Six methods for urinary protein compared

Six assays for protein in urine were compared for linearity, ease of standardization, precision, comparability of assay values, technical ease of assay, and current cost. The assays investigated were three dye-binding techniques, a recent turbidimetric technique, the trichloroacetic acid--biuret reaction, and a tannic acid protein precipitation reaction with ferric chloride. All assays suffered from standardization problems, although the biuret method showed the best analytical recovery of albumin and gamma-globulin. The tannic acid/ferric chloride method is dependent on sample pH. The turbidimetric assay exhibited the greatest imprecision; i.e., CVs were 19.5% at a protein concentration of 0.13 g/L and 6.0% at a protein concentration of 1.3 g/L. On the basis of all the factors assessed, we conclude that the Pesce/Strande Ponceau-S and the Bio-Rad Coomassie Brilliant Blue dye-binding techniques offer certain advantages over the other assays studied.