The level of urinary secretory immunoglobulin A (sIgA) of patients with IgA nephropathy is elevated and associated with pathological phenotypes

Recent studies have demonstrated deposition of secretory immunoglobulin A (sIgA) in glomeruli of some patients with IgA nephropathy (IgAN). The aim of this study is to investigate the levels of urinary sIgA in IgAN patients with different pathological phenotypes and whether it could be used as a non‐invasive biomarker for assessment of kidney injury in IgAN. Urine samples from 202 patients with IgAN were collected on the day of renal biopsy. Forty‐eight fulfilled the histopathological criteria of Haas‐I or II (group 1), 60 fulfilled Haas‐III (group 2) and 94 patients fulfilled Haas‐IV or V (group 3). Urine samples from 60 healthy sex‐ and age‐matched volunteers with negative urinalysis were collected as normal controls. Urinary sIgA was detected by sandwich enzyme‐linked immunosorbent assay and was corrected by urinary creatinine. In comparison with normal controls, the levels of urinary sIgA were significantly higher in IgAN [2·22 (0–43·82) μg/mg Cr versus 1·08 (0–16·49) μg/mg Cr, P < 0·001]. The levels of urinary sIgA were significantly higher in group 3 than that in group 2 and group 1 [3·54 (0–43·82) μg/mg Cr versus 1·63 (0–15·88) μg/mg Cr versus 0·91 (0–11·79), P < 0·001], and group 2 than group 1 (P = 0·014). The levels of urinary sIgA were associated positively with proteinuria (r = 0·443, P < 0·001), serum creatinine (r = 0·376, P < 0·001) and histopathological parameters, such as ratio of global sclerosis (r = 0·356, P < 0·001), ratio of total crescents (r = 0·339, P < 0·001) and ratios of cellular crescents (r = 0·231, P < 0·001). The levels of urinary sIgA were associated closely with histopathological phenotypes of IgAN and might be used as a non‐invasive biomarker to evaluate kidney injury in IgAN.