[Protein determination in urine--a critical review]

Seven methods for protein determination in urine were systematically checked, viz. turbidimetric assay with benzethonium chloride, turbidimetry with trichloroacetic acid of various concentrations, micellar complex formation with Coomassie Brilliant Blue G-250, formation and gel filtration of protein complexes with copper, reaction of protein tannin compounds with iron, and biuret reaction after different precipitation procedures. The evaluation was mainly based on the following criteria: identical reaction with various proteins, stable and linear absorbance with concentration, complete recovery, and greatest possible freedom from interferences. On this basis, the biuret reaction must now be regarded as the method of choice. Albumin and gamma-globulins are reliably estimated after precipitation by perchloric acid or by an ethanol-diethylether mixture, whereas all proteins, including uromucoid, may be measured after precipitation by Tsuchiya's reagent. Interference from urinary pigments can be avoided by reading versus a sample blank.