A method for the estimation of the concentration of the blue dye, T-1824, in plasma has been developed. The method is based on the fact that the dye can be reduced to a colorless compound by Na2S2O4, in alkaline solution. The extinction coefficient is determined under specified conditions before and after reduction of the dye. Under the conditions employed hemoglobin does not interfere, even if partly present as HbCO or as methemoglobin; the absorption of light by the hemoglobin is the same before and after reduction of the dye. Absorption of light by suspended lipids and plasma pigments is also unaltered by the Na2S2O4. Hence variations from sample to sample in hemolysis or degree of lipemia do not affect the accuracy with which T-1824 is determined.

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