- Patient Preparation
- Lavender (EDTA), pink (K2EDTA), or green (sodium or lithium heparin).
- Specimen Preparation
- Transport 5 mL whole blood. (Min: 3 mL)
- Storage/Transport Temperature
- Unacceptable Conditions
- Gel separator tubes. Specimens collected in sodium fluoride/potassium oxalate (gray). Hemolyzed, frozen, or room temperature specimens.
- Ambient: 3 hours; Refrigerated: 6 days; Frozen: Unacceptable
Abnormal TPMT activity: < 25 U/mL - Individuals are predicted to be at high risk of bone marrow toxicity as a consequence of standard thiopurine dosing; a dose reduction and therapeutic monitoring is recommended.
High TPMT activity: > 65 U/mL - Individuals are not predicted to be at risk for bone marrow toxicity as a consequence of standard thiopurine dosing but may be at risk for therapeutic failure due to excessive inactivation of thiopurine drugs. Individuals may require higher than the standard dose; therapeutic monitoring is recommended.
TPMT phenotype testing does not replace the need for clinical monitoring of patients treated with thiopurine drugs. Genotype for TPMT cannot be inferred from TPMT activity (phenotype). Phenotype testing should not be requested for patients currently treated with thiopurine drugs, as results will be falsely low. Current TPMT phenotype may not reflect future TPMT phenotype, particularly in patients who received blood transfusion within 30-60 days of testing. TPMT enzyme activity can be inhibited by several drugs such as: naproxen (Aleve), ibuprofen (Advil, Motrin), ketoprofen (Orudis), furosemide (Lasix), sulfasalazine (Azulfidine), mesalamine (Asacol), olsalazine (Dipentum), mefenamic acid (Ponstel), thiazide diuretics, and benzoic acid inhibitors. TPMT inhibitors may contribute to falsely low results; patients should abstain from these drugs for at least 48 hours prior to TPMT testing. Falsely low results may also occur as a result of inappropriate specimen handling.
|Component Test Code*||Component Chart Name||LOINC|
- TPMT Enzyme
- TPMT Erythrocytes