Since their appearance more than a decade ago, anti-tumor necrosis factor (TNF) inhibitors have demonstrated beneficial activity in the treatment of inflammatory bowel diseases (IBD). However, more than one-third of patients present primary resistance, and one more third become resistant over time. One of the main factors associated with loss of response is the immunogenicity of anti-TNF biologics leading to the production of antibodies targetting the TNF inhibitor, namely anti-drug antibodies (ADAbs), that accelerate drug elimination from the serum and decrease its therapeutic activity. In this study the investigators propose a medico-economic evaluation of the measurement of anti-TNF agents and anti-drug antibodies serum concentrations in the management of patients with inflammatory bowel disease treated with anti-TNFalpha inhibitors. 280 patients with Crohn's disease (CD) or ulcerative colitis (UC) will be included and randomized in 2 groups with or without drug and ADAbs monitoring. In the monitored group, in case of loss of response, the clinician will use biological informations to adapt the treatment following a simple treatment algorithm. In the unmonitored group, drug and ADAbs measurements will not be transmitted to the clinician. Clinical and economical benefits of the biological monitoring will be evaluated after a follow-up period of two years.

280 patients with Crohn's disease (CD) or ulcerative colitis (UC) will be recruited in 13 french centers and randomized in 2 groups with or without drug and ADAbs monitoring.

Anti-TNF inhibitors and ADAbs will be simultaneously measured in the patients serum by ELISA (LisaTracker - Theradiag) at weeks 6 and 24, months 4, 8, 12, 16, 20 and 24.

In the unmonitored group, drug and ADAbs measurements will not be transmitted to the clinician.

In the monitored group, in case of loss of response the clinician will use immunomonitoring data to adapt the treatment following a simple treatment algorithm:

• loss of response with therapeutic serum trough level of anti-TNF switch to another biologic
• loss of response with subtherapeutic serum trough level of anti-TNF but without detectable ADAbs increase doses and/or shorten interval between infusions
• loss of response with subtherapeutic serum trough level of anti-TNF and detectable ADAbs switch to another anti-TNF if inefficient switch to another biologic.

Clinical and economical benefits of the biological monitoring will be evaluated after a follow-up period of two years:

• primary outcome: Cost-utility analysis of the immunomonitoring strategy using incremental cost effectiveness ratio
• secondary outcome: Evaluation of the quality of life (IBDQ score)

Inclusion Criteria:

• patients with Crohn's disease or ulcerative colitis
• Treated with anti-TNF

Exclusion Criteria:

• anti-TNF contraindication
• Previous treatment with anti-TNF