Back to test list

TPMT screen

Test category:

Pharmacogenomic - Toxicity

Use of test

Purpose:

The protein thiopurine methyltransferase breaks down the thiopurine-based immunosuppressive drugs. Variants in the protein's gene, TPMT, can reduce the activity of the protein, resulting in toxic levels of the drug and bone marrow suppression. The drugs involved are azathioprine and 6-mercaptopurine (6-MP). This test identifies almost all patients at risk of neutropenia from these medications.

Other side effects such as GIT upset (with or without pancreatitis) are not identified by this test.

Utility:

The laboratory report will define the gene variants present in a patient, and identify persons at increased risk of profound neutropenia.

Ethical considerations:

This test has a role in identifying patients who may benefit from avoiding a specific drug or dose, but it provides little useful information for people (including relatives) who are not taking the medications of concern. Testing of relatives is generally not recommended.

Methodology:

Analysis of the TPMT gene.

Requesting the test

Ordering:

This test can be requested by any medical practitioner.

Sample required:

4 mL blood in EDTA. Specimens may be collected by the requesting practitioner or at any Sonic Healthcare collection centre (see link below).

To help ensure the quality of the test, a genetic test should be done with a dedicated sample whenever possible i.e. a sample collected specifically for that test rather than a sample that is used for multiple tests.

We recommend that the patient or another adult check the labelling of request forms and sample tubes. 

>Link to Sonic Healthcare collection centres

Special instructions:

Please list current and proposed medications on the request form. Please highlight any medications that have caused problems in the past.

Turnaround time:

5 business days.

Price:

This test has a Medicare rebate which, subject to the requirements of the Medicare descriptor being met, may cover all or part of the cost.

Rebate:

The Medicare details, including descriptor and schedule fee, are listed under MBS item 73327.