Effect of Laxatives on Fecal Calprotectin Testing
Glycerin enemas and other laxatives do not significantly interfere with fecal calprotectin test results and can be used when stool collection is needed. While the AGA guidelines acknowledge various sources of test variability, laxative use is not identified as a confounding factor that affects calprotectin measurement 1.
Key Factors That DO Affect Calprotectin Results
The 2023 AGA guidelines on biomarkers for ulcerative colitis management extensively reviewed factors influencing fecal calprotectin accuracy, but laxatives are notably absent from this list 1.
Documented Sources of Variability
Inter- and intra-assay variability represents the primary concern for test reliability:
- Between different assays: 2.5- to 5-fold differences can occur when testing the same stool sample, though this variability is reduced at lower calprotectin ranges (which are most clinically relevant) 1
- Within the same patient: 13% to 114% variation using the same assay on different stool samples from the same patient 1
- Within the same stool sample: 3% to 31% variation when testing different regions 1
- At lower calprotectin levels: Only 8-9% variation within samples, compared to 18-33% at higher levels 1
Disease-related factors that genuinely affect test performance:
- Disease extent and location: Calprotectin correlates more strongly with extent of inflamed surface (r = 0.86) than severity alone 1
- Proctitis: Weaker performance (r = 0.54) compared to left-sided colitis (r = 0.75) or extensive colitis (r = 0.78) 1
- Isolated proctitis: Calprotectin may be unable to accurately identify active disease 1
Clinical Implications for Test Ordering
Use the same assay consistently for a given patient when comparing results over time, as calprotectin assays are not interchangeable 1.
If results are borderline or unexpected, repeat testing or proceed to endoscopic evaluation for confirmation, given the substantial within-stool and within-day variations 1.
Other conditions that elevate calprotectin (and should be considered in interpretation):
Common Pitfalls to Avoid
Do not attribute elevated calprotectin to laxative use as a reason to dismiss the result—this is not a documented source of interference 1.
Do not delay stool collection due to concerns about laxative interference, as obtaining an adequate sample is more important than avoiding laxatives 4, 5.
Do recognize that confidence in any single measurement may be limited due to natural biological variation, particularly if the result is unexpected or borderline 1.