What is the Thiopurine Methyltransferase (TPMT) Test?

The Thiopurine Methyltransferase (TPMT) test measures the activity of the TPMT enzyme, which plays a critical role in the metabolism of thiopurine medications. These drugs are commonly used to treat autoimmune conditions, inflammatory bowel disease, and some blood disorders.

People with low or absent TPMT activity cannot safely break down thiopurine drugs, leading to accumulation and potential toxicity. Testing enzyme activity before treatment allows therapy to be tailored safely.

The test is performed on a blood sample and is considered essential before initiating thiopurine-based therapy.

Why do I need a TPMT test?

Thiopurine medications can suppress bone marrow if not metabolised correctly. TPMT testing helps identify individuals at increased risk of severe side effects before treatment begins.

You may need this test if you:

• Are due to start azathioprine, mercaptopurine, or thioguanine
• Have inflammatory bowel disease (Crohn’s or ulcerative colitis)
• Have an autoimmune condition requiring immunosuppression
• Are being assessed for long-term thiopurine therapy
• Have a family history of drug sensitivity
• Need safer dose planning before treatment

Testing significantly reduces the risk of serious blood-related complications.

Benefits of the test

1. Improves medication safety
   Identifies patients at risk of thiopurine toxicity.
2. Prevents severe bone marrow suppression
   Helps avoid dangerous reductions in blood cell counts.
3. Guides dose selection
   Supports personalised dosing or alternative therapy decisions.
4. Essential before starting treatment
   Recommended prior to thiopurine initiation.
5. One-time baseline assessment
   Enzyme activity is genetically determined and stable.
6. Reduces adverse drug reactions
   Helps prevent avoidable side effects.
7. Supports long-term treatment planning
   Enables safer immunosuppressive therapy.
8. Simple blood-based test
   Requires only a single blood sample.

Step-by-step: How we offer testing

Clinic-based testing (UK-wide)

• Choose your nearest UK clinic
• Attend your appointment
• A clinician draws a blood sample
• Sample is sent to the laboratory
• Results are issued once analysis is complete

Home or hotel visit (+£60, UK-wide)

• Book a mobile phlebotomist
• Blood sample taken at your home or hotel
• No need to attend a clinic
• Sample securely transported to the laboratory

Your results explained

Low or absent TPMT activity

• Very low enzyme function
• UK standard: Markedly reduced activity
• High risk of severe toxicity if standard doses are used

Intermediate TPMT activity

• Reduced but present enzyme function
• UK standard: Heterozygous activity range
• Increased toxicity risk; dose reduction often required

Normal TPMT activity

• Normal enzyme function
• UK standard: Normal activity range
• Standard thiopurine dosing usually tolerated