PGx azathioprine

The result is reported as the TPMT and NUDT15 diplotype with the metaboliser phenotype derived for each gene, from normal through intermediate to poor. For these genes the phenotype is assigned directly from the function classes of the two alleles, not from an activity score. Intermediate status in both genes forms a distinct compound intermediate metaboliser phenotype, new in the 2025 guideline update, carrying greater risk than either alone.

A negative result reflects only the alleles tested and does not exclude reduced function from rare or uncharacterised variants; erythrocyte TPMT enzyme-activity (phenotype) testing can identify deficiency caused by variants outside the targeted genotype, but no equivalent routine enzyme assay is established for NUDT15. The result reports constitutional germline status from blood and is not a somatic test of tumour tissue.

The product information for mercaptopurine (Xaluprine, the nearest centrally authorised thiopurine, as azathioprine holds no central EU authorisation) states that patients with little or no inherited TPMT activity and those with an inherited NUDT15 variant are at increased risk of severe toxicity. The considers TPMT and NUDT15 genotyping essential for drug safety and requires it before azathioprine therapy begins. finds substantial evidence linking these genotypes to thiopurine response and reports that pre-emptive, genotype-guided dose adjustment reduces thiopurine-induced adverse effects without compromising efficacy.

For current, actionable guidance, consult the maintained resources: the prescribing information for azathioprine; and the and thiopurine guidelines, held with the other pharmacogenomic annotations at .

Results are interpreted in the individual clinical context, and the treatment decision rests with the treating physician.

Content reviewed against the current guidelines on 28 July 2026. Pharmacogenomic classifications are periodically revised; the linked databases above hold the day-current version.

• Maillard M, Schwab M, Whirl-Carrillo M, Moyer AM, Suarez-Kurtz G, Pui CH, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for Thiopurine Dosing Based on TPMT and NUDT15 Genotypes: 2025 Update. Clinical pharmacology and therapeutics. 2026;119(4):916-927. doi: 10.1002/cpt.70209. PMID: 41618934. PMCID: PMC12997511. Correction in: Clinical pharmacology and therapeutics. 2026;119(6):1671-1672. doi: 10.1002/cpt.70298. PMID: .
• European Medicines Agency. Xaluprine (mercaptopurine) EPAR product information, last updated 25 September 2025 (revision 19).
• Coenen MJH, Nijenhuis M, Soree B, de Boer-Veger NJ, Buunk AM, Guchelaar HJ, et al. Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between TPMT/NUDT15 and thiopurines. European journal of human genetics : EJHG. 2026;34(1):152-160. doi: 10.1038/s41431-025-01986-8. PMID: . PMCID: PMC12816135.