Overview.
Inflammatory bowel disease (IBD) — comprising Crohn's disease and ulcerative colitis — affects approximately 0.3% of the Western population and is increasing globally. While endoscopy remains the diagnostic gold standard, non-invasive markers now play a central role in screening, monitoring treatment response, and predicting relapse. Fecal calprotectin is the single most valuable non-invasive marker, distinguishing IBD from IBS with high accuracy and tracking mucosal inflammation without repeated endoscopy.
IBD is a group of chronic, relapsing inflammatory disorders of the GI tract. Crohn's disease can affect any part of the GI tract (mouth to anus) with transmural, skip-lesion inflammation. Ulcerative colitis affects only the colon and rectum with continuous mucosal inflammation starting at the rectum. Both are mediated by dysregulated immune responses to the gut microbiome in genetically susceptible individuals. Lab markers track the inflammatory burden and treatment effectiveness without repeated invasive procedures.
Prevalence: IBD affects approximately 6.8 million people globally. Prevalence is 0.3% in Western countries and rising in newly industrialised nations. Crohn's and UC are roughly equally prevalent. Peak onset is 15-30 years, with a second smaller peak at 50-70 years.
Medical name: Inflammatory Bowel Disease — Biomarker Assessment
Symptoms.
Early warnings
- Chronic diarrhoea lasting >4 weeks
- Rectal bleeding or blood in stool
- Abdominal pain with weight loss
- Unexplained iron deficiency anaemia
- Elevated fecal calprotectin (>250 μg/g)
- Perianal disease (fistulae, abscesses) — strongly suggests Crohn's
Classic symptoms
- Chronic diarrhoea (often bloody in UC; may not be bloody in Crohn's)
- Abdominal pain and cramping
- Rectal urgency and tenesmus (UC)
- Weight loss and malnutrition
- Fatigue
- Perianal fistulae and abscesses (Crohn's)
- Extraintestinal manifestations: joint pain (arthritis), eye inflammation (uveitis), skin lesions (erythema nodosum, pyoderma gangrenosum)
- Nocturnal diarrhoea (distinguishes organic from functional cause)
Progression
IBD follows a relapsing-remitting course. Uncontrolled inflammation leads to complications: strictures (Crohn's), fistulae (Crohn's), toxic megacolon (UC), colorectal cancer (both, with UC > Crohn's colitis). Modern treat-to-target strategies aim for endoscopic and histological remission, using biomarkers to guide escalation before symptoms worsen. Calprotectin rising above 250 μg/g often precedes clinical relapse by weeks.
Risk factors.
- Family history of IBD (first-degree relative = 5-20x risk)
- Ashkenazi Jewish heritage
- Smoking (increases Crohn's risk; paradoxically may be protective in UC)
- History of appendectomy (protective for UC in some studies)
- Urban living and Western diet
- NSAID use (can trigger flares)
- Previous enteric infection
Lab interpretation.
Key biomarkers
- Fecal Calprotectin — Markedly elevated (>250 μg/g) in active IBD; <50 μg/g effectively rules out intestinal inflammation; used for screening and monitoring (primary)
- CRP / hs-CRP — Elevated in active IBD, especially Crohn's; less sensitive than calprotectin; normal CRP does not exclude active mucosal inflammation (primary)
- ESR — Elevated in active IBD; slower to change than CRP; useful for tracking chronic inflammation trends (secondary)
- Hemoglobin — Often low (anaemia of chronic disease + iron deficiency from GI blood loss) (secondary)
- Ferritin — Low in iron deficiency; may be falsely normal/elevated as an acute-phase reactant during active inflammation (secondary)
- Albumin — Low in severe IBD (protein-losing enteropathy, malnutrition, inflammation) (supportive)
Diagnostic criteria
- Fecal calprotectin <50 μg/g: IBD very unlikely (NPV >95%) — supports IBS diagnosis
- Fecal calprotectin 50-250 μg/g: indeterminate — repeat in 4-6 weeks or consider endoscopy if clinically suspicious
- Fecal calprotectin >250 μg/g: strongly suggests intestinal inflammation — endoscopy referral recommended
- Endoscopy with biopsy remains the gold standard for IBD diagnosis and classification
- CRP/ESR support but do not replace calprotectin — CRP can be normal in active mucosal inflammation, especially in UC
Recommended panels
When & next steps.
When to test
- Chronic diarrhoea (>4 weeks) to distinguish IBD from IBS
- Known IBD: monitoring treatment response and predicting relapse
- Before and after medication changes in IBD
- Rectal bleeding or blood in stool (after excluding haemorrhoids)
- Unexplained iron deficiency anaemia with GI symptoms
- Family history of IBD with persistent GI symptoms
If suspected
- Order fecal calprotectin (stool sample — not a blood test)
- Concurrent blood work: CBC, CRP, ESR, CMP (albumin), iron studies
- If calprotectin >250 μg/g → gastroenterology referral for endoscopy
- If calprotectin <50 μg/g → IBD effectively excluded; consider IBS workup
If confirmed
- Endoscopy with biopsy for disease classification (Crohn's vs UC), extent, and severity
- Treat-to-target: aim for endoscopic remission, not just symptom control
- Monitor with calprotectin every 3-6 months; rising calprotectin often precedes relapse
- Iron studies and supplementation — iron deficiency is the most common nutritional complication
- Bone density assessment (corticosteroid use + malabsorption)
- Colorectal cancer surveillance colonoscopy starting 8 years after diagnosis in extensive colitis
FAQs.
Can blood tests alone diagnose IBD?
No. Blood tests (CRP, ESR, CBC) support the diagnosis and monitor activity, but endoscopy with biopsy is required for definitive diagnosis. Fecal calprotectin (a stool test) is the best non-invasive screening tool, but a positive result still needs endoscopic confirmation.
What's the difference between IBS and IBD in lab tests?
Fecal calprotectin is the key differentiator. IBD causes calprotectin >250 μg/g (mucosal inflammation). IBS has normal calprotectin (<50 μg/g) — it's a functional disorder without structural inflammation. CRP/ESR are also typically normal in IBS.
How often should calprotectin be checked in known IBD?
Every 3-6 months during stable remission. After medication changes, recheck in 8-12 weeks. A rising calprotectin (from baseline normal to >250 μg/g) often predicts clinical relapse 2-3 months in advance, allowing pre-emptive treatment escalation.
Can IBD affect organs outside the gut?
Yes — up to 40% of IBD patients have extraintestinal manifestations: inflammatory arthritis (most common), eye inflammation (uveitis, episcleritis), skin conditions (erythema nodosum, pyoderma gangrenosum), liver disease (primary sclerosing cholangitis, especially in UC), and osteoporosis.