Lab Test

TPMT Enzyme

thiopurine methyltransferase, thiomethytransferase, 6 mercaptopurine

Test Codes

EPIC: LAB6343, Beaker: XTMPT, ARUP #92066


Send Outs


This test is used to detect those individuals with low TPMT (approximately 10% of the population) who will have excessive and potentially lethal myelosuppression when given standard doses of azathioprine or 6-MP.

Specimen Collection Criteria

Collect (preferred specimen): One Dark Green-top Lithium or Sodium Heparin tube.
Also acceptable: One Lavender-top EDTA tube.

Physician Office/Draw Specimen Preparation

Do not centrifuge. Refrigerate (2-8°C or 36-46°F) specimens in original collection tube immediately after collection.

Preparation for Courier Transport

Transport: 4.0 mL whole blood, refrigerated (2-8°C or 36-46°F). (Minimum: 3.0 mL)

Rejection Criteria

  • Frozen specimen.
  • Hemolyzed specimen.
  • Gel separator tubes
  • Room temp specimen.
  • Specimens not collected and processed as indicated.

In-Lab Processing

Do not centrifuge. Refrigerate (2-8°C or 36-46°F) specimens in original collection tubes immediately after collection.

Transport: 4.0 mL whole blood, refrigerated (2-8°C or 36-46°F). (Minimum: 3.0 mL)


Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): 3 hours
Refrigerated (2-8°C or 36-46°F): 6 days
Frozen (-20°C/-4°F or below): Unacceptable

Specimen Storage in Department Prior to Disposal:

Specimen retention time is determined by the policy of the reference laboratory. Contact the Send Outs Laboratory with any questions.


Sent to ARUP Laboratories, Salt Lake City, UT.


Sunday – Saturday.
Results available in 3-5 days.

Reference Range

By report.

Test Methodology

Enzymatic/Quantitative Liquid Chromatography – Tandem Mass Spectrometry (LC-MS/MS).


By report.

Clinical Utility

Azathioprine (Imuran) and 6-Mercaptopurine (6-MP, Perinethol) are thiopurine drugs that are used to treat neoplasms and a variety of rheumatologic, dermatologic, or neurologic diseases in which the immune system is thought to play a role. Although thiopurines are very useful drugs, they are also potentially toxic. The metabolic conversion of azathioprine and 6-MP to purine nucleotides and their subsequent incorporation into DNA is important in its therapeutic efficacy but also plays a role in its toxicity. The enzyme thiopurine methyltransferase (TPMT) inactivates these thiopurines by thiol methylation. A balance is established between competing metabolic pathways such that sufficient drug is converted to the active antimetabolite for therapeutic efficacy but not so much as to lead to myelotoxicity (bone marrow suppression).

CPT Codes



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