Fast and reliable quantification of busulfan in blood plasma using two-channel liquid chromatography tandem mass spectrometry: Validation of assay performance in the presence of drug formulation excipients

•A fast LC–MS/MS method for the quantification of busulfan in plasma was developed.•Busulfan was quantified across a wide concentration range using only 20 μL of plasma.•The drug vehicle polyethylene glycol 400 may be quantified simultaneously.•Ion suppression from drug vehicles was addressed to maximize analytical performance.•The assay performs well in routine practice with ISO 15189 accreditation.