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Protein Error of Indicators

Testing for protein in the urine is based on the phenomenon called the "protein error of indicators" (ability of protein to alter the color of some acid-base indicators without altering the pH). In a solution that does not contain protein, tetrabromphenol blue, buffered at a pH of 3, is yellow. However, in the presence of protein, particularly albumin, the color changes to green, then blue, depending upon the concentration. This method is more sensitive to albumin than to globulin, detecting as little as 5 mg albumin/dL urine. Bence Jones protein and mucoprotein are examples of globulin components that are sometimes present in urine, but are not distinguishable by the chemical reagent strip method for urine protein.

False-positive results can occur when testing for urine protein. A urine specimen that has remained at room temperature for an extended period of time may produce a false-positive protein result on a reagent strip. A false-positive may also occur in the presence of bacterial contamination, alkaline medication, quaternary ammonium compounds, such as disinfectants or drugs, and with skin cleansers containing chlorhexidine.