Could measurement of urinary albumin-protein ratio be useful in my patient with renal insufficiency and proteinuria?

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)?  

Liked this post? Download the app on your smart phone, and sign up below to catch future pearls right into your inbox, all for free! Thank you!

Subscribe to Blog via Email

Enter your email address to subscribe to this blog and receive notifications of new posts by email.

References

  1. 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/
  2. 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
  3. 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
  4. 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
  5. 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
Could measurement of urinary albumin-protein ratio be useful in my patient with renal insufficiency and proteinuria?

How should I interpret a positive result for serum cryoglobulins?

Cryoglobulins (CGs) are immunoglobulins that precipitate in the blood under cold conditions (<37◦ C) and redissolve upon warming1.  The term “cryoglobulinemia” is commonly used to describe patients with a systemic inflammatory syndrome that is often associated with small-to-medium vessel vasculitis due to cryoglobulin-containing immune complexes. Although some patients with cryoglobulinemia may be asymptomatic, most present with a range of diseases characterized by fatigue, arthralgia, skin rashes or necrosis, purpura, neuropathy, bowel wall ischemia and/or glomerulonephritis and kidney failure.

Wintrobe and Buell are credited for first describing cryglobulinemia in 1933 when assessing a patient who ultimately was found to have multiple myeloma2. Since then the spectrum of diseases associated with CG has expanded to also include seemingly disparate conditions such as hepatitis C, autoimmune disorders and monoclonal gammopathy of undetermined significance (MGUS).  A commonly cited classification scheme for CG is shown (Table)3.   It should be emphasized that some CGs may not fit neatly into this scheme.

In our patient, the positive CG serum test should be interpreted in the clinical context in which it was obtained while searching for risk factors as well as signs and symptoms that may be associated with cryoglobulinemia.

 

Table. Classification of cryoglobulinemia

Category Description Examples
Type I Isolated monoclonal immunoglobulin, either IgM or IgG (less commonly IgA or free immunoglobulin light chains Multiple myeloma, Waldenström’s macroglobulinemia, monoclonal gammopathy of undetermined significance (MGUS)
Type II Mixture of monoclonal IgM and polyclonal IgG Hepatitis C, HIV, other viral infections
Type III Polyclonal mixture IgM and IgG Autoimmune disorders, hepatitis C

References

  1. Takada S, Shimizu T, Hadano Y, et al. Cryoglobulinemia (review). Mol Med Rep 2012;6:3-8
  2. Wintrobe MM, Buell MV. Hyperproteinemia associated with multiple myeloma. Bull Johns Hopkins Hosp 52: 156-165, 1933
  3. Brouet JC, Clauvel JP, Danon F, et al. Biological and clinical significance of cryoglobulins. Am J Med 1974; 57:775-88.

 

Contributed by Kirstin Scott, Medical Student, Harvard Medical School

How should I interpret a positive result for serum cryoglobulins?