The factors underlying the large interindividual variability in response to glucocorticoids in Giant Cell Arteritis are poorly understood. The investigators hypothesize that a part of this variability is related to pharmacokinetic factors determined by genetic polymorphism: hepatic clearance involving cytochromes P450 of the subfamily 3A (CYP3A) and drug efflux leukocyte conditioned by P-glycoprotein involved in multidrug resistance drugs (ABCB1). The investigators have designed a multicentric prospective pharmacokinetical and pharmacogenetic cohort study to assess the link between prednisolone clearance and the relapse risk in giant cell arteritis.

Inclusion Criteria:

• Diagnosis of GCA, meeting at least 3 of the following 5 American College of Rheumatology (ACR) criteria for the diagnosis of GCA:

  1. At least 50 years of age at disease onset
  2. New onset or new type of localized pain in the head
  3. Temporal artery abnormality (i.e., temporal artery tenderness to palpation or decreased pulsation unrelated to arteriosclerosis of cervical arteries)
  4. ESR of greater than 40 mm in the first hour by the Westergren method
  5. Temporal artery biopsy showing vasculitis characterized by a predominance of mononuclear cell infiltration or granulomatous inflammation, usually with multinucleated giant cells

• Corticoid treatment since less than 14 days

• Signed informed consent

• Affiliation to the social security system

Exclusion Criteria:

• Dementia
• Predictable non observance
• Neoplasia since less than 5 years