The utility of urine specific gravity in combination with urinary qualitative protein testing for detecting abnormal...

Although qualitative tests are often used for urinary protein evaluation in primary care and health checkups, their results are affected by urine concentration, and the possibility of false-positive or false-negative results can hinder appropriate referral to a nephrologist. We aimed to investigate whether the consideration of urine specific gravity (uSG) improves the diagnostic performance of proteinuria evaluation using qualitative analysis.

Urine protein levels (mg/gCr), qualitative data, and uSG were extracted over the past 5 years in our nephrology outpatient department. Qualitative results were classified as (-) to (4+), quantitative as A1 < 150, A2 < 500, A3 ≥ 500, A4 ≥ 1000. The second test was performed 1 year later.

In total, 37% of (-) were A2 or higher and 14% of (±) were A3 or higher. In (±), 57% in uSG < 1.010 was A3 or higher, and 72% in uSG > 1.020 was A1. The AUC, with urine specific gravity as the explanatory variable and proteinuria of A2 or higher as the outcome, was 0.878 (95% CI 0.871-0.884). The result did not change significantly even after excluding severe hematuria, multiple myeloma and urine pH ≥ 7.5. The percentage of A1 was 37% for the first quantification of two consecutive (±) and 31% for only one (±).

Current qualitative tests can overestimate or underestimate the actual urinary protein levels. Consideration should be given to increasing the diagnostic performance of qualitative tests in combination with uSG.