Urinary protein

Total protein concentration in urine can be estimated at the bedside using chemically impregnated plastic strips. However, these reagent strips are unable to react to many of the heterogeneous proteins found in human urine. All these semiquantitative screening tests measure only total protein or albumin concentration. Their sensitivity and specificity can be markedly influenced by fluid intake, the state of diuresis, and the resulting urinary concentration. Laboratory quantitative protein tests are best performed in 24-h urine. There is a high degree of correlation between 24-h urinary protein excretion and protein-to-creatinine ratios in random single voided urine samples in patients with a variety of renal diseases ( Ginsberg.efal 1983), suggesting that this ratio is useful as a screening test for renal disease. It has been suggested that protein-to-creatinine ratios above 3.0 mg/mg or below 0.2 mg/mg indicate protein excretion rates of more than 3.0 g/24 h or less than 0.2 g/24 h respectively.

Only a small amount of protein appears in the urine as a result of normal tubule secretion. Two renal mechanisms can give rise to a higher urine protein level. First, a disruption in the capillary wall barrier can lead to a glomerular origin of proteinuria. Second, tubule damage or dysfunction can inhibit the normal absorptive capacity of the proximal tubule and lead to tubule proteinuria. Therefore proteinuria may suggest the presence of glomerulonephritis, vasculitis, or tubule necrosis. Quantifying urine protein excretion may help to distinguish glomerular from tubular proteinuria. Since protein excretion is in the nephrotic range (> 3 g/24 h), a glomerular source is almost certain. Quantification of urine protein excretion may also provide useful prognostic information and assist in monitoring the response to therapy.

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