Overview.
Inflammatory bowel disease (IBD) encompasses two main conditions — Crohn's disease and ulcerative colitis — characterized by chronic relapsing inflammation of the gastrointestinal tract. IBD affects approximately 6.8 million people worldwide and is a lifelong condition requiring ongoing monitoring. The key advance in recent years is the validation of non-invasive biomarkers — particularly fecal calprotectin and CRP — for monitoring disease activity and distinguishing IBD from irritable bowel syndrome (IBS), reducing the need for repeated endoscopy. The 2023 AGA biomarker guidelines formally established the role of these tests in IBD management.
Crohn's disease (CD) can affect any part of the GI tract from mouth to anus, causing transmural (full-thickness) inflammation with a characteristic skip pattern. Ulcerative colitis (UC) is limited to the colon and rectum, causing continuous mucosal inflammation beginning at the rectum. Both conditions involve dysregulated immune responses to intestinal microbiota in genetically susceptible individuals, with environmental triggers. The inflammation of BOTH conditions is driven by Th1/Th17-mediated pathways, and both respond to similar classes of medications (aminosalicylates, corticosteroids, immunomodulators, biologics).
Prevalence: IBD affects ~6.8 million people globally. In North America: Crohn's disease ~150-200 per 100,000; ulcerative colitis ~200-250 per 100,000. Peak onset: 15-30 years (with a second peak at 50-70 years). Incidence is rising in newly industrialized countries. IBD is more prevalent in industrialized nations; the 'hygiene hypothesis' and western diet are implicated in rising global incidence.
Medical name: Crohn's Disease and Ulcerative Colitis
Symptoms.
Early warnings
- Persistent diarrhoea lasting more than 4 weeks, especially with blood or mucus
- Recurrent abdominal pain or cramping that is not relieved by bowel movements
- Unexplained weight loss or poor growth in children
- Fatigue out of proportion to other symptoms
- Perianal disease (pain, discharge, skin tags, fissures) — hallmark of Crohn's
Classic symptoms
- Crohn's disease: right lower quadrant pain, non-bloody or semi-formed diarrhoea, perianal disease, mouth ulcers, weight loss
- Ulcerative colitis: bloody diarrhoea (cardinal symptom), rectal urgency and tenesmus, mucus in stool, lower abdominal cramping
- Extraintestinal manifestations (both): peripheral arthropathy, sacroiliitis, erythema nodosum, pyoderma gangrenosum, uveitis
- Primary sclerosing cholangitis (PSC): associated in ~5% of IBD patients, more strongly with UC
Progression
IBD is a relapsing-remitting condition. Without adequate treatment, repeated episodes of inflammation cause cumulative bowel damage. In Crohn's disease, this leads to strictures (bowel narrowing), fistulae, abscesses, and eventual bowel resection in 60-80% of patients over 20 years. In UC, cumulative mucosal inflammation increases colorectal cancer risk — a 5-10x increase in patients with long-standing extensive colitis, warranting regular surveillance colonoscopy. Biological therapy has significantly improved remission rates and reduced complications and surgery.
Risk factors.
- Family history of IBD (first-degree relative: 10-25% risk)
- Personal or family history of other autoimmune conditions
- Smoking: doubles the risk of Crohn's disease; protective in ulcerative colitis
- Prior appendectomy (protective in UC, not CD)
- Early life antibiotic exposure
- Western dietary pattern: high processed food, low fibre
- NSAID overuse (can trigger flares)
- Stress (triggers flares; not a direct cause)
Lab interpretation.
Key biomarkers
- Fecal Calprotectin (FC) — Elevated in active IBD (>150-200 μg/g indicates mucosal inflammation); low in IBS (<50 μg/g); NOTE: this is a stool test, not a blood test (primary)
- CRP (C-Reactive Protein) — Elevated in active inflammation; more sensitive in Crohn's than UC; used alongside fecal calprotectin for monitoring (primary)
- ESR (Erythrocyte Sedimentation Rate) — Elevated in active disease; less sensitive than CRP; can be elevated by anaemia (secondary)
- CBC (Complete Blood Count) — Anaemia (iron deficiency or anaemia of chronic disease), reactive thrombocytosis (platelets >450 × 10⁹/L) suggests active inflammation, leucocytosis in severe disease (secondary)
- Albumin — Hypoalbuminaemia indicates malnutrition or severe active disease; important prognostic marker (secondary)
- Ferritin / Iron Studies — Iron deficiency common from GI blood loss and malabsorption; ferritin may appear falsely normal/elevated as acute phase reactant during active disease (secondary)
Diagnostic criteria
- Initial diagnosis requires endoscopy with biopsy — IBD cannot be diagnosed from blood tests alone
- Histopathology: chronic active colitis (UC) or transmural granulomatous inflammation (CD)
- Fecal calprotectin: primary non-invasive test to differentiate IBD from IBS; sensitivity 85.8%, specificity 91.7% (Dajti et al., 2023; PMID: 37823411)
- CRP and ESR: support active inflammation assessment; elevated in active IBD but not in IBS
- Disease monitoring in known IBD: biomarker-based strategy (FC + CRP) can defer routine endoscopy in patients in clinical remission (AGA 2023 guidelines)
Recommended panels
When & next steps.
