The primary endpoint will be evaluated through the following variables: PUCAI score, IFX levels, and steroid treatment.

Clinical response to IFX will be evaluated through the PUCAI score. The response will be considered clinically significant if PUCAI points continue maintained below 30 during the study period. The IFX response will also be determined by IFX serum levels. A therapeutic IFX level, i.e. for achieving an adequate clinical response, is established above 6 μg/mL. Finally, the necessity, or not, of a steroid treatment during the study period will also be indicative of successful efficacy with GMA.

The trial has the following objectives:

Primary objective (PO):

The primary endpoint will be evaluated through the following variables: PUCAI score, IFX levels, and steroid treatment.

Clinical response to IFX will be evaluated through the PUCAI score. The response will be considered clinically significant if PUCAI points continue maintained below 30 during the study period. The IFX response will also be determined by IFX serum levels. A therapeutic IFX level, i.e. for achieving an adequate clinical response, is established above 6 μg/mL. Finally, the necessity, or not, of a steroid treatment during the study period will also be indicative of successful efficacy with GMA.

Secondary objectives (SO):

1. Quantification of IFX trough and ATI levels, and description of the IFX dose at baseline, 12 and 40 weeks of follow-up.
2. Measurement of PUCAI score at baseline, 12 and 40 weeks of follow-up.
3. Patients maintaining clinical response off steroids, 12 and 40 weeks of follow-up.
4. Patients experiencing flares-ups during the study period.
5. Quantification of fecal calprotectin level at baseline, 12 and 40 weeks of follow-up. Fecal calprotectin is associated with clinical remission with levels higher than 150 μg/g.
6. Measurement of C-reactive protein, erythrocyte sedimentation rate, hemoglobin, albumin, platelet levels at baseline, 12 and 40weeks of follow-up.
7. Quantification of leucocyte counts in peripheral blood at baseline, 12 and 40 weeks of followup.
8. Monitoring of AEs during the study period

Inclusion Criteria:

1. Patients under 18 years of age and weighing ≥25 kg at the time of study initiation.
2. Patients with diagnosis of UC.
3. Patients who started IFX treatment due to the lack of response to corticosteroids following an UC flare-up (steroid-refractory UC).
4. Patients who have received IFX between 12 and 16 weeks prior to the study initiation.
5. Patients who have showed a clinical response to IFX at the time of study initiation (defined as a reduction of at least 15 points in PUCAI score and being maintained below 30 points).
6. Patients with therapeutic IFX blood levels (above 6 μg/mL) at the time of study initiation.
7. Patient´s legal guardian must be willing and able to give written informed consent, and the patient must be willing to give written informed assent (if applicable as determined by the Ethics Committee) and comply with the Study visit Schedule.

Exclusion Criteria:

1. Patients who have received another anti-TNF prior to entry in the study.
2. Patients with a peripheral circulation count of less than 2,000 granulocytes per μL.
3. Pregnant and lactating of childbearing potential patients.
4. Participation in another study or use of any experimental therapy within 30 days before day 1 of Study initiation.