A Sensitive Assay of 5-fluorouracil in Plasma by Gas Chromatography-mass Spectrometry

1 A gas chromatographic-mass spectrometric method was developed for determining 5-fluorouracil in plasma, using methylated thymine as an internal standard. 2 5-fluorouracil was extracted from plasma by a novel procedure which removed plasma components interferring with the sensitivity of the assay. The method included heating the plasma, washing with ether and extracting the drug under optimum conditions. 3 The sensitivity of the assay was 10 ng/ml plasma, sufficient to determine the low concentrations of 5-fluorouracil found in plasma during continuous infusion of the drug in patients receiving chemotherapy for cancer.