Thiopurine Methyltransferase Enzyme (TPMT) Activity

thiopurine methyltransferase, thiomethytransferase, 6 mercaptopurine, Thiopurine Methyltransferase, TPMT Enzyme, Azathioprine Toxicity, Mercaptopurine Toxicity, TPMT Deficiency

Test Codes

EPIC: LAB1232000, Beaker: TMPT ENZYME, Quest: 18831

Department

Send Outs

Instructions

Blood is collected prior to azathioprine (Imuran) or 6-mercaptopurine (6-MP, Purinethol) therapy for screening purposes. The draw time is at physician's discretion.

Specimen Collection Criteria

Collect (preferred specimen): Two Lavender-top EDTA tubes.

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: Two 4.0 mL whole blood, refrigerated (2-8°C or 36-46°F). (Minimum: 2.0 mL x 2). Must be shipped within 24 hours.

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). Must be shipped within 24 hours.

Storage

Specimen Stability for Testing:

Room Temperature (20-26°C or 68-78.8°F): 48 hours
Refrigerated (2-8°C or 36-46°F): 9 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.

Laboratory

Sent to Quest Diagnostics, Wood Dale, IL.

Performed

Sunday – Thursday.
Results available in 5-7 days.

Reference Range

>12    Normal.
4-12    Heterozygote or low metabolizer.
    <4    Homozygote Deficient Range.

Test Methodology

Chromatography/Mass Spectrometry

Clinical Utility

TPMT Activity – Help prevent hepatotoxicity from thiopurine therapy. This test helps identify individuals at increased risk of hepatotoxicity from thiopurine dose escalation. Patients with a TMPT activity of 4-12 nmol 6-MMP/hr/mL RBC (heterozygote/low metabolizer) are at increased risk and may require a lower dose of thiopurine drug.

CPT Codes

84433

Contacts

Last Updated

11/19/2025