Thiopurine Methyltransferase Testing - TPMT

Thiopurine drugs such as azathioprine (AZA), 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG) are widely used in the treatment of acute lymphoblastic leukemia (ALL), autoimmune diseasesinflammatory bowel disease, and posttransplant organ rejection. Patients with abnormal thiopurine methyltransferase (TPMT) enzyme activity due to genetics and/or drug-drug interactions have an increased risk of toxicity when given thiopurines.

  • Diagnosis
  • Monitoring
  • Background
  • Lab Tests
  • References
  • Related Topics
  • Videos

Indications for Testing

  • Patients being considered for thiopurine therapy in order to
    • Detect risk for severe myelosuppression with standard full dosing of thiopurine drugs
    • Individualize dosing of thiopurine drugs
  • Patients who have experienced an adverse reaction to thiopurine therapy

Laboratory Testing

  • TPMT genotyping
    • Detects *2, *3A, *3B, and *3C alleles
    • Variant alleles are correlated with risk of severe myelosuppression with standard dosing of thiopurine drugs
    • Does not replace need for clinical monitoring
  • Pretherapeutic or posttherapeutic use
    • Detect common genotypes
    • Detect enzyme phenotype
  • Should not be used in patients with history of allogeneic bone marrow transplant
  • Therapeutic drug monitoring (metabolic phenotype)
    • Thiopurine drug metabolites
      • Thiopurine metabolite concentrations may be appropriate for individuals with deficient or high TPMT activity to monitor and optimize dose


  • Prevalence of phenotype
    • Low thiopurine methyltransferase (TPMT) activity – ~0.3%
    • Intermediate TPMT activity – ~10%
    • Normal TPMT activity – ~90%
    • High TPMT activity – unknown


  • TPMT gene – autosomal co-dominant inheritance
  • >20 TPMT deficiency alleles identified to date
    • TPMT deficiency alleles account for 95% of low-to-intermediate activity states
      • *2 (c.238G>C; p.Ala80Pro)
      • *3A (c.[460G>A;719A>G]; p.[Ala154Thr;Tyr240Cys])
      • *3B (c.460G>A; p.Ala154Thr)
      • *3C (c.719A>G; p.Tyr240Cys)
    • Homozygous or compound heterozygous
      • Associated with very low/no TPMT enzyme activity and high risk for drug-related toxicity with conventional thiopurine doses
    • Heterozygous
      • Associated with intermediate TPMT enzyme activity and increased risk for drug-related toxicity with conventional thiopurine doses
    • No variants detected
      • Predictive of *1 functional alleles
      • Predicts normal TPMT enzyme activity and normal risk for thiopurine drug-related toxicity


  • AZA, 6-MP, and 6-TG are inactive prodrugs used to treat a variety of different disease states and are metabolized by three different enzymes into three different 6-thioguanine nucleotides for activity
    • AZA is metabolized to 6-MP (one of most commonly prescribed)
      • 6-MP is converted into two pharmacologically inactive metabolites
        • 6-thiouric acid by the enzyme xanthine oxidase (XO)
        • 6-methylmercaptopurine (6-MMP) by thiopurine S-methyltransferase (TPMT)
          • Primary metabolic route for inactivation of nucleotides is catalyzed by TPMT
      • 6-MP is also converted into active thioguanine nucleotides by the enzyme hypoxanthine guanine phosphoribosyltransferase (HGPRT) to exert therapeutic cytotoxic effects
      • When 6-MP is converted to inactive metabolites, the amount of 6-TG nucleotides reduces, which balances the amount of 6-TG nucleotides needed to achieve required cytotoxicity for therapeutic treatment
      • When TPMT enzyme activity is low, proportionately more 6-MP is converted into cytotoxic 6-TG nucleotides, which increases risk for toxicity
    • Accumulation of excessive nucleotides inhibit purine synthesis, most dramatically noted in the bone marrow, inhibiting cell proliferation and contributing to myelosuppression
    • Reduced drug dosing may prevent myelosuppression in patients with intermediate and low TPMT activity
      • Individuals with very low/no TPMT enzyme activity typically experience severe myelosuppression when receiving conventional thiopurine doses
      • An estimated 30-60% of individuals with intermediate TPMT activity who receive conventional thiopurine doses experience moderate to severe myelosuppression
    • Thiopurine dosing guidelines (Clinical Pharmacogenetics Implementation Consortium [CPIC])
    • TPMT can be inhibited by common drugs
      • NSAIDs
        • Ibuprofen
        • Ketoprofen
        • Naproxen
        • Mefenamic acid
      • Diuretics
        • Furosemide
        • Thiazides
      • Ulcerative colitis drugs
        • Sulfasalazine
        • Mesalamine
        • Olsalazine
  • Factors to consider if deciding when TPMT should be ordered
    • Disease state being tested
    • Starting dose
    • Need for immediate full dose
    • Previous documented tolerance of thiopurine medication at steady state doses
Tests generally appear in the order most useful for common clinical situations. Click on number for test-specific information in the ARUP Laboratory Test Directory.

