• When do you need to measure thiopurine metabolites?
Measuring the nucleotides 6-TGN and 6-MMP is only useful in patients who fail thiopurine treatment and who are unable to enter and maintain a steroid-free remission despite an adequate dose and duration of immunomodulator therapy. Conversely, if your patient is well and tolerating thiopurine therapy, then metabolite measurements do not provide any additional useful clinical information.
• What do the results mean and how do we use them?
Measuring thiopurine metabolites effectively clarifies the reasons patients are not responding to immunomodulator therapy, and it identifies 4 groups of treatment failures (Table 9-2). The first group are patients with negligible or undetectable levels of both metabolites. These patients are likely to be noncompliant and can be questioned and managed accordingly. Confirming compliance before switching from thiopurine therapy is a justifiable reason for ordering metabolite testing. The second group are patients with low but detectable levels of both 6-TGN and 6-MMP. These patients are under-dosed and should tolerate dose escalation. The third group are patients with low 6-TGN levels and high 6-MMP levels. These patients have an unfavorable metabolite profile and are thiopurine “resistant.” Further dose escalation is unlikely to be successful. The last group are patients with therapeutic, or high, levels of both 6-TGN and 6-MMP, and ongoing active disease. This group is truly thiopurine “refractory” and will require a different class of therapeutic agent.4
• How often do you need to measure thiopurine metabolites?
The first set of metabolite measurements, which establishes the patient’s genetically determined thiopurine metabolite profile, is the most important, and these can be performed after approximately 4 weeks of therapy. The confirmation of therapeutic metabolite levels by any subsequent dose adjustments should ideally be determined by a further set of metabolite levels 2 to 4 weeks later. Subsequent metabolite levels are thereafter only necessary if disease remission cannot be established or maintained at any time.
http://www.healio.com/gastroenterology/curbside-consultation/%7B67faf0e0-974c-4f86-a430-d735ef89a8d7%7D/how-do-you-monitor-patie