Test Highlight: Test Live on August 21, 2023

Test Code Test Name
3016503 Thiopurine Metabolites in Red Blood Cells

Frequently Asked Questions

How is thiopurine metabolite testing used in thiopurine drug therapy?

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.

Which test does ARUP offer to measure thiopurine metabolites?

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.

How are the results of the Thiopurine Metabolites in Red Blood Cells test interpreted?

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.

Thiopurine Metabolites in Red Blood Cells: Results Interpretation
6-TGN Concentration (pmol/8x108 RBCs) Interpretation


Possible reduced response to therapy

235-450 Within therapeutic range


Possible increased risk for leukopenia and myelotoxicity

6-MMPN Concentration (pmol/8x108 RBCs) Interpretation


Within therapeutic range


Possible increased risk for hepatotoxicity

Is ARUP’s Thiopurine Metabolites in Red Blood Cells test equivalent to tests offered by other laboratories?

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.

How is ARUP’s Thiopurine Metabolites in Red Blood Cells test performed?

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.

What other testing does ARUP offer for use in thiopurine drug therapy?

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

Where can I find more information about thiopurine drug therapy and ARUP’s assays?