In a reproductive‑age woman with known bowel endometriosis, can the disease cause an elevated fecal calprotectin level?

Can Bowel Endometriosis Cause Elevated Fecal Calprotectin?

Yes, bowel endometriosis can theoretically cause elevated fecal calprotectin, but this is not a reliable or specific finding for endometriosis, and an elevated level should prompt evaluation for inflammatory bowel disease (IBD) rather than being attributed to endometriosis alone.

Key Clinical Reasoning

Fecal calprotectin is a neutrophil-derived protein that reflects mucosal inflammation in the gastrointestinal tract 1. While endometriosis involving the bowel wall can cause inflammation 2, 3, the evidence does not support using fecal calprotectin as a diagnostic or monitoring tool for endometriosis.

Critical Evidence Points

Lack of Correlation with Endometriosis:

• A direct study examining serum calprotectin levels found no correlation between calprotectin and the severity of endometriosis (r = -0.16, p = 0.278) 4. While this studied serum rather than fecal calprotectin, it demonstrates that calprotectin does not reliably track with endometriosis disease burden.

Specificity Problem:

• Fecal calprotectin is elevated in conditions causing mucosal inflammation, particularly IBD (ulcerative colitis and Crohn's disease), infectious enteritis, and colorectal cancer 1. The AGA guidelines extensively validate fecal calprotectin for IBD monitoring, with cutoffs of >150 mg/g indicating active intestinal inflammation 5, 6.

Important Clinical Overlap:

• Women can have both endometriosis and IBD, and this co-diagnosis is associated with profound gut dysbiosis and more severe disease manifestations 7. Bowel endometriosis can mimic IBD clinically, presenting with abdominal pain, altered bowel habits, and gastrointestinal symptoms 2, 3, 8.

Clinical Algorithm for Elevated Fecal Calprotectin in Women with Known Bowel Endometriosis

1. If fecal calprotectin is >150 mg/g:

   • Do NOT attribute this to endometriosis alone
   • Pursue colonoscopy to evaluate for IBD, as this threshold has 81% sensitivity and 72% specificity for detecting endoscopically active inflammation 5
   • Test stool for Clostridioides difficile and other enteric pathogens 6
   • Consider that the patient may have concurrent IBD and endometriosis 7

2. If fecal calprotectin is <150 mg/g:

   • This makes significant mucosal inflammation from IBD unlikely (false negative rate of 8.5% in intermediate pretest probability scenarios) 5
   • Symptoms are more likely attributable to endometriosis or functional bowel issues
   • Transvaginal ultrasound or MRI remains the appropriate diagnostic approach for bowel endometriosis 2

3. If repeat testing shows persistently elevated levels (>150 mg/g on testing 3-6 months apart):

   • Endoscopic evaluation is warranted to rule out IBD 6

Critical Pitfalls to Avoid

• Do not assume elevated fecal calprotectin is "just from endometriosis" - this can lead to missed IBD diagnosis with significant morbidity implications
• Do not use fecal calprotectin to diagnose or monitor endometriosis - it lacks sensitivity and specificity for this condition 4
• Remember that bowel endometriosis primarily affects the serosa and muscularis layers, not the mucosa where calprotectin would be released 2, making significant calprotectin elevation from endometriosis alone mechanistically unlikely
• Recognize that diagnostic delays in both conditions are common (7-10 years for bowel endometriosis 2), so maintaining high clinical suspicion for both diseases is essential

Bottom Line

While bowel endometriosis theoretically could cause some degree of intestinal inflammation, an elevated fecal calprotectin (>150 mg/g) in a woman with known bowel endometriosis should trigger evaluation for IBD rather than being dismissed as secondary to endometriosis. The test performs well for IBD detection but has no validated role in endometriosis diagnosis or management.