TPMT (Thiopurine methyl transferase)
Chemical Pathology
Background Information
- Thiopurine drugs are widely used in the treatment of inflammatory and autoimmune diseases, leukaemia and prevention of rejection post organ transplant.
- Thiopurines are catabolised to inactive metabolites by thiopurine methyl transferase (TPMT), which in effect reduces concentrations of the active metabolite, 6-thioguanine nucleotides (6TGN).
- TPMT activity exhibits genetic variation and measurement of TPMT activity should be performed prior to starting thiopurine drugs.
- In addition to TPMT activity detemination, genetic analysis is also carried out.
- Patients with undetectable activity are generally not treated with thiopurine drugs due to the increased risk of severe side effects including myelosuppression.
- Recent blood transfusions can affect results and may mask a deficient TPMT.
- Patients already receiving thiopurine drugs can show enzyme induction leading to increased TPMT activity.
Patient Preparation
- Patient should not have had recent blood transfusions (both phenotype and genotype results affected).
- TPMT activity should be determined BEFORE starting on thiopurine drugs.
Sample Requirements
For adults, 4 ml of blood taken into an EDTA tube.
Storage/Transport
Send at ambient temperature to the laboratory. If unavoidable samples can be refrigerated overnight.
Required Information
Relevant clinical details including reason for the request and whether any treatment started. Please state if any blood transfusion given in the last 4 months.
Turnaround Times
Samples are referred to an external laboratory for analysis with results expected back within 10 working days.
Reference Ranges and Result Interpretation
Ranges and interpretation reported as provided by referral laboratory.
Further Information
To learn more about TPMT visit Lab Tests Online
Page last updated: 18/09/2025 | Page last reviewed: 18/09/2025