Effects of sodium dodecylsulphate, dye concentration and paraprotein on coomassie blue dye-binding assays for protein in urine.

Various Coomassie Blue reagents, containing either increased dye concentration or added sodium dodecylsulphate, were compared with a biuret method for the assay of total protein in urine. When immunoglobulin free light chain protein or immunoglobulin paraprotein were present, results from the Coomassie Blue methods were up to 50% lower than with the biuret method; increased dye concentration did not improve comparability substantially, but the addition of sodium dodecylsulphate reduced the bias to about 20%. When neither free light chain protein nor immunoglobulin paraprotein was present, results from the Coomassie Blue methods were only about 30% lower. The addition of sodium dodecylsulphate reduced this bias to 10%. Correlations between the biuret and the Coomassie Blue method were best when the Coomassie Blue reagent contained 40 mg/L sodium dodecylsulphate (r better than 0.98 in all groups; p less than 0.001).