The purpose of this study is to determine the optimal dosage of etanercept in patients treated for idiopathic juvenile arthritis

Etanercept, a fully human soluble tumor necrosis factor (TNF) receptor fusion protein, has been demonstrated to be safe and efficacious for the treatment of patients with polyarticular JIA. However, many patients experience primary or secondary response failure, suggesting that individualization of treatment regimens may be beneficial. It has been shown that the clinical response to two anti-TNFalpha biological agents (infliximab, adalimumab) closely follows the trough drug levels and the presence of antibodies directed against the drugs. This study was undertaken to investigate whether serologic monitoring of etanercept bioavailability and immunogenicity in individual patients with JIA would be useful in optimizing treatment regimens to improve efficacy and tolerability

Inclusion Criteria:

• Patient with Juvenile Idiopathic Arthritis
• Age <18 years and> 4 years (which corresponds to the pediatric AMM)
• Patient on Etanercept treatment in the context of his usual care, whatever the dose (but the dose should remain stable in the weight during the study period). The patient must have received at least one injection of etanercept before participating in the study.
• Signature of consent by the guardians of the child
• Patient affiliate or entitled to a social security scheme

Exclusion Criteria:

• Infection progressive
• Pregnancy and lactation. For adolescents of reproductive age and sexually active, contraception should be used during the duration of treatment with etanercept
• Contraindication to treatment with Etanercept
• Refusal of the study