When to test
- Chronic diarrhoea lasting more than 4 weeks, especially with blood or mucus
- Known IBD — routine monitoring of disease activity and treatment response
- Distinguishing IBD from IBS in patients with chronic GI symptoms (fecal calprotectin is key)
- Unexplained iron deficiency anaemia with GI symptoms
- Abdominal pain with weight loss or systemic symptoms
- Before and after treatment initiation or dose adjustment to assess response
- Perianal symptoms (fissures, fistulae, skin tags) — evaluate for Crohn's disease
If suspected
- Order fecal calprotectin (stool test) to differentiate IBD from IBS
- Order: CBC, CMP, CRP, ESR, ferritin, albumin, B12 (for Crohn's), folate
- If fecal calprotectin elevated (>150 μg/g) + compatible symptoms: refer to gastroenterologist for colonoscopy with biopsies
- Stool cultures and C. difficile toxin to exclude infectious colitis before assuming IBD
- Do not start steroids or immunomodulators before diagnostic endoscopy — they alter biopsy results
If confirmed
- Treatment based on disease location, severity, and phenotype (inflammatory vs stricturing vs penetrating for CD)
- Aminosalicylates (5-ASA): first-line for mild-moderate UC; limited role in CD
- Corticosteroids: for induction of remission in moderate-severe disease (not for maintenance)
- Immunomodulators (azathioprine, 6-MP, methotrexate): for maintenance of remission
- Biological therapy (anti-TNF, anti-integrin, anti-IL-12/23 agents): for moderate-severe IBD and steroid-dependent/refractory disease
- Monitor fecal calprotectin and CRP every 6-12 months in remission; more frequently during flares
- UC: colorectal cancer surveillance colonoscopy every 1-3 years after 8-10 years of extensive disease
- CD: annual B12, folate, vitamin D; DXA scan if on long-term steroids
FAQs.
Can blood tests diagnose IBD?
Blood tests alone cannot diagnose IBD. Colonoscopy with biopsies is required for initial diagnosis — histopathology showing chronic inflammation is the gold standard. However, blood tests (CRP, CBC, albumin) and the stool test fecal calprotectin are very useful for assessing disease severity, monitoring treatment response, and distinguishing IBD from IBS once a diagnosis has been established. The 2023 AGA guidelines support using fecal calprotectin and CRP to defer routine endoscopy in patients who are in clinical remission.
How is IBD different from IBS?
IBD (Crohn's disease and ulcerative colitis) involves measurable inflammation and structural damage to the bowel wall — it can be seen on endoscopy and on blood/stool tests. IBS (irritable bowel syndrome) is a functional gut disorder where the bowel works differently but shows no visible inflammation. The key distinguishing test is fecal calprotectin: it is elevated in IBD (>150 μg/g) and normal in IBS (<50 μg/g). CRP and CBC are also typically abnormal in active IBD but normal in IBS.
What is fecal calprotectin and why does it matter?
Fecal calprotectin is a protein released by neutrophils (immune cells) when there is inflammation in the bowel lining. Because it is excreted in stool, it directly reflects mucosal (gut lining) inflammation. It is the most accurate non-invasive test for distinguishing intestinal inflammation (IBD) from functional bowel symptoms (IBS), with 85.8% sensitivity and 91.7% specificity. It is also used to monitor disease activity, predict relapse, and assess treatment response in known IBD patients.
Do I need a colonoscopy every year for IBD?
Not necessarily. The 2023 AGA guidelines support a biomarker-based monitoring strategy: if you are in symptomatic remission AND your fecal calprotectin is below 150 μg/g AND CRP is normal, routine endoscopic assessment can be deferred. Monitoring every 6-12 months with stool and blood tests is the approach for stable patients. However, if you have had extensive UC for more than 8-10 years, surveillance colonoscopy every 1-3 years is recommended for colorectal cancer screening.