A positive drug test in the pain management patient: deception or herbal cross-reactivity?

sible with LC-MS/MS for combined immunosuppressive therapies where a calcineurin inhibitor is used together with a mTOR inhibitor because both drugs can be quantified with a single assay. Because of its ease and rapidity, this method is ideally suited for the routine monitoring of these four major immunosuppressive drugs in clinical practice. Laboratories using such tests must, however, have sufficient expertise with operating LC-MS/MS systems. Furthermore, it should be considered that when switching from the commonly used immunoassays for CsA and tacrolimus to this LC-MS/MS method, the therapeutic ranges will need to be revised. As noted, a major disadvantage of the immunoassays is their cross-reactivity with variable concentrations of immunosuppressive drug metabolites present in the blood samples from transplant recipients. Because it has been shown that inactive metabolites make a major contribution to this bias (16–18), the clinical application of the LC-MS/MS method could, without appropriate adjustment of the therapeutic range, lead to a potential overdose. On the other hand, the LC-MS/MS method will provide the possibility for more accurate individualized patient dosing based on the parent drug. Rigorous reevaluation of the current therapeutic ranges for tacrolimus and CsA in cooperation with local transplant centers will therefore be necessary if a change to this highly specific method is planned.