Crohn's disease intra-abdominal abscess is a critical complication to identify before starting corticosteroids — approximately 15-20% of patients presenting with what appears to be a Crohn's flare will have an underlying abscess or septic complication (particularly in ileocolonic or post-surgical Crohn's). Starting prednisolone in the presence of an abscess suppresses the immune response to the infection, allowing the abscess to enlarge and potentially perforate — with potentially fatal consequences. The clinical red flags for abscess: localised abdominal mass (particularly right iliac fossa in ileocolonic Crohn's), fever above 38.5°C, significantly elevated CRP above 100 mg/L, or failure to respond to standard flare treatment. CT abdomen with IV contrast is the investigation of choice — it identifies abscesses, fistulae, strictures, and free perforation simultaneously. GPs and gastroenterologists should always consider CT imaging before initiating corticosteroid treatment in any Crohn's patient with fever and abdominal pain.

The perianal fistula in Crohn's disease is one of the most complex and quality-of-life-impairing complications — affecting approximately 25% of Crohn's patients, it causes chronic pain, drainage, sexual dysfunction, and social embarrassment. The management hierarchy requires an MDT approach: (1) examination under anaesthesia (EUA) with flexible sigmoidoscopy — performed by colorectal surgeon to accurately classify the fistula and exclude anorectal cancer; (2) seton suture placement — a non-cutting seton maintains drainage, prevents abscess recurrence, and allows the tract to mature before definitive surgery; (3) MRI pelvis — characterises the entire fistula anatomy including internal opening location, sphincter relationship, and secondary tracts; (4) anti-TNF therapy (infliximab or adalimumab) after EUA and seton — achieves fistula closure in approximately 50-70% of complex perianal Crohn's. GPs must recognise that perianal Crohn's requires specialist surgical + gastroenterological collaboration and should not be managed with antibiotics alone in primary care.

MR enterography (MRE) has replaced CT as the preferred imaging modality for small bowel Crohn's disease — the advantages of MRE over CT: no ionising radiation (extremely important given that Crohn's patients require repeated imaging throughout their lifetime), superior soft tissue characterisation of bowel wall inflammation (mural thickening, enhancement patterns, T2 mural signal), superior detection of fistulae and perienteric inflammation, and ability to assess bowel wall thickness and motility. MRE limitations: longer examination time (approximately 45-60 minutes), requires bowel preparation, and is less widely available out-of-hours than CT. CT abdomen is still indicated for: suspected perforation, suspected abscess (when urgent surgical decision is needed), suspected obstruction (where rapid access is essential), and any unstable patient who cannot tolerate prolonged MR scanning. GPs requesting imaging for Crohn's disease in a stable outpatient setting should request MRE rather than CT if the clinical question is disease extent and activity assessment.

Exclusive enteral nutrition (EEN) as primary induction therapy in paediatric Crohn's is a NICE recommendation that differs fundamentally from UC management — while corticosteroids remain first-line for adults with moderate-severe Crohn's, the British Society of Paediatric Gastroenterology (BSPGHAN) guidelines recommend EEN as the preferred first-line induction therapy for paediatric Crohn's disease. The reasons: corticosteroids cause growth retardation in children (by suppressing IGF-1, reducing linear bone growth), whereas EEN actually promotes growth, improves mucosal healing, and resolves inflammation without these side effects. The practical challenge: EEN requires the child to consume the entire caloric requirement (typically 1500-2500 kcal/day) as formula for 6-8 weeks — this requires significant motivation, dietitian support, and family commitment. NG tube feeding is used for children who cannot manage the required volume orally. The evidence in adults is also strong (ESPEN meta-analysis: EEN equivalent to steroids for remission induction), and EEN is an underutilised adult option.

