C4A and IgA glomerulonephritis: In observational studies, the serum C3/C4 ratio (HR 0.63, 95% CI 0.5–0.9) was found to be an independent predictor of renal outcomes in IgA nephropathy patients [60], while C4 levels (HR 2.4, 95% CI 1.6–3.8) were significantly associated with a poor prognosis among patients with IgA nephropathy [61].