Comparison between a single- and a multi-point calibration method using LC-MS/MS for measurement of 5-fluorouracil in human plasma.

When quantifying therapeutic drugs using LC-MS/MS instrumentation in clinical laboratories, batch-mode analysis with a calibration curve consisting of 6-10 concentrations for each analyte is the most widely used approach. However, this is an inefficient use of this technology since it increases cost, delays result availability and precludes random instrument access. Various alternative methods to reduce the calibrator use and improve efficiency without compromising analytical quality have been investigated, and a single-point calibration has been reported to be the simplest, least expensive and the quickest approach. This study compares a single and a multi-point calibration method using LC-MS/MS with 5-fluorouracil (5-FU) as a model drug. The method was validated for quantitative analysis of 5-FU over a concentration range of 0.05-50 mg/L. Patients undergoing cancer treatment with intravenous 5-FU had plasma 5-FU concentrations measured, and their dose adjusted in real time based on the calculated area under the time-concentration curve (AUC). Subsequently, a single point calibration method using a concentration at 0.5 mg/L was compared to the multi-point calibration method in terms of accuracy and precision. A Bland-Altman bias plot and a Passing-Bablok regression analysis showed a good agreement between the two methods (mean difference = -1.87 %, slope = 1.002, respectively) when comparing patient plasma 5-FU concentrations. The calibration method did not impact the AUC results nor the decision on 5-FU dose adjustments. Our study demonstrated that a single point calibration method produced analytically and clinically comparable results to those produced by a multi-point method when quantifying 5-FU and is feasible to be used clinically.