Protein, Total, Quantitative, 24-Hour Urine

Normal urine protein consists of albumin (≤35 mg/24 hours), other plasma proteins (ie, globulins, haptoglobin, β₂-microglobulin, and light chain). Tamm-Horsfall glycoprotein secreted by renal tubular cells may contribute ≤50 mg/24 hours. Urinary protein in normals tends to increase with age, exercise, and standing posture.

Tests requiring a 24-hour urine collection with no preservative, such as creatinine, may also be performed on the same specimen. Although quantitative protein can be run on a random specimen or timed collections less than 24 hours, 24-hour collections are preferable for evaluation of the nephrotic states and inflammatory renal disorders. Creatinine, creatinine clearance, BUN, serum protein electrophoresis, ANA, anti-DNA antibodies, HIV, hepatitis C antibody, hepatitis B antigen, and complement levels (including total complement, C₃, C₄) are among useful tests to work up patients with proteinuria. Urine electrophoresis, immunofixation and immunoelectrophoresis are useful in patients older than 35 years of age to investigate possible diagnosis of amyloidosis, myeloma, and Waldenström macroglobulinemia.

Some patients exhibit orthostatic proteinuria (ie, recumbent urine protein 100−180 mg in a 12-hour overnight urine collection and up to 1 g in the subsequent 12 hours while ambulatory). The presence of >200 mg of urinary protein in the overnight specimen or equally increased amounts of urine protein in both specimens indicates a need for further work-up.¹

Nephrotic syndromes are the causes of the most severe urinary protein losses. Nephrotic syndrome is defined now usually by degree of proteinuria (ie, proteinuria >50 mg/kg/day). After time, additional signs and symptoms occur including hypoproteinemia, hypoalbuminemia, elevation of α₂-globulin with decreased γ-globulin on electrophoresis, hyperlipidemia, and edema. Urinary albumin is a more sensitive marker of progression and regression of renal disease than urine total protein, especially when urine total protein is <300 mg/g creatinine. In most laboratories, urine albumin is available from protein electrophoresis following concentration procedures; however, this method is not sensitive to low concentrations of albumin.² Albumin, 24-Hour Urine is the preferred test.