Methods for assay of both total bilirubin and direct bilirubin in a sample. The inventive methods enable the amounts of total and direct (conjugated) bilirubin to be colorimetrically measured in about 5 minutes, and do not require the precipitation and removal of potentially interfering substances such as proteins prior to analysis. Novel stable reagents that solubilize the bilirubin and interfering substances are also provided. A novel total bilirubin reagent causes the oxidation of the total bilirubin in the sample, and the formation of a light-absorbing bilirubin chromophore detectable in the near-infrared region of the spectrum. A novel direct bilirubin reagent causes the oxidation of the direct bilirubin in the sample, and the formation of a light-absorbing bilirubin chromophore detectable in the near-infrared region of the spectrum, whereas the chromophore formed by the unconjugated bilirubin is substantially colorless.
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