Interference by venlafaxine ingestion in the detection of tramadol by liquid chromatography linked to tandem mass spectrometry for the screening of illicit drugs in human urine

Confirmation of the presence of illicit drugs in human fluids by liquid chromatography linked to atmospheric pressure ionisation tandem mass spectrometry (LC/MS/MS) is becoming increasingly popular. A rapid LC/MS/MS method is described using reversed phase gradient elution chromatography on a 50 x 3 mm base deactivated C8 column with identification of drugs based on their Multiple Reaction monitoring (MRM) transitions, retention time, and co-elution of stable isotopic analogues where available. The method is used for drug confirmation following initial screening by immunoassay, and identification of drugs such as tramadol, for which no immunoassay is available. False positive results for tramadol were observed in cases where subjects were being treated with the commonly prescribed antidepressant drug venlafaxine. This was a result of the presence of the metabolite O-desmethylvenlafaxine, which has a similar transition to tramadol. This study highlights the need to consider drug metabolites, as well as parent drugs in interferences in LC/MS/MS methods.