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Laboratory: Clinical Biochemistry  (SENDOUTS)
Test Name:
THIOPURINE METABOLITES (6-mercaptopurine, Azathioprine) - (B)
Test Code: MIS8
Clinical Information:
Description: Assay measures trough levels of 6-thioguanine nucleotides (6-TG) and 6-methyl-mercaptopurine (6-MMP).

Test Indications: 6-Mercaptopurine (Purinethol) and its imidazolyl derivative, Azathioprine (Imuran), are immunosuppressive drugs. 6-Mercaptopurine (6-MP) is indicated for remission induction and maintenance therapy of acute lymphoblastic leukemia (ALL). Azathioprine is indicated as an adjunct for the prevention of rejection in renal allograft (kidney transplant) patients, for the management of rheumatoid arthritis, and for the management of inflammatory bowel disease. Azathioprine is cleaved to 6-MP. 6-MP is metabolized via a series of enzymatic steps to 6-thioguanine nucleotides (6-TGNs), to 6-methyl-mercaptopurine (6-MMPNs) by the enzyme thiopurine methyltransferase (TPMT), and to 6-thiouric acid by the enzyme xanthine oxidase (XO). TPMT enzyme activity has large inter-individual variations which affect the efficacy, toxicity and variability of the treatment. Therapeutic drug monitoring of 6-MP metabolites (6-TGNs and 6-MMPNs) in erythrocytes may assist therapy, particularly in combination with TPMT enzyme activity or mutation analysis.
Recommended screening test for azathioprine and 6-mercaptopurine induced leucopenia is  Thiopurine Methyltransferase (TPMT), Enzyme Activity (Phenotype), Erythrocytes – (B) (test code: TPMP)

Test Approval Requirements: F260-11-02 Biochemistry Approval for Test Referral Form

Patient preparation instructions: Collect a trough specimen (within 1 hour prior to next dose).
Collection Devices:
Specimen Required:
Whole Blood: 3.0 mL
Pediatric Whole Blood: 3.0 mL

Adult and Pediatric Volume: min 3.0 mL 
Collection Information: 
1. Invert several times to mix blood.
2. Send whole blood specimen in original tube. Do not aliquot.

Special Processing: Do not centrifuge. Keep specimen cold - do not freeze.

Specimen Stability: 
Ambient: 24 hours
Refrigerated: 7 days
Frozen: unacceptable
Other comments: Grossly hemolyzed and heparin plasma are NOT suitable for testing and to be rejected.
Whole Blood: 3.0 mL

Shipping & Storage: Send in original collection tube. Store and ship whole blood at room temperature (preferred) or refrigerated. Do not freeze.
Specimen must be received at the testing site within 3 days of collection due to limited stability.

Referred Out Location: Specimen referred from HSC-MS5 to:
In-Common Laboratories (ICL)
57 Gervais Drive
North York, Ontario
M3C 1Z2
Telephone: (416) 422-3000
Test Name: Thiopurine Metabolites, Whole Blood
Testing location: CA, USA
Reference Values:
Name Units Reference Interval
6-TG pmol / 8x10(8) RBC 235 – 400
6-MMP pmol / 8x10(8) RBC < 5700
These results are useful in assessing a patient's metabolism of azathioprine (AZA) or 6-mercaptopurine (6-MP).
- A 6-thioguanine (6-TG) level greater than 235 pmol/8x10(8) RBC has been associated with remission and very high levels have been associated with leucopenia.
- 6-methylmercaptopurine (6-MMP) levels greater than 5700 pmol/8x10(8) RBC may be associated with hepatoxicity.

Method of Analysis: Liquid Chromatography/Tandem Mass Spectrometry (LC/MS/MS)
Within 2 Weeks
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