Development and Validation of a UHPLC-MS/MS Method to Identify Interference from Qiliqiangxin Capsule and Deslanoside in Digoxin Therapeutic Drug Monitoring: A Comparison with Immunoassay

BACKGROUND: Digoxin is a cardioactive drug with a narrow therapeutic range, making therapeutic drug monitoring (TDM) essential in clinical practice. We aimed to establish an ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method for the simultaneous determination of digoxin, deslanoside, and three cardiac glycosides from Qiliqiangxin capsule (periplogenin, periplocymarin, and periplocin) in human plasma, and to identify their interference in digoxin TDM by comparison with the enzyme multiplied immunoassay technique (EMIT). METHODS: Chromatographic separation was performed on a reverse-phase column with gradient elution. Mass spectrometry detection was performed using an electrospray ionization source and multiple reaction monitoring mode. Sample pretreatment was conducted using liquid-liquid extraction. RESULTS: The five analytes exhibited good linearity across the range of 0.05-10 ng/mL. Intra- and interbatch accuracy ranged from 90.6% to 105.1%, with imprecision below 10.6%. Application of this method to 314 TDM samples showed that digoxin concentrations measured using UHPLC-MS/MS and EMIT were consistent (r = 0.972) for patients taking digoxin alone. However, owing to cross-reactivity, EMIT overestimated digoxin concentrations in patients receiving only Qiliqiangxin capsule or deslanoside compared with UHPLC-MS/MS. CONCLUSIONS: The developed UHPLC-MS/MS method was highly sensitive and time-efficient, making it a reliable tool for digoxin TDM and identifying interference from Qiliqiangxin capsule and deslanoside.