Bromcresol green assay is nonspecific for rat plasma albumin

Bromcresol green (BCG) assay has been used in microvascular studies to determine albumin in rat plasma. The purpose of this study was to demonstrate that BCG overestimates rat plasma albumin partly because BCG binds to transferrin, a beta-globulin. The light absorbance of a transferrin-BCG reagent solution is shown to increase with time; appreciable binding occurs within a few seconds. Pure transferrin produced BCG assay results (P less than 0.001) that could be expressed as pseudoalbumin concentrations. Albumin and transferrin solutions of equal concentrations were mixed in equal parts; a plot of albumin concentration determined by BCG vs. actual albumin concentration had a slope greater than the expected value of 1.0 (P less than 0.001). Plasma samples were obtained from six rats and assayed for albumin. Electrophoresis yielded a plasma albumin of 2.97 +/- 0.11 g/dl, whereas BCG assay yielded a significantly (P less than 0.01) greater value of 3.58 +/- 0.07 g/dl. We conclude that BCG assay estimates of albumin-to-globulin ratio (A/G), in which G is determined from the difference between total protein and albumin, are especially subject to error.