Renal handling of the third (C3) and fourth (C4) components of the complement system in the nephrotic syndrome.

Complementuria is a common finding in patients with heavy proteinuria from a variety of causes, and was detected in 23 out of 34 nephrotic subjects. Mean excretion of C3 and C4 in these patients was 49 +/- 22 and 14 +/- 3 mg/24h, respectively. The renal handling of complement appears to be largely molecular weight (MW) dependent, an inverse relationship between the sieving coefficient and MW of transferrin, IgG, C3, and C4 obtaining, in nephrotic patients irrespective of the nature of their glomerulopathy or degree of renal function. Furthermore, glomerular sieving of C3 and C4 was not significantly different in patients with immune glomerular injury associated with extensive glomerular complement deposition, from that in patients with non-immune glomerulopathy, suggesting that no unique mechanism exists for the transglomerular passage of complement from serum into the urine of the former group. The finding of a large increase of sieving of C3 and C4 in nephrotic patients with end-stage renal failure may indicate a failure by atrophic tubules to reabsorb and catabolize filtered complement.