About the estimation of albuminuria based on proteinuria results

What is it about?

When estimating urinary albumin to creatinine ratio (ACR) from proteinuria to creatinine ratio (PCR), there may be a correlation between albuminuria and proteinuria in a setting where the majority of proteinuria cases are due to glomerular proteinuria (particularly in a diabetic-dominated cohort). These comparisons cannot be applied in all patients. Benzethonium chloride denatures proteins in urine, resulting in a suspension that can be qualified. In contrast to the statements of Résimont, all protein fractions are precipitated. Despite all efforts, proteinuria assays probably never will get standardized. Besides, Tamm Horsfall protein (the most abundant protein in normal urine) and Bence Jones proteinuria (a rare problem in the elderly) should be considered. The physiologically occurring Tamm Horsfall protein reacts in the benzethonium method, as evidenced by the reported reference values. The non-specificity of the benzethonium method protein may cause a large error in the proteinuria-based estimation of albuminuria in the low proteinuria range. In the low ACR range, the PCR/ACR ratio is high, which can be explained by the presence of Tamm-Horsfall proteins. Also linearity issues of the benzethonium assay have been reported in the high range. In EQA schemes for proteinuria, occasionally inter-vendor differences can be observed. In rare cases, the presence of Bence Jones proteinuria in the elderly may result in proteinuria results that cannot be reliably extrapolated to albumin results. Furthermore, factitious proteinuria (Munchausen syndrome) may lead to erroneous albumin estimation. When using economic arguments to avoid testing for albuminuria (proteinuria tests are less expensive than immunochemical albuminuria tests), it should be noted that quantitative albuminuria can be achieved using inexpensive urine test strips in conjunction with a sensitive urine test reader. Because total urinary protein is an ill-defined analyte that can never be standardized satisfactorily, a consensus has developed over the years that total urinary protein should be replaced by urinary albumin. Much effort has gone into developing a separate standard for urinary albumin for this purpose. In conclusion, care should be taken when estimating albuminuria out of proteinuria data, in particular in the case of low-range proteinuria or in patients presenting with Bence Jones proteinuria.

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