The fecal calprotectin test measures a protein found in a stool sample to determine the level of inflammation within the gastrointestinal tract. Calprotectin serves as a reliable, non-invasive biomarker that helps medical professionals assess whether digestive symptoms are caused by an inflammatory condition. An elevated result acts as an alarm signal, indicating the migration of specific immune cells into the bowel lining due to active inflammation. This simple test provides a measurable, quantitative insight into the internal state of the digestive system.
Calprotectin’s Role in the Immune System
Calprotectin is a complex of two proteins, S100A8 and S100A9, belonging to the S100 family of calcium-binding proteins. This complex is highly concentrated within neutrophils, a type of white blood cell, making up as much as 60% of the cell’s soluble protein content. Neutrophils are the immune system’s first responders, rapidly migrating to sites of injury or infection.
When the intestine becomes inflamed, neutrophils travel through the gut wall to the site of damage. Calprotectin is then released from these activated or dying neutrophils into the intestinal lumen. Because the protein is stable and resistant to degradation by digestive enzymes, it can be easily detected and measured in a stool sample.
The protein functions as an antimicrobial agent by binding essential transition metals, such as zinc and manganese. By sequestering these metals, calprotectin starves invasive bacteria of necessary nutrients, contributing to the body’s innate immune defense. Therefore, the amount of calprotectin in the feces directly correlates with the number of neutrophils that have infiltrated the gastrointestinal lining.
Why the Fecal Calprotectin Test is Used
The clinical application of the fecal calprotectin test is to differentiate between inflammatory and non-inflammatory gastrointestinal disorders. Many digestive diseases, such as Inflammatory Bowel Disease (IBD) and Irritable Bowel Syndrome (IBS), share similar symptoms like abdominal pain and diarrhea. However, their underlying causes and treatments are vastly different.
IBD, including Crohn’s disease and Ulcerative Colitis, is characterized by chronic inflammation, while IBS is a functional disorder without structural inflammation. A normal calprotectin result strongly suggests symptoms are caused by a non-inflammatory issue like IBS, excluding IBD as the likely cause. This distinction helps streamline the diagnostic process.
The test’s non-invasive nature and high sensitivity for detecting mucosal inflammation offer an advantage over traditional diagnostic methods. A normal fecal calprotectin result can help avoid the need for a colonoscopy, which is a costly and invasive procedure. It acts as an effective screening tool to determine which patients require intensive investigation, such as endoscopy, and which can be managed conservatively.
Understanding Test Results and Numerical Ranges
Results from a fecal calprotectin test are reported in micrograms of calprotectin per gram of stool (\(\mu\)g/g). Interpretation is based on cut-off points that indicate the likelihood of active inflammation. Although ranges vary depending on the laboratory and assay used, they follow a predictable pattern for clinical decision-making.
A result below 50 \(\mu\)g/g is considered within the normal range, indicating inflammation is unlikely. This finding suggests that digestive symptoms are consistent with a functional disorder, such as IBS, rather than IBD. This threshold is often used to rule out active inflammatory disease.
The intermediate or “gray” zone spans from 50 \(\mu\)g/g up to 200 \(\mu\)g/g. A result in this range is inconclusive and may be due to low-grade inflammation, a transient issue like a mild infection, or certain medications. Professionals may recommend retesting in several weeks or pursuing other diagnostic avenues to clarify the source of the elevation.
Levels exceeding 200 \(\mu\)g/g, and particularly those above 250 \(\mu\)g/g, suggest active intestinal inflammation. This elevated result indicates an organic disease process, such as IBD, and usually prompts a referral for an endoscopic procedure like a colonoscopy. Endoscopy allows visual examination of the bowel and obtaining tissue biopsies for a definitive diagnosis. Higher values correlate with more severe inflammation.
Clinical Conditions That Raise Calprotectin Levels
While the fecal calprotectin test is frequently associated with Inflammatory Bowel Disease (IBD), many conditions can cause protein elevation. Chronic high levels are attributed to IBD, specifically Crohn’s disease and Ulcerative Colitis, where active inflammation drives a continuous influx of neutrophils into the bowel. The magnitude of the result reflects the current severity and extent of the IBD.
Temporary elevations occur with infectious colitis, which is inflammation caused by bacterial, viral, or parasitic gastrointestinal infections. Once the infection clears, the calprotectin level returns to the normal range, distinguishing this from the persistent nature of IBD. This temporary elevation highlights that the test indicates inflammation, but not the specific cause.
Certain medications also cause low to moderate increases in calprotectin. Regular use of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), such as ibuprofen or naproxen, can induce mild irritation in the gut lining, leading to a measurable rise. Other localized inflammatory conditions, including diverticulitis or microscopic colitis, can also result in elevated levels due to localized neutrophil recruitment.
Gastrointestinal malignancy, such as colorectal cancer, can also elevate fecal calprotectin, though this is a less common cause. Since the test measures inflammation generally, any condition that damages the intestinal mucosa and recruits neutrophils will cause an elevation. A medical provider must interpret the result within the context of a patient’s full clinical picture and symptoms.