Thiopurine S‑Methyltransferase (TPMT) Genotype
Also known as: Azathioprine Toxicity, Mercaptopurine Toxicity, TPMT Deficiency
Use
Thiopurine S‑Methyltransferase (TPMT) Genotype testing identifies genetic variants in the TPMT gene that affect enzyme activity, which is critical for metabolism of thiopurine drugs such as azathioprine and mercaptopurine. Determining a patient’s TPMT genotype helps guide dosing to reduce risk of toxicity and optimize therapeutic efficacy.
Special Instructions
Insurance authorization is required prior to specimen draw for outpatients; no special forms are needed. Performing laboratory schedules testing on Wednesday and Friday. Test requires physician attestation of patient consent if ordering facility is in certain states (AK, DE, FL, GA, IA, MN, NV, NJ, NY, OR, SD, VT) or test is performed in MA.
Limitations
This test detects approximately 95% of TPMT mutations; some intermediate or slow metabolizer variants may not be identified. Analytical performance has been validated under CLIA; the assay has not been FDA cleared or approved.
Methodology
PCR-based (PCR)
Biomarkers
LOINC Codes
- 36922-3
- 79713-4
Result Turnaround Time
5-6 days
Related Documents
For more information, please review the documents below
Specimen
Whole Blood
Volume
5 mL
Minimum Volume
3 mL
Container
EDTA (lavender‑top) tube; acceptable: sodium heparin (green‑top) or ACD solution A/B (yellow‑top) tube
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 8 days |
| Refrigerated | 8 days |
| Frozen | Unacceptable |
Other tests from different labs that may be relevant