Can total urinary protein measurements predict microalbuminuria?

We re-addressed the question of whether routine total urinary protein determinations can be used to predict the presence of microalbuminuria by studying 61 patients who attended a diabetic clinic and tested negative or had one positive protein by dipstick. Total urinary protein was measured by the Biorad dye-binding method in undialyzed urine (UND), in dialyzed urine (DIAL), and in dialyzed urine in which albumin and globulins were separated, measured separately with albumin and globulin standards and the results added together to obtain total urinary protein (A + G). The results were compared with albumin measurements obtained by radioimmunoassay (RIA). Compared to DIAL, urinary protein measurements were 43% higher with A + G and 22% higher with UND. Microalbuminuria correlated moderately with UND (r =0.81) and better with the other methods (r=0.87 for DIAL, r=0.91 for A + G). None of the methods predicted microalbuminuria reliably. Taking a protein-to-creatinine ratio of 0.15 and an albumin-to-creatinine ratio of 0.03 as upper limits of normal, we found that UND had a 72% positive predictive value (28% false positives) and 85 % negative predictive value (15% false negatives). DIAL had 90% positive predictive value (10% false positives) and 78% negative predictive value (22% false negatives). A + G had 65% positive predictive value (35% false positives) but 91% negative predictive value (9% false negatives). A + G, which uses the correct standards, would be the most suitable method for screening, having the least number of false negatives, but has more false positives because it is more sensitive. In practice, most routine chemical laboratories find it expedient to use only UND, but physicians interpreting the results of this method should be aware of its limitations.