The Crohn's disease biologic drug monitoring (TDM) for infliximab and adalimumab has become a standard of care in gastroenterology shared care — the principle is that therapeutic outcomes correlate with drug levels, and subtherapeutic levels predict loss of response (secondary failure) before clinical symptoms develop. The target trough levels: infliximab trough >3 µg/mL at week 14 (before the 5th infusion) is associated with sustained remission at 1 year; adalimumab trough >5-8 µg/mL. Loss of response in Crohn's on biologics requires: (1) TDM to measure trough level; (2) anti-drug antibody (ADAb) assay; (3) decision matrix: low level + low/no ADAbs = dose intensify (shorten interval or increase dose); low level + high ADAbs = switch to different anti-TNF (adalimumab if on infliximab, or switch mechanism to vedolizumab/ustekinumab); adequate level + low ADAbs = switch mechanism (pharmacodynamic failure — not pharmacokinetic). GPs receiving Crohn's patients from hospital follow-up should be aware of this framework when escalation is being considered.

The SONIC trial (Study of Biologic and Immunomodulator Naive Patients in Crohn's Disease) established the principle of combination biologic + immunomodulator therapy as superior to either monotherapy in moderate-severe Crohn's — infliximab + azathioprine combination achieved corticosteroid-free remission in approximately 57% of patients at 26 weeks, compared to approximately 44% with infliximab alone and approximately 30% with azathioprine alone. The mechanism: azathioprine reduces the formation of anti-drug antibodies against infliximab (by suppressing B-cell function), maintaining higher trough infliximab levels and longer drug survival. Practically: any patient being started on infliximab for moderate-severe Crohn's should receive combination therapy with azathioprine or mercaptopurine unless there is a specific contraindication (TPMT deficiency, thiopurine intolerance, lymphoma history). Monotherapy with infliximab alone has higher rates of immunogenicity and secondary loss of response.

Post-operative Crohn's disease recurrence prevention is a clinically important area where smoking cessation has the largest single modifiable effect — the postoperative recurrence rate in Crohn's disease is approximately 70-80% endoscopic recurrence at 1 year and 30-40% clinical recurrence within 2-3 years without preventive treatment. Risk factors for early recurrence: smoking (doubles recurrence risk and is the single most impactful modifiable factor), perforating disease (fistulating or abscess history), multiple previous resections, and involved resection margins (R1 margins). Post-resection colonoscopy at 6-12 months (Rutgeerts score) stratifies endoscopic recurrence severity and guides treatment escalation. GPs co-managing post-surgical Crohn's patients must: reinforce smoking cessation at every contact, ensure post-resection prophylactic treatment is prescribed (metronidazole short-term, then azathioprine ± infliximab for high-risk), and arrange colonoscopy surveillance referral.

The methotrexate teratogenicity risk management in Crohn's is one of the most critical prescribing safety responsibilities in gastroenterology shared care — methotrexate is an FDA Category X teratogen (known to cause embryotoxicity and teratogenesis — neural tube defects, craniofacial abnormalities, cardiac defects) and must never be used during pregnancy. The safeguards that must be in place for any patient of childbearing potential on methotrexate: (1) Two forms of effective contraception (e.g., COCP + condom, or IUS) documented in the clinical record at every prescription review; (2) Negative pregnancy test documented before initiation; (3) Clear documented counselling that pregnancy is prohibited on MTX and for 3 months after stopping; (4) For male patients: document counselling that methotrexate impairs spermatogenesis and is contraindicated during active attempts at conception (stop 3 months before planned conception). GPs co-prescribing methotrexate in shared care must have this contraceptive counselling documented at each prescription — this is an NPSA patient safety alert requirement.

The Crohn's disease post-resection recurrence monitoring with colonoscopy at 6-12 months is the Rutgeerts score system — after ileocaecal resection (the most common elective Crohn's surgery), colonoscopy at 6-12 months post-operatively grades recurrence at the neo-terminal ileum and anastomosis using the Rutgeerts score: i0 = no lesions (low recurrence risk — maintain current treatment); i1 = ≤5 aphthous lesions (low-moderate risk — monitor closely); i2 = >5 aphthous lesions with normal mucosa between lesions or lesions confined to anastomosis (moderate risk — consider step-up to thiopurine or biologic); i3 = diffuse aphthous ileitis with diffusely inflamed mucosa (high risk — step up to biologic); i4 = diffuse inflammation with large ulcers, nodules, or strictures (very high risk — biologic essential). GPs should ensure post-resection Crohn's patients have colonoscopy booked at 6-12 months post-surgery as part of the shared care framework.