Optimized, end-to-end thiopurine testing

For children who require thiopurine medications (such as azathioprine and 6-mercaptopurine) to treat inflammatory bowel disease (IBD), understanding how thiopurines are metabolized is vital to choosing the most effective treatment. Because some pediatric patients display disparities in thiopurine metabolization, laboratory testing is important to prevent thiopurine toxicity.

Mayo Clinic Laboratories’ full-spectrum thiopurine testing uses emerging technologies that provide accurate results to guide treatment decisions.

Enzyme testing

Pre-therapy testing can identify patients at risk for bone marrow suppression or organ toxicity from thiopurines due to low thiopurine methyltransferase (TPMT) activity and a diminished ability to rid the body of the medication’s active form. Enzyme activity testing can detect both deficient and hyperactive TPMT activity and identify individuals at risk for therapeutic resistance.

Our TPMT enzyme activity evaluation uses three enzyme substrates in separate reactions to determine TPMT’s ability to metabolize thiopurines. This approach improves the assay’s specificity and reduces potential inconclusive results encountered when using only one substrate.

A likelihood of having carrier status or deficient or hyperactive TPMT is determined through a bioinformatics approach that integrates results from the three enzyme reactions using Collaborative Laboratory Integrated Reports (CLIR). This Mayo Clinic-developed, multivariate, pattern recognition software combines covariate adjusted results of different tests into a set of precision reference ranges that physicians can use to better distinguish between false positives and true positive results.

TPMT3 | Thiopurine Methyltransferase Activity Profile, Erythrocytes

  • Measures enzyme activity in patient's blood to determine thiopurine metabolization.
  • Measures TPMT activity using three enzyme substrates in separate reactions, which do not interfere with each other and compete for TPMT.
  • Reduces inconclusive results obtained when using only one substrate to measure TPMT activity.
  • Integrates results from the three enzyme reactions into a likelihood score of having reduced, deficient, or hyperactive TPMT activity determined through bioinformatic analysis using Collaborative Laboratory Integrated Reports (CLIR).

Genotype testing for thiopurine metabolization

Enzyme and genotype testing can be used together to assess patient risk prior to therapy initiation. TPMT3 enzyme testing can detect individuals with increased metabolism and rare variants not included in the genotyping test. However, genotyping is required to test for genetic variations in NUDT15, an enzyme that helps break down thiopurines in the body and has been shown to affect thiopurine toxicity.1,2

TPNUQ | Thiopurine Methyltransferase (TPMT) and Nudix Hydrolase (NUDT15) Genotyping, Varies

  • Uses patients’ genetic information to determine thiopurine metabolism.
  • Evaluates for nuances in TPMT and NUDT15, which can have genetic variants strongly associated with thiopurine-related toxicity.
  • Key in detecting predictive decreased or no activity alleles know to be more prevalent in Asian, Hispanic, and some Native American populations.3
  • Noninvasive, child-friendly specimen collection option – saliva sample.

TPMT3 and TPNUQ are complementary tests that:

  • Provide lifelong information on TPMT activity and TPMT and NUDT15 genetic variations.
  • Can be used in current and future thiopurine treatment.
  • When used with THIO, significantly improves likelihood of medication efficacy, decreasing the risk of severe or adverse reactions.
  • Offers results that could be important for family members, including parents and siblings who might have one or both of the same TPMT allele.

Test in Focus

Ann Moyer, M.D., Ph.D., discusses TPNUQ, Mayo Clinic Laboratories' genotyping test for identifying patients at risk for thiopurine toxicity. Used prior to therapy initiation, our assay evaluates for nuances in both TPMT and NUDT15, which have associations to thiopurine metabolization.

Testing after therapy initiation

Metabolite monitoring after initiation of therapy enables clinicians to optimize therapy and identify elevated metabolite concentrations that may result in toxicity. Additionally, clinicians should order testing as needed for dose changes, flare-ups, signs of toxicity, and suspicion of noncompliance, as well as in patients who do not respond to therapy as expected.

THIO | Thiopurine Metabolites, Whole Blood

Recommended time points for testing:

  • Four weeks after starting therapy, to ensure patient compliance and evaluate for early risk of toxicity.
  • 12-16 weeks (after TGN metabolites have reached steady-state).
  • Annually.

Learn more about how to order these tests at your institution.


  1. Moriyama T, et al. NUDT15 Polymorphisms Alter Thiopurine Metabolism and Hematopoietic Toxicity. Nat Genet. 2016;48(4):367-73.
  2. Kakuta Y, et al. Pharmacogenetics of Thiopurines for Inflammatory Bowel Disease in East Asia: Prospects for Clinical Application of NUDT15 Genotyping. J Gastroenterol. 2018;53(2):172-80.
  3. Tanaka Y, Saito Y. Importance of NUDT15 Polymorphisms in Thiopurine Treatments. J Pers Med. 2021;11(8):778. Published 2021 Aug 10. Doi:10.3390/jpm11080778.