TPMT Genotyping

Thiopurine methyltransferase (TPMT) is a phase two drug-metabolizing enzyme expressed in the kidney, red blood cells, liver and other tissues. TPMT plays a role in the elimination of thiopurine drugs, including mercaptopurine, azathioprine and thioguanine.

Impaired TPMT function can cause life-threatening myelosuppression, a condition in which bone marrow activity is decreased, and other toxicities in patients given normal doses of thiopurines. Impaired function has also been associated with cisplatin-induced ototoxicity in children, although this is not well understood. Individuals with decreased TPMT activity may require thiopurine dosage reductions of 50 to 90 percent to tolerate therapy and reduce the risk of adverse reactions.

Indications for Testing

Patients who are initiating thiopurine medications or who are experiencing adverse effects or low efficacy may be good candidates for TPMT testing. Testing can confirm the presence of genotypes that affect the metabolism of drugs that are metabolized by this pathway. Assessment of medical necessity for testing is up to the provider’s clinical judgment.

Insurance Coverage

TPMT testing is covered by Medicare and most insurance providers. The CPT code for TPMT testing is 81401 (provided as guidance only).

Specimen Information

Click here to order testing supplies or call Client Services at 800-837–8362.

  • Buccal Swabs: 4 sterile buccal swabs
  • Blood: 5-10cc whole blood lavender-top EDTA or yellow-top ACD-A tubes

Turnaround Time: 7 business days, faster turnaround available for clinical trials