Pharmacokinetic Study of Rituximab Induction Regimen in ANCA-associated Vasculitis

Inclusion Criteria: * Age \> 18 years * Granulomatosis with polyangiitis or microscopic polyangiitis (according to Chapel Hill criterions), with or without detectable ANCA * Decision taken to start an induction regimen with rituximab * Informed and having signed the study consent form * If of child-bearing potential, female patients will have use an effective method of contraception during RTX (rituximab) treatment and in the 12 months following RTX treatment stop * no-breast-feeding during RTX treatment and in the 12 months following RTX treatment stop Exclusion Criteria: * Other primary or secondary systemic vasculitis * Incapacity or refusal to sign the informed consent form * Incapacity or refusal to adhere to treatment or perform the follow-up examinations required by the study * Allergy, documented hypersensitivity or contraindication to the medications used in the present study (corticosteroids, rituximab) * severe active infection * Patient with severe heart failure (stage NYHA IV) or any other unstable heart disease Pregnancy, except for cases where the expected benefit oj treatment seems to surpass the potential risks * Patients with active hepatitis B * Any live vaccine within four weeks prior to the first infusion of RTX