Significance of urinary C3 excretion in glomerulonephritis

The third component of complement (C3) was measured in the urine of 98 patients with a variety of renal diseases. Renal biopsy was performed on 83 of the patients and examined by light, electron, and immunofluorescence microscopy. Urinary C3 was detected in cases of membranous glomerulonephritis, mesangiocapillary glomerulonephritis, rapidly progressive glomerulonephritis, and renal amuloidosis. It was not detected in minimal lesion glomerulonephritis; in cases of proliferative glomerulonephritis it was detected only in those showing histological evidence of a progressive lesion. Concentrations were low or undetectable in cases of non-immunological renal diseases. There was a good correlation between urinary C3 concentrations and the deposition of C3 in glomerular capillary walls, as seen by immunofluorescence microscopy, and there was no correlation with the degree or selectivity of proteinuria. Urinary C3 excretion appears to be an accurate indicator of continuing activity of disease. It is suggested that the presence of C3 in urine is due to complement fixation by immune complexes in glomerular capillary walls, and that urinary C3 estimations have potential applications in the study of glomerulonephritis.