Laboratory: Clinical Biochemistry
Test Name:
THIOPURINE METHYLTRANSFERASE (TPMT) PHENOTYPING (RBC) - (B)
Test Code: TPMP
Clinical Information:
The recommended screening test for azathioprine and 6-mercaptopurine induced leucopenia is Thiopurine Methyltransferase Phenotyping (RBC) (test code:  TPMP).
 
Test performed at: 
Health Sciences Centre by DSM Biochemical Genetics Laboratory using an enzyme activity assay with the product detected by UPLC/fluorescence.
 
Blood is stable refrigerated for up to 7 days.
Multiple red blood cell transfusions pre-test may produce a false indication of a patient’s RBC-TPMT phenotype.  It is strongly encouraged to test RBC-TPMT phenotype prior to red blood cell transfusions.  If not possible, TPMT genotyping for only the common variants may be used to screen for patients at high risk of AZA or 6-MP-induced leucopenia.  Please see TPMT genotyping.
Collection Devices:
Transport specimen to lab on ice (to prevent specimen from freezing, wrap specimen in paper towel and place on ice). Patients should be tested prior to azathioprine or 6-MP administration.
Specimen Required:
Whole Blood: 4.0 mL
Pediatric Whole Blood: 1.0 mL

Minimum volumes: Adult 1.0 mL Pediatric 0.5 mL
Referral:

Send whole blood specimen (following instructions in Lab Notes section below) to:
Chemistry Laboratory
MS551 Thorlakson Building
820 Sherbrook Street
Winnipeg, MB  R3A 1R9
 
Contact Phone Number for Lab 204-787-1534
Requisition:
Reference Values:
Low activity:  < 10 nmol/g Hb/h
Intermediate activity:  10 - 40 nmol/g Hb/h
Normal activity:  > 40 nmol/g Hb/h
Patients with low RBC-TPMT activity are at high risk of AZA or 6-MP induced leucopenia.
 
Falsely low RBC TPMT enzyme activity occurs with patients who are taking aminosalcylates at the time of sample collection.
Availability:
Within 1 Week
See Also:
More Information:
DO NOT CENTRIFUGE.  DO NOT FREEZE.  SHIP SPECIMEN WITH ICE or COLD PACK TO 
PREVENT BLOOD FROM FREEZING, DO NOT PLACE SPECIMEN DIRECTLY ON ICE.