Urinary albumin to protein ratio: more of the same or making a difference?

In this issue, Smith et al. [1] consider whether the simple ratio of urine albumin to total protein (uAPR) provides a measure of low molecular weight (LMW) proteinuria that distinguishes tubular versus glomerular pathology. In a retrospectively studied cohort of 1011 proteinuric patients [protein:creatinine ratio (PCR) >30 mg/mmol], uAPR was correlated with simultaneous findings on urine protein immunofixation electrophoresis (uPEI). A single, blinded observer categorized the electrophoresis pattern as being either tubular (predominantly LMW), glomerular (a dominant albumin band), mixed or overflow (light chain monoclone) proteinuria. The LMW protein b2-microglobulin (b2M) and the tubular enzyme N-acetyl-b-D-glucosaminidase (NAG) were also measured in a subgroup (n 1⁄4 248) of outpatients and their predictive performance at uPEI classification compared to that of uAPR. Renal biopsy results were available to confirm the diagnostic utility of uAPR for only 68 patients. uAPR gave a reasonable prediction of a ‘tubular’ uPEI proteinuria pattern in ROC curve analysis (AUC 0.84), which was similar to that of b2M and NAG. It also performed better than 1293