Raised Calprotectin: When You Need a Bowel MRI

Faecal calprotectin is a protein released by white blood cells in the gut lining. When it's raised, it's telling you there's inflammation in the bowel. The question is: what's causing it, and how much of the bowel is affected? Small bowel MRI (MR enterography) is one of the best tools for answering both questions.

What Calprotectin Levels Mean

Calprotectin is measured from a stool sample. The normal range and clinical interpretation:

• Less than 50 µg/g: Normal. Inflammatory bowel disease (IBD) is very unlikely.
• 50–100 µg/g: Borderline. May warrant repeat testing. Could be mild inflammation from any cause.
• 100–250 µg/g: Mildly elevated. Possible IBD, infection, or NSAID-related inflammation. Further investigation usually recommended.
• 250–500 µg/g: Moderately elevated. IBD is increasingly likely. Endoscopy and/or imaging typically indicated.
• Greater than 500 µg/g: Significantly elevated. Active IBD is probable. Investigation should be expedited.
• Greater than 1,000 µg/g: Severely elevated. Active, significant bowel inflammation. Urgent investigation needed.

Calprotectin is excellent at distinguishing inflammatory bowel disease (Crohn's, ulcerative colitis) from irritable bowel syndrome (IBS). IBS doesn't raise calprotectin because it's a functional condition — the gut is misbehaving, but there's no inflammation. A normal calprotectin in someone with bowel symptoms makes IBD very unlikely and can spare them invasive investigations.

Why MRI for the Bowel?

Colonoscopy — a camera examination of the large bowel — is the gold standard for assessing the colon. But colonoscopy can't see the small bowel (the 6 metres of intestine between the stomach and the colon), and Crohn's disease frequently affects the small bowel, particularly the terminal ileum (the last section before the colon).

MR enterography (small bowel MRI) fills this gap. It provides detailed images of the entire small bowel without radiation, showing:

• Wall thickening: Inflamed bowel has a thicker wall than normal. MRI measures this precisely.
• Enhancement pattern: After gadolinium contrast injection, inflamed bowel wall lights up on MRI — the degree and pattern of enhancement indicates how active the inflammation is.
• Strictures: Narrowed sections of bowel, which can occur from chronic inflammation and scarring. MRI shows whether a stricture is inflammatory (potentially treatable with medication) or fibrotic (may need surgery).
• Fistulae: Abnormal connections between the bowel and other structures (skin, bladder, other bowel loops). These are a hallmark of Crohn's disease and require specific treatment.
• Abscesses: Collections of pus near the bowel wall, a complication of penetrating Crohn's disease.
• Skip lesions: Crohn's disease characteristically affects patches of bowel with normal bowel in between. MRI maps the distribution of disease throughout the entire small bowel.
• Mesenteric changes: The mesentery (the tissue connecting the bowel to the body wall) shows specific changes in Crohn's disease — fat wrapping around the bowel (creeping fat), enlarged lymph nodes, and engorged blood vessels.

The Investigation Pathway

When calprotectin is raised and IBD is suspected, the typical investigation sequence is:

1. Repeat calprotectin to confirm it's persistently elevated (not a one-off from a recent infection)
2. Blood tests: CRP, ESR, full blood count, albumin, liver and kidney function, iron studies, vitamin B12, folate — these provide a broader picture of inflammation, nutritional status, and organ function
3. Colonoscopy with biopsies: To assess the colon and terminal ileum directly
4. Small bowel MRI (MR enterography): To assess the rest of the small bowel that colonoscopy can't reach

In practice, colonoscopy and MRI are often requested together when Crohn's disease is suspected, because Crohn's can affect any part of the gut from mouth to anus, and a complete assessment requires both.

MR Enterography: What to Expect

Small bowel MRI requires a bit more preparation than a standard MRI scan:

• Oral preparation: You'll drink about 1–1.5 litres of a special solution (usually mannitol or polyethylene glycol based) over 45–60 minutes before the scan. This distends the small bowel, making it easier to see. The taste is neutral — not pleasant, but manageable.
• Fasting: You'll need to fast for 4–6 hours before the scan.
• Anti-spasmodic injection: You'll usually receive an injection of buscopan (hyoscine butylbromide) to reduce bowel movement during the scan, improving image quality.
• Contrast injection: Gadolinium contrast is injected intravenously during the scan to highlight areas of inflammation.
• Scan duration: Approximately 30–45 minutes in the scanner.

Crohn's Disease vs Ulcerative Colitis

Both are types of inflammatory bowel disease, but they behave differently — and MRI helps distinguish them:

Crohn's disease

• Can affect any part of the GI tract, but most commonly the terminal ileum and colon
• Patchy distribution (skip lesions)
• Full-thickness wall involvement (transmural inflammation)
• Can cause strictures, fistulae, and abscesses
• MRI is essential for assessment because of small bowel involvement

Ulcerative colitis

• Only affects the colon (and sometimes the very end of the small bowel — backwash ileitis)
• Continuous inflammation starting from the rectum and extending proximally
• Superficial inflammation (mucosa and submucosa)
• Doesn't typically cause fistulae or strictures
• Primarily assessed by colonoscopy, but MRI can be useful for assessing disease extent and complications

Monitoring Known IBD

If you've already been diagnosed with Crohn's disease, MRI is used for ongoing monitoring:

• Assessing response to treatment — is the inflammation getting better?
• Detecting complications — new strictures, fistulae, or abscesses
• Pre-surgical planning — mapping exactly where disease is active to guide surgical decisions
• Post-surgical monitoring — checking for recurrence at the surgical join (anastomosis)

Because MRI uses no radiation, it can be repeated as often as needed without cumulative exposure — a significant advantage for young patients who may need monitoring for decades.

Other Causes of Raised Calprotectin

While IBD is the most common cause of significantly raised calprotectin, other conditions can elevate it:

• Infectious gastroenteritis: A recent stomach bug can raise calprotectin temporarily. It should normalise within 4–6 weeks.
• NSAID use: Regular use of anti-inflammatory drugs like ibuprofen can cause gut inflammation and raise calprotectin.
• Coeliac disease: Usually causes mild elevation, not the high levels seen in IBD.
• Colorectal cancer: Can raise calprotectin, though this is typically investigated with colonoscopy rather than MRI.
• Microscopic colitis: Inflammation visible only under the microscope, not on MRI or colonoscopy appearance.

If your calprotectin is raised, the next step is understanding why. Small bowel MRI provides a detailed, radiation-free look at the part of your gut that other investigations can't easily reach — and it's often the test that confirms or rules out Crohn's disease.