Fecal Calprotectin: A Marker of Intestinal Inflammation
Fecal Calprotectin (FC) is a non-invasive tool used to evaluate gastrointestinal symptoms. This biomarker is a protein released into the stool when inflammation is present inside the intestinal tract. Because it is easily measured from a small stool sample, it serves as an effective initial screening test in gastroenterology.
Calprotectin is a protein complex highly concentrated within neutrophils (a type of white blood cell). When the intestinal lining becomes inflamed, such as in Inflammatory Bowel Disease (IBD), neutrophils migrate to the site of damage. These immune cells release calprotectin into the bowel lumen, where it is carried out in the stool. Measuring the amount of calprotectin in the feces directly correlates to the intensity of active inflammation in the gut.
Defining the Calprotectin Thresholds for IBD
The main purpose of measuring fecal calprotectin in a symptomatic patient is to help distinguish between a functional disorder, like Irritable Bowel Syndrome (IBS), and an organic inflammatory disease, such as Crohn’s Disease. Results are reported in micrograms of calprotectin per gram of stool (\(\mu\)g/g), and specific numerical cutoffs act as guidelines for clinical action.
A result below 50 \(\mu\)g/g is considered normal and suggests that active intestinal inflammation is highly unlikely. This low level effectively rules out Inflammatory Bowel Disease (IBD) and strongly suggests symptoms are caused by a non-inflammatory condition like IBS.
The indeterminate or “gray zone” for calprotectin levels generally falls between 50 \(\mu\)g/g and 200 \(\mu\)g/g. A result in this range indicates mild inflammation that is not severe enough to definitively point to IBD. Patients in this zone may require a repeat test or closer clinical observation, as the inflammation could be transient or due to a milder, self-limiting cause.
Levels greater than 200 \(\mu\)g/g (or sometimes 250 \(\mu\)g/g, depending on clinical guidelines) are strongly suggestive of active intestinal inflammation. At this high level, the probability of having an organic disease like Crohn’s Disease or Ulcerative Colitis is significant. A result in this high range usually prompts a gastroenterologist to recommend a colonoscopy and biopsy to confirm the diagnosis and assess the disease extent.
Why Elevated Levels Do Not Guarantee Crohn’s Disease
While a high fecal calprotectin level is strongly associated with IBD, it is not a specific diagnostic test for Crohn’s Disease alone. Calprotectin is a generalized marker of neutrophil activity, meaning any condition causing significant inflammation or damage to the intestinal lining can lead to an elevated reading.
Acute infectious gastroenteritis is a common cause of temporarily high calprotectin levels. The resulting immune response leads to neutrophil migration and calprotectin release, which usually resolves once the infection clears. Certain medications, particularly Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) like ibuprofen, can also cause inflammation and irritation in the gut lining, resulting in elevated calprotectin.
Other gastrointestinal conditions, such as diverticulitis, microscopic colitis, and colorectal cancer, can also cause the localized inflammation necessary to raise calprotectin levels. These non-IBD causes often result in levels that fall within the intermediate or lower end of the elevated range compared to the severe inflammation seen during an active Crohn’s flare. Consequently, the test is used as a filter to identify patients who need a more invasive procedure, like a colonoscopy, to determine the exact cause of the inflammation.
Monitoring Disease Activity Post-Diagnosis
Once Crohn’s Disease has been formally diagnosed, the role of fecal calprotectin shifts from diagnostic screening to long-term disease management and monitoring. The test provides a simple, non-invasive method for tracking how well a patient is responding to therapy. A successful treatment plan, which aims to reduce inflammation, should result in a significant drop in calprotectin levels over time.
Monitoring the calprotectin level can also help predict a potential relapse before symptoms become clinically obvious. A sustained or steadily rising FC level can indicate that inflammation is returning, allowing a doctor to adjust medication before the patient experiences a full disease flare. A goal of modern Crohn’s management is achieving “mucosal healing,” meaning the inflammation has resolved at the deepest tissue level.
Fecal calprotectin levels are used as a surrogate marker for this healing, often with the goal of achieving a level below 100 \(\mu\)g/g, or even below 50 \(\mu\)g/g. This correlates well with endoscopic evidence of deep remission. Tracking this biomarker provides a clear, quantitative measure of disease control that is less invasive than repeated colonoscopies.