Tubular urinary indexes reliably distinguish between primary tubulointerstitial and primary glomerular diseases in patients referred for kidney biopsy.

Aims: Kidney biopsy remains the gold standard for accurately diagnosing renal diseases. Urinalysis and assessment of renal function are the cornerstones for assessment of patients prior to biopsy. There is significant overlap in the results of routine urine parameters (proteinuria, erythrocyturia, leukocyturia) among different kidney diseases, which hinders the possibility of adequately estimating disease etiology prior to the biopsy. The aim of our study was to assess whether diverse markers of glomerular and tubular proteinuria - urinary albumin, IgG, alpha-1-microglobulin (alpha-1-m) and N-acetyl-beta-D-glucosaminidase (NAG) - are capable of distinguishing between patients with primary tubulointerstitial (TID) and primary glomerular disease (GLOM). Methods: Our study is a retrospective, single-center, consecutive case series of patients referred for kidney biopsy. We analyzed routine urinalysis results performed on a second morning urine sample immediately prior to the biopsy. Results: Patients with TID had significantly higher values of alpha-1-m and NAG, with lower values of albumin and IgG in the urine compared to patients with GLOM. Three tubular urinary indexes had high sensitivity and specificity for distinguishing TID from GLOM: NAG/albumin, alpha-1-m/proteinuria, and alpha-1-m/albumin, with the highest values in the latter index (96.6% and 98.2%, respectively, cut-off point >= 0.33). Conclusions: Prior to kidney biopsy, tubular urinary indexes may present a valuable tool in distinguishing patients with TID from patients with GLOM.