Thiopurine S-methyltransferase (TPMT) is an enzyme involved in the metabolism of drugs called thiopurines, such as the immunosuppressant azathioprine, used to treat inflammatory bowel disease (IBD). In the general population, there is wide variation in TPMT enzyme activity that is largely genetically determined by the TPMT gene. While normal levels of TPMT enzyme activity are found in 89% of people, 11% have intermediate activity and approximately 0.3% have little or no activity. People who lack TPMT enzyme activity generally cannot tolerate treatment with thiopurines and often develop severe bone marrow toxicity that can be life threatening. TPMT genotyping determines the most common TPMT gene alleles that result in intermediate or deficient levels of the TPMT enzyme. TPMT phenotyping determines the level of TPMT enzyme activity found in red blood cells. TPMT genotyping and phenotyping have the potential to be used as a screening test to identify IBD patients who are at an increased risk of bone marrow complications so that treatment decisions can be made before they are prescribed drugs that contain thiopurine.