Clinics and Departments

Laboratory Services

Lab Dept:

Chemistry

Test Name:

THIOPURINE METHYLTRANSFERASE, RBC

General Information

Lab Order Codes:

TMT

Synonyms:

TPMT (Thiopurine Methyltransferase); Myelotoxicity

CPT Codes:

83789 – Mass Spectrometry,Tandem Mass Spectrometry ,NOS

Test Includes:

Thiopurine Methyltransferase level reported in U/mL RBC.

Logistics

Test Indications:

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).

Lab Testing Sections:

Chemistry - Sendouts

Referred to:

Mayo Medical Laboratories (MML Test#: TPMT/80291)

Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

2 – 4 days

Special Instructions:

N/A

Specimen

Specimen Type:

Whole blood

Container:

Green top (Sodium Heparin) tube
Alternate: Lavender top (EDTA) or Green top (Lithium Heparin) tube

Draw Volume:

5 mL (Minimum: 3 mL) blood

Processed Volume:

Same as Draw Volume

Collection:

Routine venipuncture

Special Processing:

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.

Patient Preparation:

None

Sample Rejection:

Gross hemolysis; mislabeled or unlabeled specimens

Interpretive

Reference Range:

Reference ranges apply to all ages

Normal

≥15.0 U/mL RBC

Low normal

10.1 – 14.9 U/mL RBC

Carrier

6.0 – 10.0 U/mL RBC

Deficient

0.0 – 5.9 U/mL RBC

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.

Critical Values:

N/A

Limitations:

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.

Methodology:

Enzymatic End Point/Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

References:

Mayo Medical Laboratories December 2012

Updates:

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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