Should we test TPMT enzyme levels before starting azathioprine?

Thiopurine methyltransferase (TPMT) is the main enzyme responsible for inactivating toxic products of azathioprine (AZA) metabolism. Patients with homozygous deficiency of this enzyme have no enzyme activity and ideally should not be given AZA. Patients with heterozygous deficiency have 50% of enzyme activity and have been shown to respond well and tolerate half a standard dose.