A spot urine test for determination of albumin-protein ratio (uAPR) may be useful in distinguishing glomerular vs tubulointerstitial source of proteinuria. A low (<0.4) uAPR, defined as urinary albumin to creatinine ratio(uACR)/urinary protein to creatinine ratio (uAPR) is more suggestive of a tubulointerstitial renal disease and less suggestive of glomerular pathology.1-3
A 2012 study involving simultaneous measurements of urinary albumin and total protein in over 1000 proteinuric patients found a relatively high (0.84) area under curve (AUC) in a receiver operating characteristic curve analysis for uAPR (vs 0.74 for uACR and 0.54 for uPCR) in discriminating between tubular and non-tubular proteinuria pattern on urine protein electrophoresis and immunofixation. An uAPR cut-off of <0.4 was found to be 88% sensitive and 99% specific for the diagnosis of primary tubulointerstitial disorders on renal biopsy.1
Due to the limitations of this study (including a relatively small subset of patient who had renal biopsy), a related editorial concluded that a low uAPR gives a “reasonable prediction of a tubular electrophoretic proteinuria”, but that it warrants further validation. Nevertheless, uAPR could potentially be useful in patients with moderate proteinuria (>300 mg/day to <3 g/day) who have not had renal biopsy and where assessment of likelihood of tubulointerstitial vs glomerular source of proteinuria is desired.3 Interestingly, the utility of uAPR in predicting non-glomerular source of hematuria has also been reported.4
Bonus pearl: Did you know that the negatively-charged glomerular capillary wall repels negatively charged albumin thus preventing its filtration (charge-barrier) (5)?
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- Smith ER, Cai MMX, McMahon LP, et al. The value of simultaneous measurement of urinary albumin and total protein in proteinuric patients. Nephrol Dial Transplant 2012;27:1534-41. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4035283/
- Fraser SDS, Roderick PJ, McIntyre NJ, et al. Assessment of proteinuria in patients with chronic kidney disease stage 3: albuminuria and non-albumin proteinuria. PLOS ONE 2014;9:e98261. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4035283/pdf/pone.0098261.pdf
- Ellam T, Nahas ME. Urinary albumin to protein ratio: more of the same or making a difference. Nephrol Dial Transplant 2012;27:1293-96. https://www.ncbi.nlm.nih.gov/pubmed/22362784
- Ohisa N, Yoshida K, Matsuki R, et al. A comparison of urinary albumin-total protein ratio to phase-contrast microscopic examination of urine sediment for differentiating glomerular and nonglomerular bleeding. Am J Kidney Dis 2008;52:235-41. https://www.ajkd.org/article/S0272-6386(08)00828-7/pdf
- Venkat KK. Proteinuria and microalbuminuria in adults: significance, evaluation, and treatment. S Med J 2004;97:969-79. https://internal.medicine.ufl.edu/files/2012/07/5.18.05.04.-Proteinuria-review.pdf