• Proportion of patients with response at week 12, i.e. CDAI decrease of 70 or more for patients with luminal disease, or reduction of 50 percent or more from base line in the number of draining fistulas for patients with fistulising disease. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Total expenses during the study. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]

• Mean change compared to baseline in WPAI score at week 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Mean change compared to baseline in IBDQ score at week 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Mean change compared to baseline in CDAI score at week 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Mean change compared to baseline in PDAI score at week 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]
• Proportion of patients in remission (CDAI less than 150 for luminal disease, closure of all fistulas for fistulising disease) at week 12. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]

To compare treatment outcome in patients with Crohn's disease with secondary loss of response to infliximab (i.e. initial good response follow by loss of response) treated according to current standards based only on clinical features versus treatment based on serum levels of infliximab and anti-infliximab antibody (Ab) status.

• Procedure: Measurement of serum infliximab and anti-infliximab antibodies
• Procedure: Treatment according to current standards without knowledge of serum infliximab and anti-infliximab Ab status

• Ainsworth MA, Bendtzen K, Brynskov J. Tumor necrosis factor-alpha binding capacity and anti-infliximab antibodies measured by fluid-phase radioimmunoassays as predictors of clinical efficacy of infliximab in Crohn's disease. Am J Gastroenterol. 2008 Apr;103(4):944-8. Epub 2007 Nov 19.
• Bendtzen K, Ainsworth M, Steenholdt C, Thomsen OO, Brynskov J. Individual medicine in inflammatory bowel disease: Monitoring bioavailability, pharmacokinetics and immunogenicity of anti-tumour necrosis factor-alpha antibodies. Scand J Gastroenterol. 2009 Jan 13;:1-8 [Epub ahead of print]
• Bendtzen K, Geborek P, Svenson M, Larsson L, Kapetanovic MC, Saxne T. Individualized monitoring of drug bioavailability and immunogenicity in rheumatoid arthritis patients treated with the tumor necrosis factor alpha inhibitor infliximab. Arthritis Rheum. 2006 Dec;54(12):3782-9.
• Svenson M, Geborek P, Saxne T, Bendtzen K. Monitoring patients treated with anti-TNF-alpha biopharmaceuticals: assessing serum infliximab and anti-infliximab antibodies. Rheumatology (Oxford). 2007 Dec;46(12):1828-34.

Inclusion Criteria:

1. Patient must be able to understand the information given to him/her and give written informed consent.
2. Definitive diagnosis of Crohn's disease (confirmed by recent radiological, endoscopic and/or histological evidence according to international criteria) .
3. Age between 18 and 65 years.
4. Previous good response to at least 3 doses (5mg/kg) of infliximab (as judged by the treating physician).
5. Loss of response to standard doses of infliximab (as judged by the treating physician).
6. Last infliximab infusion given at least 4 weeks before inclusion.
7. For patients with luminal disease, the CDAI should be above 220 points at inclusion.
8. For patients with fistulising disease only, at least one draining perianal fistula (confirmed by radiography, MR, ultrasound or physical examination) should be present.

Exclusion Criteria:

1. Any contraindication to continued infliximab treatment
2. Short bowel syndrome
3. Bowel resection within 12 weeks of inclusion.
4. Current or recent history of severe, progressive, and/or uncontrolled renal, hepatic, haematological, gastrointestinal, endocrine, pulmonary, cardiac, neurological, or cerebral disease.
5. Pregnancy
6. History of alcohol or drug abuse within the prior year
7. Patients who do not meet concomitant medication criteria.
8. Any other condition, which in the Investigator's judgment would make the patient unsuitable for inclusion in the study.

• Crohn-Colitis Foreningen
• Det Strategiske Forskningsråd