Protein, Total, Urine

The Protein, Total, Urine test (urine total protein; 24-hour urine protein) is ordered to confirm and quantify proteinuria. It supports evaluation of primary glomerular disorders, including minimal change disease, membranous nephropathy, and focal segmental glomerulosclerosis, as well as secondary renal involvement from systemic conditions such as diabetes mellitus, systemic lupus erythematosus, and amyloidosis. It also assists with kidney risk assessment in individuals at increased likelihood of chronic kidney disease. This measurement is further used to monitor renal status during exposure to medications with nephrotoxic potential, including aminoglycosides, amphotericin B, cisplatin, cyclosporine, NSAIDs, ACE inhibitors, sulfonamides, penicillins, thiazide diuretics, and furosemide.

Under normal physiology, urinary protein excretion is minimal because of glomerular size and charge selectivity with efficient proximal tubular reabsorption. Typical daily protein loss is approximately 40–80 mg, and excretion greater than 150 mg/day constitutes proteinuria, which is usually albumin-predominant. Proteinuria may be transient and functional, occurring with fever, strenuous exercise, stress, acute infection, or dehydration, or it may be orthostatic in young individuals. Overflow proteinuria reflects excessive filtration of low–molecular weight proteins such as hemoglobin, myoglobin, or monoclonal light chains. Renal proteinuria is categorized as glomerular, due to basement membrane injury and common in diabetic nephropathy and primary glomerulopathies, or tubular, due to impaired proximal tubular reabsorption and typically less than 2 g/day. Marked protein loss (>3–3.5 g/day) can lead to hypoalbuminemia, reduced oncotic pressure, and edema. Persistent microalbuminuria is associated with increased ischemic heart disease risk. A random urine total protein is a screening approach and does not subtype the pattern of proteinuria, whereas a 24-hour urine collection permits quantitation of excretion. Identification of specific proteins such as Bence Jones protein requires targeted testing, for example urine protein electrophoresis.