Thiopurine Methyltransferase, RBC
Also known as: TPMT RBC
Use
This phenotyping test is used to assess the risk of severe myelosuppression with standard dosing of thiopurine drugs in individuals for whom thiopurine therapy is being considered. It identifies individuals with low and intermediate Thiopurine Methyltransferase (TPMT) activity who may be at risk for myelosuppression when exposed to standard doses of thiopurines, including azathioprine and 6-mercaptopurine. TPMT is the main metabolic pathway for the inactivation of thiopurine drugs in the bone marrow, and low TPMT activity can lead to increased conversion of 6-mercaptopurine into the cytotoxic 6-thioguanine nucleotides.
Special Instructions
This test must be performed prior to the initiation of thiopurine therapy. Patients should abstain from drugs that inhibit TPMT activity, such as naproxen, ibuprofen, and certain other medications, for at least 48 hours before testing. Specimens should be collected in EDTA or heparin tubes (lavender, pink K2EDTA, or green sodium/lithium heparin).
Limitations
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) testing. The current TPMT phenotype may not reflect future phenotype, especially in patients with recent blood transfusions. Several medications can inhibit TPMT activity, leading to false low results. Specimens should not be hemolyzed, frozen, or collected using gel separator tubes.
New York Approved
Methodology
Mass Spectrometry
Biomarkers
LOINC Codes
- 53819-9
Result Turnaround Time
3-5 days
Related Documents
For more information, please review the documents below
Specimen
Whole Blood
Volume
5 mL
Minimum Volume
3 mL
Container
Lavender (EDTA), pink (K2EDTA), or green (sodium or lithium heparin).
Patient Preparation
Abstain from TPMT inhibitors 48 hours prior to testing.
Causes for Rejection
Gel separator tubes; Specimens collected in sodium fluoride/potassium oxalate (gray); Hemolyzed, frozen, or room temperature specimens.
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 3 hours |
| Refrigerated | 6 days |
| Frozen | Unacceptable |
Other tests from different labs that may be relevant