Protein–Osmolality Ratio for the Quantitative Assessment of Proteinuria from a Random Urinalysis Sample

Abstract

Proteinuria evaluation is important for the differential diagnosis of renal disease and the assessment of prognosis and response to therapy. Qualitative proteinuria correlates poorly with quantitative 24-hour protein levels because of hydration and methodologic problems associated with observer error. The authors automated quantitative urinary protein and osmolality measurements and evaluated whether a urinary protein-osmolality ratio (mg/L/mOsm/kg) in the random urinalysis predicts the 24-hour urine protein level. They compared this ratio with the (1) 24-hour protein levels in 78 normal women and 53 normal men; (2) 24-hour protein levels in 129 proteinuric patients; and (3) urinary protein-creatinine ratio in 131 normal patients and 105 proteinuric patients. The normal urinary protein-osmolality ratio was less than 0.12 (sensitivity, 96%; specificity, 93%). This compared to a urinary protein-creatinine ratio of less than 0.05 (sensitivity, 96%; specificity, 90%). A urinary protein-osmolality ratio of greater than 2.5 indicates a level of more than 3.0 g/24 hour (sensitivity, 91%; specificity, 98%). The urinary protein-osmolality ratio = 1.12x + 0.05 (r = 0.88), where x = 24-hour protein. The authors conclude that the quantitative urinary protein-osmolality ratio is better than a qualitative urinalysis and urinary protein-urinary creatinine ratio for detecting or assessing abnormal proteinuria and allows for a reasonable prediction of 24-hour protein levels.