TPMT Enzyme Activity Test

Description of the Test

• The TPMT (Thiopurine Methyltransferase) enzyme activity test measures the activity level of the TPMT enzyme in red blood cells.

• TPMT plays a critical role in breaking down thiopurine medications such as azathioprine, mercaptopurine (6-MP), and thioguanine (6-TG).

• Identifying TPMT activity helps predict how a patient will metabolize these drugs, guiding safe and effective dosing.

Purpose / Clinical Significance

• Prevents severe drug toxicity by identifying patients at risk for myelosuppression or bone marrow suppression due to low TPMT activity.

• Aids in tailoring dosage of immunosuppressants and chemotherapy drugs to the individual's metabolic capacity.

• Ensures safe administration of thiopurines in treatment plans for autoimmune diseases, leukemia, and inflammatory bowel conditions.

• Helps differentiate between genetic polymorphisms affecting enzyme function and acquired enzyme suppression.

Principle

• The test detects the enzymatic activity of TPMT in red blood cells through biochemical or genetic analysis.

• In biochemical testing, red blood cells are incubated with a thiopurine substrate, and the product is measured to determine TPMT activity.

• Alternatively, molecular methods can identify mutations in the TPMT gene that reduce enzyme activity.

Whom and Why Should Take This Test

• Patients who are about to start treatment with azathioprine, mercaptopurine, or thioguanine.

• Individuals with a history of severe side effects or blood cell abnormalities after taking thiopurine drugs.

• Those with autoimmune diseases such as Crohn’s disease, ulcerative colitis, or rheumatoid arthritis receiving immunosuppressive therapy.

• Children and adults undergoing maintenance therapy for leukemia or other cancers.

• Patients with family history of TPMT deficiency or drug intolerance.

Specimen Requirements

• Requires a whole blood sample, typically collected in an EDTA (lavender-top) tube.

• Sample should be kept cool and transported promptly to maintain enzyme integrity.

• Fasting is not required, but prior blood transfusion may interfere with results.

Common Methods

• Enzymatic activity assay performed on red blood cells using liquid chromatography or spectrophotometry.

• Genetic testing using PCR to detect known TPMT gene mutations associated with reduced or absent enzyme function.

• Some labs may offer both enzyme activity and genotyping for comprehensive analysis.

Interpretation of Results

• High TPMT activity suggests a patient is likely to tolerate standard thiopurine doses.

• Intermediate activity (heterozygous carriers) may require reduced dosage to avoid toxicity.

• Low or absent activity indicates a high risk of severe toxicity; alternative therapies or significantly reduced doses are recommended.

• Results must be correlated with clinical condition, treatment history, and CBC monitoring.

Reference Range

• Normal TPMT activity typically ranges between 15–26 units/mL RBCs, but the exact range may vary by laboratory.

• Activity is categorized as normal (high), intermediate (heterozygous), or low/absent (homozygous deficient).

Limitations

• Recent blood transfusions can mask a true TPMT deficiency by introducing donor red blood cells with normal activity.

• The test does not predict non-TPMT-related adverse drug reactions.

• Enzyme activity levels may be affected by handling errors or delayed sample processing.

• Genetic testing may not detect all rare or novel TPMT variants.

Follow-up Testing

• CBC with differential to monitor for early signs of bone marrow suppression during thiopurine treatment.

• Repeat TPMT activity testing may be considered in certain clinical scenarios or after transfusion.

• Thiopurine metabolite testing may be used to monitor ongoing drug metabolism and adherence.

Conclusion
The TPMT enzyme activity test is a vital pre-treatment screening tool that ensures patient safety when prescribing thiopurine medications. By identifying those with low or absent enzyme activity, this test helps prevent potentially life-threatening complications such as myelosuppression.