Protein error of indicators

The dipstick test for total protein includes a cellulose test pad impregnated with tetrabromphenol blue and a citrate pH 3 buffer. The reaction is based on the protein error of indicators phenomenon in which certain chemical indicators demonstrate one color in the presence of protein and another in its absence. Thus tetrabromphenol blue is green in the presence of protem at pH 3 but yellow in its absence. The color is read after exactly 60s and the test has a lower detection limit of 150 to 300mg/L, depending on the type and proportions of protein present. The reagent is most sensitive to albumin and less sensitive to globulins, Bence Jones protein, mucoproteins, and hemoglobin. 

The first useful application of the protein error of indicators arose from the early studies of Klotz (K22, K23). In the presence of low concentrations of bovine albumin, but not of 7-globulin or gelatin, the absorption spectra of a number of different dye solutions have been found to undergo pronounced shifts. Albumin-binding data and absorption maxima of many anionic azo dyes have been listed (M3). Three of these dyes have been utilized for protein estimation. 

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