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.

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 Set Up
Monday - Friday
Analytic Time

4 days

Soft Order Code
MiChart Code
TPMT Genotype
  • ThiopurineMethyltransf'ase,RBC
  • Thiopurine Methyltransferase (TPMT), Phenotype
  • TPMT Enzyme
  • TPMT Phenotype
  • Thiopurine Methyltransferase (TPMT), Erythrocytes, Enzyme Activity
  • TPMT
  • TPMT Activity Profile
Reference Laboratory
Mayo TPMT3
Special Testing
Specimen Requirements
Collection Instructions

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

Special Handling

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.

CPT Code
Fee Code
NY State Approved