Recommendations: The TPMP test should only be done once on each patient – TPMT activity does not change. Repeat testing is not done for in province patients.
Patient preparation instructions: 
1) TPMT phenotyping should not be done once a patient is taking Azathiopurine, Thioguanine or 6-Mercaptopurine. The assay for TPMT phenotyping measures the enzyme activity by producing drug metabolite in the assay, so once a patient is taking thiopurine drugs, they are producing the metabolite in vivo and the result of the assay would be falsely elevated. 

2) TPMT activity must be measured prior to RBC transfusion to avoid false indication. If not possible, TPMT genotyping for only the common variants may be used to screen for patients at high risk of thiopurine drug induced toxicity.

3) TPMT enzyme activity can be inhibited by several drugs and may contribute to falsely low results. Patients should discontinue drugs for at least 48 hours prior to TPMT testing: Naproxen, Ibuprofen, Ketoprofen, Furosemide (Lasix), Sulfasalazine (Azulfidine), Mesalamine (Asacol), Olsalazine (Dipentum), Mefenamic acid (Ponstel), Trimethoprim (Proloprim), Methotrexate, Thiazide diuretics, and Benzoic acid inhibitors.

Patients should be tested prior to azathioprine or 6-MP administration.

Individuals with low TPMT activity are at high risk of bone marrow toxicity with standard thiopurine dosing.
Method of Analysis: Enzymatic / UPLC-Fluorescence