Modification of the colorimetric assay for chloramphenicol in the presence of bilirubin

The authors recently found that bilirubin interferes with the colorimetric chloramphenicol assay. Levels of serum bilirubin alone (4 to 6 mg/dl) resulted in apparent concentrations of chloramphenicol which appear to be in the therapeutic range. Concentrations of serum bilirubin greater than 8 mg/dl resulted in levels of apparent chloramphenicol associated with toxicity (>50 μg/ml). Small amounts of activated charcoal added to the isoamyl acetate extraction step of the assay eliminated this interference by bilirubin.