Test Overview

Test Methodology

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Test Usage

Detection of individuals with low thiopurine methyltransferase activity who are at risk for excessive myelosuppression or severe hematopoietic toxicity when taking AZA and for detection of individuals with hyperactive thiopurine methyltransferase activity who have therapeutic resistance to thiopurine drugs and may develop hepatotoxicity if treated with these drugs.

Reference Range*

6-Methylmercaptopurine: 3.00-6.66 nmol/mL/hr; 6-Methylmercaptopurine riboside: 5.04-9.57 nmol/mL/hr; 6-Methylthioguanine riboside: 2.70-5.84 nmol/mL/hr.

* Reference ranges may change over time. Please refer to the original patient report when evaluating results.

Test Limitations

TPMT activity is measured in RBCs. If a patient has had a recent blood transfusion, his true enzyme activity may not be accurately reflected. TPMT enzyme activity can be inhibited by several drugs such as: naproxen (Aleve), ibuprofen (Advil, Motrin), ketoprofen (Orudis), furosemide (Lasix), sulfasalazine (Azulfidine), mesalamine (Asacol), olsalazine (Dipentum), mefenamic acid (Ponstel), trimethoprim (Proloprim), methotrexate, thiazide diuretics, and benzoic acid inhibitors. TPMT inhibitors may contribute to falsely low results: patients should abstain from these drugs for at least 48 hours prior to TPMT testing. Patients with acute lymphoblastic leukemia (ALL) may have lower TPMT activities before treatment and higher activities following treatment.

Test Details

Days Test Performed

Monday - Friday

Analytic Time

4 days

Soft Order Code


MiChart Code

TPMT Genotype



Thiopurine Methyltransferase (TPMT), Phenotype
TPMT Enzyme
TPMT Phenotype
Thiopurine Methyltransferase (TPMT), Erythrocytes, Enzyme Activity
TPMT Activity Profile

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Laboratory Reference

Mayo TPMT3

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Specimen Requirements

Offsite Collection Instructions

Collect specimen in a lavender top tube (preferred); green top (sodium or lithium heparin) is acceptable. Send intact whole blood, refrigerated.

Special Handing

If TPMT Enzyme Activity is requested, the specimen will be sent to Mayo Medical Laboratories unless the Prometheus assay is specifically requested.

Normal Volume

5 mL whole blood

Minimum Volume

3 mL whole blood

Additional Information

Test sent to Mayo Medical Laboratories.

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