Interference of Thiouracil in the Ferric Chloride-sulfuric Acid Cholesterol Reaction.

I T IS IJSUAL PRACTICE to determine both butanol-extractable iodine (BET) and cholesterol on a given sample of serum. Recently, Man proposed adding thiouracih in approximately 0.OO1M concentration to stored sera as a preservative for BET determinations (1). An impression was gained in the author’s laboratory that sera so treated with thiouracil often yielded cholesterol values that appeared to be somewhat lower than anticipated. The cholesterol procedure employed at the time of this observation was that of Zak et al. (2). Investigation revealed that the Zak ferric chloride-sulfuric acid reaction for cholesterol gives erroneously low results in the presence of thiouracil. Figure 1 shows the absorption curve of the color reaction (Curve 1), and also that the addition of increasing amounts of thiouracil (TU) (curves 2-6) suppresses the 550 m absorption maximum of the color complex. Addition of 0.13 mg. of thiouracil per milliliter of serum (approximately 0.001M) causes an average decrease in the apparent cholesterol value of 30-40 mg. per 100 ml. of serum. In Man’s paper (1) no attempt was made to ascertain the exact concentration of thiouracil, but “the practice was to add a minute particle, as small an amount of thiouracil as could be dropped from a pointed