How to Test for Fecal Calprotectin
Fecal calprotectin is measured by collecting a stool sample (typically a spot sample from a single bowel movement) and analyzing it using either enzyme-linked immunosorbent assay (ELISA) or chemiluminescence immunoassay (CLIA) methods, with results typically reported in mg/g or μg/g.
Sample Collection
Practical Collection Method
- Collect a spot stool sample from a single bowel movement—no need for 24-hour collection or multiple samples 1
- Sample size is small (typically a few grams) and can be collected in a standard stool collection container 1
- Timing considerations: In patients with active disease and frequent bowel movements, fecal calprotectin values can vary widely throughout the day (coefficient of variation up to 40-114%) 2. The first morning stool does NOT consistently provide the highest or lowest value 2
- For patients with active ulcerative colitis, avoid basing critical treatment decisions on a single measurement due to high within-day variability 2
Sample Preparation
- Modern extraction methods use:
- Small sample size with high dilution
- Dissociating agents in extraction solution
- Closed disposable tubes to minimize contamination 1
- Extraction yield is approximately 78% of total calprotectin 1
- Calprotectin is extremely stable in feces, making it practical for clinical use 3
Laboratory Analysis Methods
Standard Laboratory Methods
Two primary assay types are used:
ELISA (Enzyme-Linked Immunosorbent Assay)
CLIA (Chemiluminescence Immunoassay)
Point-of-Care and Home Testing
- Rapid lateral flow assays are available for quantitative measurement 6, 4
- Home testing with smartphones (CalproSmart): Patients can perform extraction, apply to lateral flow device, photograph with smartphone, and receive results within seconds 4
Clinical Interpretation Framework
Key Cutoff Values from Guidelines
The AGA guidelines 7 establish three standard cutoffs (allowing ±50 mg/g variation):
- <50 mg/g: Very low inflammation likelihood
- <150 mg/g: Preferred cutoff for ruling out active inflammation in symptomatic remission
- >250 mg/g: High specificity for active inflammation
Context-Specific Interpretation
For UC patients in symptomatic remission 7:
- Fecal calprotectin <150 ± 50 mg/g reliably rules out active inflammation (low certainty evidence)
- Normal fecal calprotectin has high negative predictive value (NPV 89.8-97.3%) 6
For Crohn's disease monitoring 8:
- Similar cutoffs apply (>50, >150, >250 mg/g)
- Useful for detecting endoscopic inflammation and postoperative recurrence
For chronic diarrhea screening 6:
- Optimal cutoff 93.8 μg/g for any colonoscopic abnormality (AUC 0.830)
- Cutoff 118.4 μg/g for colorectal inflammation (AUC 0.896)
- Cutoff 157.0 μg/g specifically for IBD (AUC 0.929)
Important Caveats
Method-Specific Considerations
- Different assays may yield different absolute values 5—use method-specific cutoffs when available
- CLIA methods tend to report slightly higher values than ELISA 5
- Ensure consistency in testing method for serial monitoring
Factors Affecting Results
- No interference from foods or common oral medications 1
- Uneven distribution in stool is possible, but spot measurements are generally reliable 1
- Within-day variability is high in active disease—consider repeat testing if results don't match clinical picture 2
When NOT to Rely on Single Measurement
- Patients with overt active UC and frequent bowel movements 2
- When results conflict with clinical presentation—repeat in 3-6 months 7
- For critical treatment escalation decisions in severe disease 2
Pediatric Considerations
Fecal calprotectin is validated in children with IBD, correlating closely with both colonoscopic (r = 0.75-0.85) and small bowel inflammation on 99Tc scanning (r = 0.80) 3. The same measurement principles apply.