Objective:The aim of our study is to develop an LC-MS/MS method using isotope internal standard for the determination of vancomycin in human blood serum and to validate its clinical value.Method:We conducted a methodological evaluation study using serum samples from 221 hospitalized patients (142 males and 79 females; mean age (59.31±15.32) years) who received treatments of vancomycin at the Sir Run Run Shaw Hospital of Zhejiang University between March 2021 and June 2022. In addition, thirty clinical residual serum samples from healthy individuals (15 males and 15 females; mean age (35.65±9.86) years) undergoing physical examination were used for methodological evaluation. The method was established using AB Sciex Triple Quad 4500 MD liquid chromatography-tandem mass spectrometer and chromatographic separation was carried out using a Phenyl-Hexyl column with gradient elution. The mobile phase was composed of 0.1% formic acid in water and methanol; the column temperature was 40℃; Vancomycin-[d12] TFA salt was used as the internal standard (IS). The sensitivity, specificity, linearity, accuracy, imprecision, matrix effect, and carry-over of the method were evaluated.Results:The detection limit of vancomycin was 0.2 mg/L and the lowest limit of quantification was 0.5 mg/L. It showed good linearity ( R2=0.998 4) in the 1 to 50 mg/L concentration range. Accuracy (recovery rate 87.45%-112.69%), intra-day and inter-day imprecision ( CV 4.91%-7.69%), internal standard standardized matrix factor (90.22%-104.29%). Carryover pollution was negligible. Of the 221 patients, the mean trough concentrations of vancomycin in serum was (13.15±8.56) mg/L. Conclusion:The LC-MS/MS method for the detection of serum vancomycin established in our laboratory meets the requirements of the reference method, and can be used for the monitoring of clinical therapeutic drugs.