What Is Calprotectin?
Calprotectin is a calcium- and zinc-binding protein that makes up roughly 60% of the cytoplasmic protein content of neutrophils — the white blood cells that rush to sites of infection or inflammation. When neutrophils migrate into the intestinal lumen (the inside of the gut), they release calprotectin into the stool. A fecal calprotectin (FC) test measures the concentration of this protein in a stool sample, providing a non-invasive proxy for intestinal neutrophilic inflammation.
Why Clinicians Use It
The principal clinical use of fecal calprotectin is to distinguish inflammatory bowel disease (IBD) from irritable bowel syndrome (IBS) without requiring colonoscopy. Meta-analyses show that FC has a pooled sensitivity of approximately 93% and specificity of 94% for detecting IBD in adults with chronic gastrointestinal symptoms. Because IBS does not involve significant neutrophilic infiltration, FC levels in IBS patients typically remain below 50 µg/g — the conventional cut-off for a negative result.
Interpreting the Numbers
Most laboratories report fecal calprotectin using three bands:
Less than 50 µg/g: Normal. IBD is unlikely. IBS or other non-inflammatory conditions are more probable.
50–200 µg/g: Borderline. A repeat test after 4–6 weeks is often recommended. Mild elevations can occur with NSAID use, infections, and colorectal polyps.
Greater than 200 µg/g: Elevated. Further investigation (usually colonoscopy with biopsies) is warranted to evaluate for IBD, microscopic colitis, or other mucosal inflammation.
What the Test Does Not Tell You
Fecal calprotectin is excellent at detecting the presence of neutrophilic inflammation, but it cannot specify the cause. Elevated levels occur in IBD, infectious colitis, NSAID enteropathy, diverticulitis, and colorectal cancer. It is also less reliable in the upper GI tract — small-bowel Crohn's disease can produce misleadingly low readings if the ileal disease is mild or fibrotic rather than actively neutrophilic.
Age matters too. Infants and young children normally have higher fecal calprotectin levels, making adult reference ranges inappropriate for paediatric populations.
Monitoring, Not Just Diagnosis
Beyond initial diagnosis, serial FC measurements are increasingly used to monitor IBD disease activity, guide treat-to-target strategies, and predict relapse. A rising FC trend — even while symptoms remain stable — may signal subclinical mucosal inflammation, prompting pre-emptive treatment escalation before a clinical flare occurs.