What is the 6-Thioguanine Nucleotides Test?

The 6-Thioguanine Nucleotides (6-TGN) test measures active metabolites produced when the body processes thiopurine medications such as azathioprine, mercaptopurine or thioguanine. These metabolites are responsible for the immunosuppressive effect of the drugs.

Unlike standard drug level testing, 6-TGN measurement reflects how effectively the medication is being converted into its active form inside red blood cells. This provides a more accurate assessment of therapeutic response and helps explain treatment failure or side effects.

The test is widely used in patients with inflammatory bowel disease, autoimmune disorders and certain haematological conditions where thiopurine therapy is prescribed.

Why do I need a test for 6-Thioguanine Nucleotides?

You may need this test if you are taking thiopurine medication and your condition is not responding as expected, or if you are experiencing side effects. Individuals metabolise thiopurines differently, meaning standard dosing does not always produce effective or safe drug levels.

Testing helps determine whether a dose is too low to be effective, within the therapeutic range, or high enough to increase toxicity risk. It is especially useful when balancing disease control against side effects such as low blood counts or liver abnormalities.

Symptoms and scenarios where testing is recommended:

• Poor response to azathioprine or mercaptopurine
• Disease flares despite treatment
• Suspected medication toxicity
• Low white blood cell counts
• Abnormal liver blood tests during treatment
• Dose adjustment or optimisation
• Long-term thiopurine monitoring
• Suspected non-adherence to medication

Benefits of the 6-Thioguanine Nucleotides Test

1. Optimises thiopurine treatment effectiveness
   Confirms whether active drug levels are sufficient for disease control.
2. Reduces risk of medication toxicity
   Identifies levels associated with bone marrow suppression or liver damage.
3. Explains treatment failure
   Helps determine whether lack of response is due to under-metabolism or resistance.
4. Supports personalised dosing
   Enables tailored dose adjustments rather than trial-and-error changes.
5. Improves long-term disease management
   Helps maintain remission in chronic inflammatory conditions.
6. Identifies poor medication adherence
   Very low levels may indicate missed or inconsistent dosing.
7. Complements TPMT and NUDT15 testing
   Functional monitoring alongside genetic risk assessment.
8. Provides actionable clinical guidance
   Results directly inform treatment decisions.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a blood sample.
3. The sample is analysed for 6-TGN levels.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your gastroenterologist, haematologist or prescribing clinician.

Your Results Explained

Low Levels

Low 6-TGN levels indicate insufficient active metabolite exposure.
This may result in poor disease control and ongoing inflammation.
It can be caused by under-dosing, rapid metabolism or non-adherence.
Typical therapeutic concern: below 235 pmol/8×10⁸ RBC.

Normal / Therapeutic Levels

Therapeutic levels indicate effective immunosuppression with balanced safety.
This range is associated with optimal treatment response in many patients.
Typical therapeutic range: 235–450 pmol/8×10⁸ RBC.

High Levels

High 6-TGN levels increase the risk of bone marrow suppression and infection.
Patients may develop low white blood cell counts or other toxic effects.
Levels above ~450 pmol/8×10⁸ RBC often prompt dose reduction or closer monitoring.