Thiopurine Methyltransferase, RBC
Ordering Recommendation
Preferred screening test to detect individuals at high risk for myelosuppression when exposed to standard dose of thiopurines. INTENDED FOR PRETHERAPEUTIC EVALUATION ONLY.
Enzymatic/Quantitative Liquid Chromatography-Tandem Mass Spectrometry
Mon, Wed, Fri
3-4 days  
New York DOH Approval Status
This test is New York DOH approved.
Submit With Order
Specimen Required
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  
Reference Interval
Normal TPMT activity:  25-65 U/mL - Individuals are predicted to be at low risk of bone marrow toxicity as a consequence of standard thiopurine therapy; no dose adjustment is recommended.

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.  
Interpretive Data
The TPMT, RBC assay is used as a screen to detect individuals with low (abnormal) TPMT activity who may be at risk for excessive myelosuppression when exposed to standard doses of thiopurines, including azathioprine (Imuran) and 6-mercaptopurine (Purinethol). TPMT is the primary metabolic route for inactivation of thiopurine drugs in the bone marrow. When TPMT activity is low, it is predicted that proportionately more 6-mercaptopurine can be converted into the cytotoxic 6-thioguanine nucleotides that accumulate in the bone marrow causing excessive toxicity. The activity of TPMT is measured by the nanomoles of 6-methylmercaptopurine (inactive metabolite) produced per 1 mL of packed red blood cells, (U/mL).

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.

Compliance Statement B: This test was developed and its performance characteristics determined by ARUP Laboratories. The U.S. Food and Drug Administration has not approved or cleared this test; however, FDA clearance or approval is not currently required for clinical use. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions.
This assay measures only enzyme activity.
CPT Code(s)
Component Test Code*Component Chart NameLOINC
0092067Thiopurine Methyltransferase53819-9
* Component test codes cannot be used to order tests. The information provided here is not sufficient for interface builds; for a complete test mix, please click the sidebar link to access the Interface Map.
  • TPMT
  • TPMT Enzyme
  • TPMT Erythrocytes