What is TPMT testing, and why is it clinically relevant?

• A. TPMT testing identifies predisposition to diabetes.
• B. TPMT testing measures telomere length.
• C. TPMT activity/genotype guides dosing of thiopurines to avoid myelosuppression.
• D. TPMT testing predicts response to beta-blockers.

Answer: (C)

TPMT testing looks at how well the enzyme thiopurine S-methyltransferase works, which is key for metabolizing thiopurine drugs such as azathioprine and 6-mercaptopurine. People vary in TPMT activity due to genetic differences, with some having normal activity, some intermediate, and some very low activity. If someone with low TPMT activity receives standard doses, they can accumulate toxic metabolites and develop dangerous bone marrow suppression (myelosuppression). By identifying a patient’s TPMT activity or genotype, clinicians can tailor the thiopurine dose: normal activity usually allows standard dosing, intermediate activity requires a reduced dose, and very low activity may prompt avoiding thiopurines or using substantial monitoring. Testing can be done by measuring enzyme activity in blood (phenotype) or by detecting TPMT gene variants (genotype), with common variants like TPMT3A and TPMT3C linked to reduced activity. This practice is clinically important because it helps prevent severe hematologic toxicity while enabling effective use of thiopurines in conditions such as inflammatory bowel disease and certain cancers.