Leflunomide (Arava®) is a drug used in the treatment of rheumatoid arthritis. It acts by preventing the body from producing the antibodies responsible for symptoms of the disease (inflammation, edema, stiffness, joint pain). Leflunomide is a prodrug, a drug that is administered in an inactive form that must be converted into an active metabolite by the liver. This active metabolite is present in the blood for a long period of time (half-life greater than 2 weeks), and significant levels of metabolite can be measured up to 2 years after the treatment is stopped. This test measures levels of active Lefunomide metabolite in the blood, and results are expressed in micrograms of metabolite per millilitre of blood (µg/mL).
Metabolite levels between 50 and 100 µg/mL are associated with an adequate therapeutic response in many patients, but individual variations are frequent. Given the long half-life of the metabolite and its toxic effects (teratogen) on the fetus, testing is useful when pregnancy is desired. Levels below 0.02 µg/mL (repeated after 2 weeks) are desirable in both spouses and can be reached more quickly using neutralizing agents (charcoal, cholestyramine).