External-standard high-performance liquid chromatographic method for quantitative determination of furosemide in plasma by using solid-phase extraction and on-line elution.

A rapid and simple external-standard high-performance liquid chromatographic (HPLC) method has been developed for the determination of the concentration of furosemide in plasma. The analyte is extracted with a C-2 ethyl sorbent. On-line elution of the analyte into the HPLC system is accomplished with an advanced automated sample processor (Varian). Furosemide is quantified by fluorescence detection within a linear range of 25 to 1000 ng/mL (average correlation coefficient, 0.9998), with a limit of detection of 1.8 ng/mL. Both internal- and external-standard procedures were evaluated, and the external-standard procedure demonstrated superior characteristics. The external-standard procedure was precise to within a relative standard deviation of 8% and accurate with less than 3% error throughout the concentration range studied. The external-standard HPLC method was used to analyze the concentration of furosemide in greater than 1000 plasma samples obtained from patients with either normal kidney function or renal failure who had received furosemide either orally or intravenously in an experimental setting.