Urine should be at room temperature prior to testing with the reagent strip method.

Clinical Significance

Normal urine contains up to 150 mg (1 to 14 mg/dL) of protein each day. This protein originates from the ultrafiltration of plasma and from the urinary tract itself. Proteins of low molecular weight ([MW] <40,000) readily pass through the glomerular filtration barriers and are reabsorbed. Because of their low plasma concentration, only small quantities of these proteins appear in the urine. In contrast, albumin, a moderate-molecular-weight protein, has a high plasma concentration. This fact, combined with its ability (although limited) to pass through the filtration barriers, accounts for the small amount of albumin present in normal urine. Actually, less than 0.1% of plasma albumin enters the ultrafiltrate, and 95% to 99% of all filtered protein is reabsorbed. High-molecular-weight proteins (>90,000) are unable to penetrate a healthy glomerular filtration barrier. The end result is that the proteins in normal urine consist of about one-third albumin and two-thirds globulins. Among proteins that originate from the urinary tract itself, three are of particular interest: (1) uromodulin (also known as Tamm-Horsfall protein), which is a mucoprotein synthesized by the distal tubular cells and involved in cast formation; (2) urokinase, which is a fibrinolytic enzyme secreted by tubular cells; and (3) secretory immunoglobulin A,which is synthesized by renal tubular epithelial cells.2

The presence of an increased amount of protein in urine, termed proteinuria, is often the first indicator of renal disease. For most patients with proteinuria (prerenal and renal), the protein present at an increased concentration is albumin, although to varying degrees. Protein reabsorption by the renal tubules is a nonselective, competitive, and threshold-limited (Tm) process. Basically, when an increased amount of protein is presented to the tubules for reabsorption, the tubules randomly reabsorb the protein in a rate-limited process. As the quantities of proteins other than albumin increase and compete for tubular reabsorption, the amount of albumin excreted in the urine also increases. Proteinuria results from (1) an increase in the quantity of plasma proteins that are filtered, or (2) filtering of the normal quantity of proteins but with a reduction in the reabsorptive ability of the renal tubules. Early detection of proteinuria (i.e., albumin) aids in identification, treatment, and prevention of renal disease. However, protein excretion is not an exclusive feature of renal disorders, and other conditions can also present with proteinuria.

Proteinuria can be classified into four categories: prerenal or overflow proteinuria, glomerular proteinuria, tubular proteinuria, and postrenal proteinuria. This differentiation is based on a combination of protein origination and renal dysfunction; together, they determine the types and sizes of proteins observed in the urine (Table 6.5).

Table 6.5