Urinary excretion of β2-microglobulin as a prognostic marker in immunoglobulin A nephropathy.

β2-microglobulin (β2-MG) is freely filtered at the glomerulus and subsequently reabsorbed and catabolized by proximal renal tubular cells. Urinary β2-MG is an early and sensitive biomarker of acute kidney injury; however, its utility as a biomarker of immunoglobulin A nephropathy (IgAN) is unclear. We included urinary β2-MG levels in the routine laboratory examination of all inpatients with biopsy-proven IgAN at our hospital from 2006 to 2010. We retrospectively analyzed the correlation between β2-MG levels and clinical parameters as a prognostic biomarker of IgAN. A total of 51 patients (30 males, 21 females; mean age, 33.01 ± 12.73 years) with IgAN were included in this study. Initial demographic, clinical, and laboratory data for all patients are listed. The mean initial estimated glomerular filtration rate and 24-hour urine protein levels were 94.69 ± 34.78 mL/min/1.73 m(2) and 1.28 ± 1.75 g/day, respectively. The mean level of urinary β2-MG was 1.92 ± 7.38 µg/mg creatinine. There was a significant correlation between initial serum creatinine (iSCr), urine protein creatinine ratio (UPCR), and the level of β2-MG (r = 0.744, r = 0.667, p < 0.01). There was also a significant correlation between renal function tests and the level of urinary β2-MG (p < 0.01). Cox regression analysis showed that albumin, β2-MG, iSCr, and UPCR were significant predictors of disease progression in IgAN. Urinary β2-MG levels showed a significant correlation with renal function and proteinuria in IgAN. Thus, we propose that urinary β2-MG may be an additional prognostic factor in patients with IgAN.