A comparative study of inosine triphosphatase activity in fresh erythrocytes and dried blood spots.

Inosine triphosphatase (ITPase) is one of the enzymes involved in thiopurine metabolism. Polymorphisms in the ITPA gene, resulting in a decreased activity of ITPase, are associated with adverse drug reactions during thiopurine therapy. Measuring ITPase activity is therefore of utmost importance. We compared measurement of ITPase activity in fresh erythrocyte lysates and eluates of dried blood spots (DBS). Upon storage up to 2 years at room temperature a fast decrease of ITPase activity in DBS was observed. The residual activity declined from 70% of the activity measured in fresh erythrocyte lysate 3 days after spotting to ~5% of after 2 years. Patients with an activity lowering polymorphism in the ITPA gene could still be detected in DBS after longer periods of storage at room temperature, using DBS of patients with a normal wild type activity, which were stored over the same period as reference. However, routine determination of ITPase activity in fresh erythrocyte lysates is preferred. Measurement of ITPase activity⏐59