IBD: Crohn’s Disease and Ulcerative Colitis

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GI NOTES

IBD: Crohn’s Disease and Ulcerative Colitis

Key points

Ulcerative colitis (UC) is continuous mucosal inflammation starting in the rectum. Crohn’s disease (CD) is transmural, patchy, and can affect any part of the GI tract.
The treatment target is endoscopic improvement or mucosal healing, not just symptom relief. Fecal calprotectin helps monitor between scopes.
Mild to moderate UC: 5-ASA (rectal for proctitis and left-sided disease, oral for extensive colitis). Mesalamine is not recommended for Crohn’s disease (ACG 2025).
Moderate to severe disease: start an advanced therapy early; there is no need to fail thiopurines first. Steroids are for induction only.
Acute severe UC is an emergency: IV steroids, C. difficile testing, VTE prophylaxis, early sigmoidoscopy, and rescue therapy by day 3 if no response.

1. UC vs Crohn’s disease

	Ulcerative colitis	Crohn’s disease
Location	Rectum, extending proximally in a continuous pattern	Anywhere from mouth to anus; terminal ileum most common; rectal sparing common
Depth	Mucosal	Transmural: strictures, fistulas, abscesses
Typical symptoms	Bloody diarrhea, urgency, tenesmus	Abdominal pain, diarrhea, weight loss; perianal disease
Histology	Crypt abscesses, crypt distortion	Non-caseating granulomas (in a minority), patchy inflammation
Smoking	Protective (but never advised)	Worsens disease; stopping is a key treatment
Surgery	Colectomy is curative for colitis	Not curative; disease often recurs at the anastomosis

2. Work-up

Labs: CBC, metabolic panel, albumin, CRP, iron studies, vitamin B12 and D; fecal calprotectin.
Stool: C. difficile and enteric pathogens at presentation and with every flare.
Endoscopy: ileocolonoscopy with biopsies from each segment (including normal-looking mucosa). Upper endoscopy in CD with upper GI symptoms.
Imaging for CD: CT or MR enterography, or intestinal ultrasound; MRI pelvis for perianal disease.
Before advanced therapy: TB test (IGRA), hepatitis B and C, HIV, varicella immunity, vaccines up to date (no live vaccines once immunosuppressed), TPMT or NUDT15 before thiopurines.

3. Treatment by severity

FIGURE: TREATMENT LADDER IN ULCERATIVE COLITIS

Confirm UC; exclude infection

Mild to moderate: rectal and/or oral 5-ASA; budesonide MMX if needed

Moderate to severe: advanced therapy (biologic, JAK inhibitor or S1P modulator), steroids only as a bridge

Acute severe: admit, IV steroids, rescue infliximab or cyclosporine, surgery

Reassess at about 8 weeks (symptoms, calprotectin), then endoscopy to confirm healing

Original diagram based on the ACG 2025 ulcerative colitis guideline.

Drug class	Examples	Use	Key safety points
5-ASA	Mesalamine (rectal 1 g/day or more; oral 2 g/day or more)	UC induction and maintenance	Rare nephritis: check creatinine. Not for Crohn’s.
Corticosteroids	Prednisone, budesonide (MMX for UC, ileal-release for ileocecal CD)	Induction only (under 3 months)	Never for maintenance; bone, glucose, infection, mood
Anti-TNF	Infliximab, adalimumab, golimumab (UC), certolizumab (CD)	UC and CD; infliximab first line for perianal CD	TB and hepatitis B reactivation, infections, lymphoma (with thiopurines)
Anti-integrin	Vedolizumab	UC and CD	Gut-selective; favorable safety
IL-12/23 and IL-23 inhibitors	Ustekinumab; risankizumab, mirikizumab, guselkumab	UC and CD	Favorable safety; risankizumab can cause liver enzyme rise
JAK inhibitors	Tofacitinib (UC), upadacitinib (UC and CD)	Oral, rapid onset	Herpes zoster, VTE, lipids, cardiovascular risk in older smokers
S1P receptor modulators	Ozanimod, etrasimod	UC	Bradycardia, macular edema; check ECG and eye exam as indicated
Immunomodulators	Azathioprine, 6-MP, methotrexate	Combination with infliximab; steroid-sparing maintenance	Cytopenias, hepatotoxicity, pancreatitis, lymphoma, skin cancer

Infliximab works better combined with a thiopurine. In anti-TNF loss of response, check drug levels and antibodies; primary non-responders should switch to a different drug class.

