What Fecal Calprotectin Measures
Fecal calprotectin measures intestinal inflammation by quantifying a neutrophil-derived protein that is released into the stool when neutrophils migrate into gastrointestinal tissue in response to mucosal inflammation. 1, 2
The Biological Basis
Calprotectin is a 36 kDa calcium- and zinc-binding protein from the S100 family that comprises approximately 60% of the cytoplasmic protein content of neutrophils 3. When intestinal inflammation occurs, neutrophils infiltrate the gastrointestinal mucosa and release calprotectin, which then appears in stool at concentrations roughly six times higher than plasma levels 3. The amount of fecal calprotectin directly correlates with the degree of mucosal inflammation present 4, 5.
Clinical Interpretation Framework
The test functions as a surrogate marker of gastrointestinal inflammation with specific performance characteristics that vary by clinical context 1, 2:
In Inflammatory Bowel Disease (IBD)
For Crohn's Disease: At a cutoff of 150 ± 50 mg/g, fecal calprotectin demonstrates 81% sensitivity and 72% specificity for detecting endoscopically active disease (SES-CD score ≥3) 1
For Ulcerative Colitis: At the same 150 ± 50 mg/g cutoff, sensitivity is 71% and specificity is 69% for detecting moderate to severe endoscopic inflammation (Mayo Endoscopic Subscore 2-3) 2
Key Clinical Applications
Ruling out inflammation (high negative predictive value): A level <150 mg/g in asymptomatic patients reliably excludes active inflammation, making it particularly useful to avoid unnecessary endoscopy 1, 2. This is most effective in patients with symptomatic remission where the pretest probability of inflammation is low (approximately 15-20%) 1.
Ruling in inflammation (moderate positive predictive value): Levels >150 mg/g in patients with moderate to severe symptoms suggest active inflammation with high specificity, though the positive predictive value remains modest (12.8% in older adults, 20.9% in younger patients for IBD diagnosis) 6.
Predicting relapse: Elevated calprotectin (>200-300 mg/g) in patients with symptomatic remission increases the risk of clinical relapse at 12 months by nearly 5-fold (RR 4.79,95% CI 2.81-8.17) 1.
Important Caveats
Not specific to IBD: Elevated fecal calprotectin occurs in any condition causing intestinal inflammation, including infectious enteritis, colorectal cancer, NSAID-induced enteropathy, and even following certain gastrointestinal surgeries 4, 7. Interpretation must always incorporate clinical context.
Age considerations: While guidelines often discourage use in older adults due to higher colorectal cancer rates, recent data show fecal calprotectin maintains high sensitivity (93.8%) for IBD in patients ≥50 years, though with lower positive predictive value 6.
Symptom-dependent performance: The test performs differently based on symptom severity. In patients with mild symptoms, normal calprotectin cannot reliably rule out active inflammation, whereas in asymptomatic patients it effectively excludes disease 2.
The fundamental principle: fecal calprotectin quantifies neutrophilic intestinal inflammation, not the underlying cause, making it a sensitive but non-specific marker that requires clinical correlation for proper interpretation 4, 3, 8.