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Background: The clinical significance of a trace protein reading on urinalysis is unclear, and such a result is often ignored by the clinician. Methods: We examined 185 samples of urine with trace proteinuria by both Chemstrips and sulfosalicylic acid testing, and compared the results with those of urinary albumin and total protein concentrations. Results: Taking for the purposes of this study an arbitrary upper limit of normal of 20 mg/l for albumin and 100 mg/l for total protein concentration, we found abnormal albumin excretion in 87% and abnormal total protein excretion in 88% of trace samples. In this study, a negative urinalysis for protein excluded microalbuminuria in 87% and proteinuria in 78% of cases. Conclusion: Qualitative testing for protein by urinalysis has a high sensitivity and specificity for diagnosing or ruling out microalbuminuria. Trace proteinuria usually means microalbuminuria; negative proteinuria tends to rule it out.

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.