Serum BAFF levels were higher in IgAN patients and correlated with proteinuria and faecal metabolites. (A) Serum BAFF levels were determined by ELISA in 44 IgAN patients, 23 HSs and 8 controls. BAFF levels were significantly higher in IgAN patients with respect to HSs (P = 0.012) and non-IgA glomerulonephritis patients (controls; P < 0.0001). Serum BAFF levels in 30 IgAN patients correlated with (B) proteinuria (P < 0.001) and with different faecal metabolites: (C) 4-(1,1,3,3-tetramethylbutyl) phenol (P = 0.0027), (D) p-tert-butyl-phenol (P = 0.0003), (E) methyl neopentyl phthalic acid (P = 0.0007), (F) hexadecyl ester benzoic acid (P = 0.003) and (G) furanone A (0.024) in IgAN patients (Spearman’s correlation). ^*P ≤ 0.05, ^**P ≤ 0.005.