A prospective, non-commercial, multicenter study will be conducted in a cohort of patients initiating induction therapy and a cohort of patients with stable treatment with ustekinumab.

The present study aims to evaluate the association between serum levels (trough) of Ustekinumab and clinical, biological, endoscopic and radiological response/remission in patients with Crohn's disease, in order to determine threshold levels at induction and maintenance that may be predictors of efficacy.

Working hypothesis: Serum Ustekinumab levels in Crohn's disease patients are correlated with clinical, biochemical and endoscopic/radiological response/remission.

Patients with an established diagnosis of Crohn's disease by usual criteria (clinical, analytical, endoscopic, radiological and/or histological) who initiate treatment with Ustekinumab (induction cohort) or who are on maintenance treatment with Ustekinumab (maintenance cohort) in a stable dosis (a minimum of 6 months) will be included.

Inclusion Criteria:

• Patients with an established diagnosis of CD according to standard criteria (clinical, analytical, endoscopic, radiological, and/or histological) with inflammatory activity, who begin treatment with ustekinumab for remission induction. Activity will be considered to be the presence of clinical activity (Harvey-Bradshaw index ≥5) and/or elevated fecal calprotectin (≥ 250μg/g; ≥100 in cases of surgically treated CD) and/or the presence of endoscopic activity (SES-CD ≥3).
• Patients with a confirmed diagnosis of CD according to standard criteria (clinical, analytical, endoscopic, radiological, and/or histological) undergoing stable maintenance treatment with ustekinumab for at least 6 months.

Exclusion Criteria:

• Patients in whom ustekinumab is indicated for the prevention of post-surgical recurrence.
• Patients in whom ustekinumab is indicated for the control of perianal disease.
• Patients in whom ustekinumab is indicated for the control of extraintestinal manifestations.
• Pregnant women.
• Patients who are unable to understand the nature of the study, the procedures to be followed, or who are not able to sign an informed consent form.

Patients with an established diagnosis of CD based on standard criteria (clinical, laboratory, endoscopic, radiological, and/or histological) who are starting treatment with ustekinumab (induction cohort) or who are undergoing maintenance treatment with ustekinumab (maintenance cohort) on a stable basis (minimum of 6 months) will be included.