Thiopurine Methyltransferase, RBC 0092066
Method: Enzymatic/Quantitative Liquid Chromatography-Tandem Mass Spectrometry


Does not replace clinical monitoring

TPMT inhibitors may contribute to falsely low test results

TPMT activity should be assessed prior to treatment with thiopurine drugs

Thiopurine Methyltransferase (TPMT) Genotyping, 4 Variants 2012233
Method: Polymerase Chain Reaction/Fluorescence Monitoring


Only targeted TPMT allele variants  will be detected by this panel

Diagnostic errors can occur due to rare sequence variations

Genotyping cannot distinguish between the *1/*3A and *3B/*3C genotypes

Genotyping does not replace the need for therapeutic drug monitoring or clinical observation

Genotyping in patients who have received allogenic stem cell/bone marrow transplant  will reflect donor status

Thiopurine drug metabolism and risk for toxicity may be affected by genetic and nongenetic factors that are not evaluated by this test

Test does not assess for TPMT allele variants associated with ultra-high enzyme activity

Thiopurine Metabolites by LC-MS/MS 2014484
Method: Quantitative Liquid Chromatography/Tandem Mass Spectrometry

General References

Booth RA, Ansari MT, Loit E, Tricco AC, Weeks L, Doucette S, Skidmore B, Sears M, Sy R, Karsh J. Assessment of thiopurine S-methyltransferase activity in patients prescribed thiopurines: a systematic review. Ann Intern Med. 2011; 154(12): 814-23, W-295-8. PubMed

Chouchana L, Narjoz C, Beaune P, Loriot M, Roblin X. Review article: the benefits of pharmacogenetics for improving thiopurine therapy in inflammatory bowel disease. Aliment Pharmacol Ther. 2012; 35(1): 15-36. PubMed

DiPiero J, Teng K, Hicks K. Should thiopurine methyltransferase (TPMT) activity be determined before prescribing azathioprine, mercaptopurine, or thioguanine? Cleve Clin J Med. 2015; 82(7): 409-13. PubMed

Relling MV, Gardner EE, Sandborn WJ, Schmiegelow K, Pui C, Yee SW, Stein CM, Carrillo M, Evans WE, Hicks JK, Schwab M, Klein TE, Clinical Pharmacogenetics Implementation Consortium. Clinical pharmacogenetics implementation consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing: 2013 update. Clin Pharmacol Ther. 2013; 93(4): 324-5. PubMed

Relling MV, Gardner EE, Sandborn WJ, Schmiegelow K, Pui C, Yee SW, Stein CM, Carrillo M, Evans WE, Klein TE, Clinical Pharmacogenetics Implementation Consortium. Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing. Clin Pharmacol Ther. 2011; 89(3): 387-91. PubMed

Smith MA, Marinaki AM, Sanderson JD. Pharmacogenomics in the treatment of inflammatory bowel disease. Pharmacogenomics. 2010; 11(3): 421-37. PubMed

Medical Reviewers

Last Update: December 2017