Thiopurine drugs are used as both immunosuppressive and chemotherapeutic agents. Inherited genetic variants that result in reduced activity of the enzyme thiopurine S-methyltransferase (TPMT) are associated with cytoxicity and particularly with bone marrow failure. Multiple variants have been associated with decreased TPMT activity but 4 specific variant alleles (TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C), account for the majority (>95%) of low and intermediate activity cases. Patients with one variant allele have intermediate activity whereas those with two variants have low or no activity.

TPMT genetic testing can predict thiopurine drug toxicity in a number of conditions, including renal transplantation, rheumatoid disease, inflammatory bowel disease and lymphoblastic leukaemia. Genetic variants have also been implicated in ototoxicity associated with cisplatin treatment in children.

An MBS rebate is available for this test.