Objective. After filtration through glomeruli, β2-microglobulin is reabsorbed in proximal tubules. Increased urinary β2-microglobulin indicates proximal tubule injury and measurement of β2-microglobulin in urine is useful to determine the source of renal injury. Kidney injury molecule-1 (KIM-1) has been characterized as a selective proximal tubule injury marker. This study was designed to evaluate the correlation of urinary β2-microglobulin concentration and KIM-1 expression as evidence of proximal tubule injury. Methods. Between 2009 and 2012, 46 patients with urine β2-microglobulin (RenalVysion) had follow-up kidney biopsy. Diagnoses included glomerular and tubule-interstitial disease. Immunohistochemical staining for KIM-1 was performed and the intensity was graded from 0 to 3+. Linear regression analysis was applied to correlate the values of urinary β2-microglobulin and KIM-1 staining scores. P < 0.05 was considered statistically significant. Results. Thirty patients had elevated urinary β2-microglobulin. KIM-1 staining was positive in 35 kidney biopsies. There was a significant correlation between urinary β2-microglobulin and KIM-1 staining (P < 0.05). Sensitivity was 86.6%, specificity was 43.7%, positive predictive value was 74.2%, and negative predictive value was 63.6%. Conclusion. Increased urinary β2-microglobulin is significantly correlated with KIM-1 staining in injured proximal tubules. Measurement of urine β2-microglobulin is a sensitive assay for proximal tubule injury. 1. Introduction Acute kidney injury is a common clinical syndrome that is characterized by a rapid decline in kidney function, often triggered by glomerular disease and/or tubulointerstitial disease and associated with high morbidity and mortality [1–4]. In addition to serum creatinine, β2-microglobulin is a biomarker that can be used to determine underlying causes of acute kidney injury [5]. Serum β2-microglobulin derives from cellular membrane turnover [6], since β2-microglobulin forms the invariant light chain portion of major histocompatibility complex (MHC) class I in membranes of all cells [7–10]. As a single-chain small polypeptide (MW = 11.8？kDA), β2-microglobulin is filtered almost completely through the glomeruli of the healthy kidney [11, 12] and then reabsorbed by the renal proximal tubules. Only a small amount of β2-microglobulin can be detected in the urine under normal physiological conditions. Therefore, levels of serum and urinary β2-microglobulin reflect the functions of glomeruli and proximal tubules [13]. In patients with acute kidney injury, an
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