4. Acute severe ulcerative colitis

Truelove-Witts criteria: 6 or more bloody stools a day plus at least one of heart rate above 90, temperature above 37.8 C, hemoglobin below 10.5 g/dL, or ESR above 30 mm/h.

Admit; stool for C. difficile; pharmacologic VTE prophylaxis (bleeding is not a contraindication).
IV methylprednisolone 60 mg/day (or hydrocortisone 100 mg three to four times daily).
Flexible sigmoidoscopy with biopsies within 72 hours (ideally 24) to assess severity and exclude CMV. Avoid full colonoscopy.
Avoid opioids, anticholinergics and NSAIDs (risk of toxic megacolon). Routine antibiotics and TPN are not needed.
Day 3: if not responding, start rescue infliximab or cyclosporine. Consider higher-dose infliximab if albumin is low.
Involve colorectal surgery early: toxic megacolon, perforation, massive bleeding, or failure of rescue therapy means colectomy.

5. Crohn’s disease specifics

Perianal disease: MRI pelvis and exam under anesthesia; drain abscesses and place setons before starting an anti-TNF. Infliximab is first line; other advanced therapies are reasonable alternatives.
Strictures: inflammatory strictures may respond to medical therapy; short fibrotic strictures can be balloon-dilated; longer ones may need strictureplasty or resection.
After resection: colonoscopy 6 to 12 months later to check for recurrence (Rutgeerts score). High-risk patients (smokers, penetrating disease, prior resection) may start preventive therapy.
Smoking cessation is one of the most effective interventions in CD.

6. Health maintenance

Colorectal cancer surveillance: colonoscopy starting about 8 years after diagnosis for colitis involving more than the rectum, then every 1 to 3 years depending on risk. With primary sclerosing cholangitis, start at diagnosis and repeat yearly.
Vaccines: inactivated vaccines (influenza, pneumococcal, recombinant zoster, hepatitis B, COVID-19, HPV); avoid live vaccines while immunosuppressed.
Other: annual skin checks on thiopurines, bone health on steroids, cervical cancer screening, screening for anxiety and depression, iron deficiency and anemia.

7. High-yield pearls

Always test for C. difficile in an IBD flare; it is common and changes management.
Steroid dependence (cannot taper within about 3 months) means the maintenance plan is failing.
Fecal calprotectin above about 100 to 150 mcg/g suggests active inflammation; a normal value makes active IBD unlikely.
Extraintestinal manifestations: arthritis, uveitis, episcleritis, erythema nodosum, pyoderma gangrenosum, PSC (most often with UC).

Self-check questions

Is mesalamine recommended for maintenance of Crohn’s disease?
A patient with UC has 8 bloody stools a day, heart rate 110 and hemoglobin 9.8. What is this, and what are the first steps?
What must be done before starting an anti-TNF?
When should colorectal cancer surveillance begin in extensive UC?

Answers: (1) No; the 2025 ACG guideline discourages mesalamine for luminal Crohn’s disease. (2) Acute severe UC: admit, C. difficile test, VTE prophylaxis, IV methylprednisolone, sigmoidoscopy within 72 hours, and rescue therapy on day 3 if no response. (3) Screen for TB and hepatitis B (and C, HIV), and update vaccines. (4) About 8 years after diagnosis (at diagnosis if PSC is present).

References

Rubin DT, Ananthakrishnan AN, Siegel CA, Barnes EL, Long MD. ACG Clinical Guideline Update: Ulcerative Colitis in Adults. Am J Gastroenterol. 2025;120(6):1187-1224.
Lichtenstein GR, Loftus EV Jr, Isaacs KL, et al. ACG Clinical Guideline: Management of Crohn’s Disease in Adults. Am J Gastroenterol. 2025;120(6):1225-1264.
Turner D, Ricciuto A, Lewis A, et al. STRIDE-II: an update on the Selecting Therapeutic Targets in Inflammatory Bowel Disease initiative of the IOIBD. Gastroenterology. 2021;160(5):1570-1583.
Murthy SK, Feuerstein JD, Nguyen GC, Velayos FS. AGA Clinical Practice Update on Endoscopic Surveillance and Management of Colorectal Dysplasia in Inflammatory Bowel Diseases. Gastroenterology. 2021;161(3):1043-1051.

For educational purposes only. This note does not replace clinical judgment or local protocols. Check doses against current labeling and institutional guidance.