Thiopurine Methyltransferase (TPMT) Activity/Phenotyping, Whole Blood
Test Overview
Clinical Utility
Determining a patient’s TPMT status prior to starting therapy with a thiopurine drug is important for purposes of calculating the optimal drug dosage. Patients should be tested prior to azathioprine or 6-MP administration.
Individuals with low TPMT activity are at risk for life-threatening severe hematopoietic toxicity, while individuals with high TPMT activity present therapeutic resistance to the thiopurine drugs and may develop hepatotoxicity when taking thiopurine drugs (azathioprine, 6-mercaptopurine, or 6-thioguanine).
Result Included
Thiopurine Methyltransferase (TPMT) Activity
Specimen
Specimen Type
Whole Blood (EDTA)Volume
Sample Volume
4 mL
Pediatric
Volume
1 mL
Minimum
0.5 mL
Patient Preparation
Total hemoglobin must be measured on the same day as collection of the TPMT specimen and submitted with the test order.
TPMT activity must be measured prior to RBC transfusion to avoid false indication.
TPMT enzyme activity can be inhibited by several drugs and may contribute to falsely low results. Patients should discontinue drugs for at least 48 hours prior to TPMT testing: naproxen, ibuprofen, ketoprofen, furosemide (Lasix), sulfasalazine (Azulfidine), mesalamine (Asacol), olsalazine (Dipentum), mefenamic acid (Ponstel), trimethoprim (Proloprim), methotrexate, thiazide diuretics, and benzoic acid inhibitors.
Collection & Handling
Handling Information
Do not centrifuge. Do not freeze. Do not transfer specimen – submit in original collection tube. Tubes with multiple overlaying labels or tubes and caps wrapped with parafilm will be rejected.
Store and send cold.
Specimen must be accompanied by a hemoglobin result determined on the same collection day. Specimen must be received at ICL Mon-Thu within 4 days of collection. State collection date.
Stability
| Refrigerated | Frozen |
|---|---|
| 8 days | Unacceptable |
Rejection Criteria
| Criteria | Specification |
|---|---|
| Specimen | Frozen or centrifuged |
Performance & Interpretation
Turnaround Time
5 days
Results
-
Thiopurine Methyltransferase (TPMT) Activitynmol/g Hb/hLow Activity: <10
Intermediate 10 - 40
Normal Activity: >40
Referral Location
Canada
Interface & Setup
HL7 Interface Codes
| Order Code | Result Name | Result Codes | Units |
|---|---|---|---|
| TPMT | TPMT | 39162 | nmol/g Hb/h |
Test Version
Last Updated
2024-01-10