New colorimetric method for quantitative determination of protein in urine

Total urinary protein is rapidly precipitated at room temperature by tannic acid. The tannic acid/protein precipitate, dissolved in aqueous triethanolamine/ferric chloride solution, gives a purple-violet color of high absorptivity. Absorbance at 510 nm is linearly related to concentration from 0.05 to 1.50 A for a protein content of 0.05 to 1.50 g/liter, and less than 5 mg/liter can be detected. The CV and analytical recovery ranged from 0.5 to 1.8% and 98 to 103%, respectively. Nonprotein urinary constituents do not interfere.