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Test Name:
TEMPORARILY NON-ORDERABLE Thiopurine Methyltransferase, Erythrocytes [Mayo]
- SBMF No:
42125 - Performance Lab Name:
Mayo Medical Laboratories - Test Mnemonic:
TPMT - ABN:
Required – Not FDA-Approved - CPT Code:
83789 - LOINC Code:
19146-0 - Ref Lab Test No:
80291 - Also Known As:
Myelosuppression
Myelotoxicity
TPMT Enzyme Activity (phenotype) for Azathioprine (Imuran) and 6-MP (Purinethol) dosing - Also See:
45103 PROMETHEUS® TPMT Enzyme
44808 Thiopurine Methyltransferase, Erythrocytes [ARUP] - Spec Type:
Whole blood - Spec Container:
Green top (heparin) or lavender top (EDTA) tube - Pref Vol:
5.0 mL - Min Vol:
3.0 mL - Fasting:
No - Spec Collect:
DO NOT COLLECT
Effective February 3, 2012 this test is temporarily non-orderable due to back-ordered reagent. Expected down time is approximately 4 weeks. Refer to the "Also See" section above for alternative test options.Routine venipuncture
- Spec Process:
Gently invert tube to mix blood with the anticoagulant
Do not centrifuge
Do not remove plasma from cells
Do not transfer blood to other containers
Forward entire tube of unprocessed whole blood to the laboratory - Spec Store Transport:
Refrigerated - Spec Stability:
Room temperature (20-30°C): Unacceptable
6 days Refrigerated (2-8°C)
Frozen: Unacceptable - Spec Reject:
Specimens other than whole blood
Anticoagulants other than sodium heparin, lithium heparin, or EDTA
Severely hemolyzed sample
Frozen or room temperature sample - Methodology:
Enzymatic End Point
Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) - Use:
Detection of individuals with low thiopurine methyltransferase activity who are at risk for excessive myelosuppression or severe hematopoietic toxicity when taking azathioprine (Imuran) or 6-MP (Purinethol) - Clinical Significance:
Thiopurine methyltransferase (TPMT) deficiency is a condition in which patients treated with standard doses of the immunosuppressant azathioprine (Imuran) or the antineoplastic drug 6-mercaptopurine (6-MP, Purinethol) may develop life-threatening myelosuppression or severe hematopoietic toxicity. The metabolic conversion of azathioprine or 6-MP to purine nucleotides and the subsequent incorporation of these nucleotides into DNA plays an important role in both the therapeutic efficacy and the toxicity of these drugs. A competitive catabolic route for the metabolism of thiopurines is catalyzed by the TPMT enzyme, which inactivates them by thiomethylation. A balance must be established between these competing metabolic pathways so that; 1) sufficient amounts of drug are converted to the nucleotide to act as an antimetabolite and 2) the levels antimetabolite do not become so high as to cause potentially lethal bone marrow suppression.
TPMT deficiency is an autosomal recessive condition with an incidence of approximately 1 in 300 individuals homozygous for deleterious mutations in the TPMT gene; about 10% of the population are heterozygous carriers of TPMT mutations. Adverse effects of azathioprine or 6-MP administration can be observed in individuals who are either homozygous or heterozygous for TPMT deficiency. As such, knowing a patient's TPMT status prior to treatment with azathioprine or 6-MP is important for purposes of calculating drug dosages.
- Reference Range:
>17.0 U/mL RBC (normal)
11.9-17.0 U/mL RBC (ambiguous range)
15.4-17.0 U/mL RBC (probable low normal)
11.9-15.3 U/mL RBC (possible carrier)
6.0-11.8 U/mL RBC (carrier range)
0.0-5.9 U/mL RBC (homozygous deficient range)Reference values apply to all ages.
Interpretive Data:
Expected values for individuals in the carrier range for thiopurine methyltransferase deficiency are between 6.0 U/mL RBC to 11.8 U/mL RBC.Expected values for individuals homozygous for deleterious mutations in the TPMT gene (deficient TPMT) are < or =5.9 U/mL RBC.
Results between 11.9 U/mL RBC and 17.0 U/mL RBC are considered ambiguous: 15.4 U/mL RBC to 17.0 U/mL RBC represents probable low normal; 11.9 U/mL RBC to 15.3 U/mL RBC represents possible carrier status.
Cautions:
Results that fall within the ambiguous range (probable low normal and possible carrier) can occur because of either assay variability or biological variation. - Additional Test Info:
This test was developed and its performance characteristics determined by Laboratory Medicine and Pathology, Mayo Clinic. This test has not been cleared or approved by the U.S. Food and Drug Administration.
Reference:
"Thiopurine Methyltransferase (TPMT), Erythrocytes." 2011 Online Test Catalog, Mayo Medical Laboratories, 2011. Web. 30 July 2011 <http://www.mayomedicallaboratories.com/test-catalog/Overview/80291> - Day Run:
Mon, Wed-Fri - Time Reported:
4-6 days - Test Type:
ENZYME
