Analytical and clinical validation of an LC-MS/MS method to measure thiopurine S-methyltransferase activity by quantifying d3-6-MMP

With the inclusion of isotope labelled substrate, interfering non-enzymatic methylation no longer results in potential false assignment of abnormal patients. Furthermore, the method can be applied to patients who have already been prescribed thiopurine drugs. This new LC-MS/MS is therefore a favourable clinical routine application to test TPMT activity, as it shows excellent performance in identifying patients with TPMT deficiency.