You will probably need repeated serum urate (SU) blood tests while taking allopurinol, at least for a while. These measure the amount of uric acid in your blood. It’s this uric acid that can form crystals in your joints and trigger symptoms. For most patients, it is recommended to target an SU level of less than six milligrams per decilitre. If your number is higher than this, you might need to start taking a higher dose.