You should have levels of the enzyme thiopurine methyl transferase (TPMT) measured in your blood. This test can help to predict who is more likely to experience side effects with azathioprine or mercaptopurine, such as suppressed bone marrow function. For patients found to have reduced TPMT enzyme levels the dose of thiopurines can be lowered, while those with no detectable levels should be offered alternative treatments
You should be screened for immunity to the chickenpox virus and for infections including varicella zoster virus, hepatitis B virus, hepatitis C virus, and human immunodeficiency virus. Screening for Tuberculosis (TB) should also be considered, especially if risk factors are present such as recent visits to countries with high levels of TB, exposure to TB or no previous BCG vaccination for TB