Thiopurine Metabolites in Red Blood Cells

Thiopurine drugs, including azathioprine, mercaptopurine, and thioguanine, are used to treat many conditions. Inactivation of these drugs may be affected by variants in the TPMT and/or NUDT15 genes and impaired thiopurine methyltransferase enzyme function, leading to potential cytotoxic effects at standard thiopurine doses in some patients. Measurement of thiopurine metabolites can be used to identify risk for such effects after therapy initiation and may also be used to monitor adherence to therapy or optimize dosing. Thiopurine metabolite testing should not be used as the sole basis for dose determination. Therapeutic drug management should be based on the complete clinical picture, including the clinical indication for thiopurine treatment, the degree of myelosuppression as measured by CBCs, comedications, and the results of other tests.

ARUP offers the Thiopurine Metabolites in Red Blood Cells test for use in thiopurine drug therapy. This quantitative liquid chromatography-tandem mass spectrometry assay measures 6-thioguanine nucleotides (6-TGNs) to determine whether dosing is in the optimal range and assesses the risk for leukopenia and myelotoxicity. In addition, 6-methylmercaptopurine nucleotides (6-MMPNs) are measured to assess the potential risk for hepatotoxicity. For more information, refer to the Thiopurine Metabolites in Red Blood Cells Test Fact Sheet.

Refer to the table below for results interpretation information. Results for both metabolites should be interpreted simultaneously and in the context of dosing (e.g., the time of dosing relative to specimen collection), the results of other tests, and other clinical factors. Dosing adjustments may be considered after careful consideration of test results in context; refer to the Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for thiopurine dosing based on TPMT and NUDT15 genotypes for more information.

The Thiopurine Metabolites in Red Blood Cells test can be used for the same clinical purposes as tests offered by other laboratories; however, concentrations reported by different laboratories should not be used for longitudinal comparison. Results may vary between laboratories based on factors such as sample preparation methods and laboratory instrumentation. For example, ARUP’s assay yields results that are approximately 30% higher for 6-MMPNs than other laboratories’ tests; however, the results for 6-TGNs are congruent with those of other assays. ARUP has performed extensive studies to validate this assay and is confident in its accuracy and clinical utility.

The Thiopurine Metabolites in Red Blood Cells test is performed on whole blood, which should be sent promptly to ensure the integrity of red blood cells. Cells are separated from whole blood, washed, counted, and prepared for analysis by liquid chromatography-tandem mass spectrometry.

ARUP offers two additional tests that are useful in thiopurine drug therapy:

• Thiopurine Methyltransferase, RBC: This enzyme phenotype assay may be used before the initiation of therapy to assess risk for toxic effects of thiopurine therapy at standard doses. For more information, refer to the Thiopurine Methyltransferase, RBC Test Fact Sheet.
• TPMT and NUDT15: This test identifies TPMT and NUDT15 variants associated with impaired thiopurine drug inactivation. It may be performed before or during therapy to assess risk or investigate a toxic reaction to thiopurine drugs. For more information, refer to the TPMT and NUDT15 Test Fact Sheet.

ARUP offers a number of educational resources for clinicians.

Test Fact Sheets describe and offer additional details about specific ARUP tests:

• Thiopurine Metabolites in Red Blood Cells Test Fact Sheet
• TPMPT and NUDT15 Test Fact Sheet
• Thiopurine Methyltransferase, RBC Test Fact Sheet

ARUP Consult topics offer guidelines-based information about testing strategy:

• Therapeutic Drug Monitoring - TDM
• Germline Pharmacogenetics - PGx

These non-ARUP resources may also be useful for clinicians:

• Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for thiopurine dosing based on TPMT and NUDT15 genotypes
• TPMT and NUDT15 genotyping recommendations: a joint consensus recommendation of the Association for Molecular Pathology, Clinical Pharmacogenetics Implementation Consortium, College of American Pathologists, Dutch Pharmacogenetics Working Group of the Royal Dutch Pharmacists Association, European Society for Pharmacogenomics and Personalized Therapy, and Pharmacogenomics Knowledgebase