Background: The association between thiopurine methyltransferase (TPMT) deficiency and myelosuppression with azathioprine is well recognized. Two cases are presented that illustrate the very different outcomes that may occur with azathioprine in patients with TPMT deficiency, which affects 0.3-0.6% of caucasians. CASE 1: The first patient's TPMT deficiency was identified following hospitalization for pancytopenia attributed to azathioprine. CASE 2: The second patient was identified as deficient early in treatment and myelosuppression was avoided by treating with a greatly decreased dose (25 mg per week).
Conclusions: Testing for TPMT ideally should be performed in every patient commencing a thiopurine drug.