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Lab Dept:
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Chemistry
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Test Name:
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THIOPURINE METHYLTRANSFERASE, RBC
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General Information
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Lab Order Codes:
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TMT
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Synonyms:
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TPMT (Thiopurine Methyltransferase); Myelotoxicity
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CPT Codes:
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83789 – Mass Spectrometry,Tandem Mass Spectrometry ,NOS
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Test Includes:
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Thiopurine Methyltransferase level reported in U/mL RBC.
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Logistics
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Test Indications:
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Useful for detection of individuals with low TPMT activity who will have excessive myelosuppression or severe hematopoietic toxicity when taking azathioprine (Imuran) or 6-MP (Purinethol).
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Lab Testing Sections:
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Chemistry - Sendouts
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Referred to:
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Mayo Medical Laboratories (MML Test#: TPMT/80291)
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Phone Numbers:
Minneapolis:
Saint Paul:
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612-813-6280
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651-220-6550
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Test Availability:
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Daily, 24 hours
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Turnaround Time:
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2 – 4 days
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Special Instructions:
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N/A
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Specimen
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Specimen Type:
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Whole blood
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Container:
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Green top (Sodium Heparin) tube
Alternate: Lavender top (EDTA) or Green top (Lithium Heparin) tube
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Draw Volume:
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5 mL (Minimum: 3 mL) blood
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Processed Volume:
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Same as Draw Volume
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Collection:
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Routine venipuncture
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Special Processing:
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Lab Staff: Do Not Centrifuge. Store specimen at refrigerated temps before shipping. Stable for 6 days at 2-8 degrees centrigrade. Forward specimen to reference lab on a cold pack. Do Not Freeze. Forward promptly.
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Patient Preparation:
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None
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Sample Rejection:
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Gross hemolysis; mislabeled or unlabeled specimens
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Interpretive
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Reference Range:
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Reference ranges apply to all ages
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Normal
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≥15.0 U/mL RBC
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Low normal
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10.1 – 14.9 U/mL RBC
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Carrier
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6.0 – 10.0 U/mL RBC
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Deficient
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0.0 – 5.9 U/mL RBC
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Interpretation:
Expected values for individuals in the carrier range for thiopurine methyltransferase are between 6.0 – 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.
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Critical Values:
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N/A
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Limitations:
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There is a partial overlap for the distribution of thiopurine methyltransferase activities observed between patients with normal and heterozygous genotypes; thus, results that fall within the low normal range can occur because of assay variability or biological variation.
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Methodology:
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Enzymatic End Point/Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)
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References:
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Mayo Medical Laboratories December 2012
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Updates:
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3/8/2012: Tube type changed from Green to Lavender. Temporary change due to test issue at Mayo. Mayo is forwarding testing to Prometheus until further notification.
12/26/2012: Test reinstated at MML. Updated reference ranges.
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