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), temporary
forward to ARUP (FATPM)

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

24.0 – 44.0 U/mL
Individuals are predicted to be at low risk of bone marrow toxicity (myelosuppression) as a consequence of standard thiopurine therapy; no dose adjustment is recommended.

Intermediate:

17.0 – 23.9 U/mL
Individuals are predicted to be at intermediate risk of bone marrow toxicity (myelosuppression) as a consequence of standard thiopurine therapy; a dose reduction and therapeutic drug management is recommended.

High:

>44.0 U/mL
Individuals are not predicted to be at risk for bone marrow toxicity (myelosuppression) as a consequence of standard thiopurine dosing, but may be at risk for therapeutic failure due to excessive inactivation

Low:

<17.0 U/mL
Individuals are not predicted to be at high risk for bone marrow toxicity (myelosuppression) as a consequence of standard thiopurine dosing. It is recommended to avoid use of thiopurine drugs.

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 2015

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.
12/7/2015: Updated for temporary test referral to ARUP. Internal test as MML is unavailable.



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