C3 Nephritic factor is an autoantibody found in some patients with mesangiocapillary glomerulonephritis, dense deposit disease and/or partial lipodistrophy. This most common form of this autoantibody recognises (and stabilises) the otherwise unstable C3 cleaving enzyme of the alternative pathway of the complement system. Presence of the autoantibody leads to uncontrolled activation of C3 leading to very low C3 levels but normal C4. The presence of this complement profile (very low C3 normal C4) in patients with appropriate renal symptoms is almost characteristic of the presence of C3 Nef. Specific tests for C3 Nef are problematic and rarely necessary. They involve adding a suspected sample with normal serum and measuriung loss of functional C3.
Mesangiocapillary glomerulonephritis and/or partial lipodistrophy should be considered only if C3 concentration below normal and C4 within reference range.
If positive in patients with renal disease it strongly suggests mesangio-capillary nephritis.
Serum Separator Tube (SST)
Test serum incubated with normal serum for 30 minutes at 37°C. The degree of C3 activation is detected by electrophoresis.
Referred Test. Sample must be received in laboratory within 3 hrs of venepuncture.
Assay range notes
Positive results expressed as 1 to 3+