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Thiopurine S-methyltransferase (TPMT) Activity

Published: July 27, 2026Updated: August 25, 2026

TPMT activity measures the level of the enzyme thiopurine S-methyltransferase in red blood cells, which is crucial for metabolizing thiopurine drugs.

Quick summary

TPMT activity predicts how well a patient metabolizes thiopurine drugs.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
U/mL RBC
Clinical utility
predictive
Result kind
quantitative
FDA-cleared
Roche TPMT, Prometheus TPMT, Mayo Clinic TPMT
interpretation: {"text":"Patients may be at increased risk of severe side effects and may require a lower dose of thiopurine drugs.","label":"Low TPMT Activity"}; {"text":"Patients are likely to metabolize thiopurine drugs normally and may require standard or higher doses for therapeutic effect.","label":"Normal/High TPMT Activity"}

Thiopurine drugs, such as azathioprine, mercaptopurine, and thioguanine, are commonly used in the treatment of various conditions, including autoimmune disorders and certain types of cancer. TPMT is the enzyme responsible for metabolizing these drugs.

Measuring TPMT activity helps determine the appropriate dosage of thiopurine drugs. Patients with low TPMT activity may require a reduced dose to avoid severe side effects, while those with high activity may need a higher dose for therapeutic efficacy.

The reference range for TPMT activity is typically between 26–50 U/mL RBC. Values outside this range may indicate a need for dose adjustment or alternative treatment strategies.

Frequently asked questions

What is the significance of TPMT activity testing?
TPMT activity testing helps in personalizing thiopurine drug therapy, minimizing toxicity, and ensuring therapeutic efficacy.
How is TPMT activity measured?
TPMT activity is typically measured in red blood cells (RBC) using a validated laboratory assay.
  1. 1
    Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for Thiopurine Methyltransferase Genotype and Thiopurine DosingSource
  2. 2
    TPMT Testing and Thiopurine Drug MonitoringSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy