Thiopurine Methyl Transferase
The TPMT test measures enzyme activity that affects how thiopurine drugs are metabolised. It helps reduce the risk of serious side effects before starting treatment.
Turnaround time
5 days
Biomarkers count
1
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Details about Thiopurine Methyl Transferase
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
- Improves medication safety
Identifies patients at risk of thiopurine toxicity. - Prevents severe bone marrow suppression
Helps avoid dangerous reductions in blood cell counts. - Guides dose selection
Supports personalised dosing or alternative therapy decisions. - Essential before starting treatment
Recommended prior to thiopurine initiation. - One-time baseline assessment
Enzyme activity is genetically determined and stable. - Reduces adverse drug reactions
Helps prevent avoidable side effects. - Supports long-term treatment planning
Enables safer immunosuppressive therapy. - 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
How It Works
Simple steps to get your results
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Choose your test online and book in seconds. Select your preferred clinic location or home visit option.
Visit or Test at Home
Attend one of our UK or London clinics, arrange a home nurse visit, or use a finger-prick kit where available.
Get Your Results
Your sample is analysed by accredited UK laboratories, with secure results delivered directly to you.
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WhatsApp UsHow to Prepare for Your Test
Follow these guidelines for accurate results
Sample Timing
Where possible, attend your blood test between 7am and 12pm. Please wait until any short-term illness or infection has fully resolved before testing. Avoid intense exercise for 24–48 hours beforehand, as this can affect certain markers.
Fasting & Hydration
Fasting is not always required, but some tests may recommend it. If fasting is advised, avoid food for 8–12 hours before your appointment and drink water only. Stay well hydrated, as this helps with sample collection and accuracy.
Medications & Supplements
Continue prescribed medications unless advised otherwise by your clinician. Avoid vitamin, mineral, or biotin supplements for at least 24–48 hours before your test, as these can interfere with results. If you are unsure about any medication or supplement, please let us know before your appointment.
Hormones & Menstrual Cycle
For hormone-related tests, timing within your menstrual cycle may be important. If relevant, follow any specific guidance provided on your test page.
If you have any questions or special circumstances, our team is happy to advise before your appointment.