (FE of sodium [FENa], 0.5 ± 0.5; FE of calcium [FECa], 0.3 ± 0.3; FE of phosphate [FEPO 4 ], 14 ± 13; FE of uric acid [FEUA], 9.8 ± 5; FE of magnesium [FEMg], 1.3 ± 0.5). In the 13 nephrotic patients with MesP-NS and superimposed TIF (4.9% ± 2%), there was no difference in FE solutes from those in group 1 except for FEMg (3.3 ± 0.9; P P P 4 , 17 ± 12; P , not significant; FEUA, 16.5 ± 8; P P 4 , 47 ± 24; FEUA, 37 ± 18; FEMg, 12 ± 6). Thus, the results imply that (1) normal tubular transport reflects an underlying intact tubulointerstitial structure, whereas tubular dysfunction indicates an underlying tubulointerstitial disease, and (2) FEMg is the most sensitive index to detect an early abnormality of tubular structure and function.