Fecal calprotectin is a protein biomarker found in stool that provides a non-invasive measure of inflammation within the gastrointestinal tract. Calprotectin is released primarily by neutrophils, which migrate to the gut lining when inflammation is present. Because the protein is resistant to degradation by intestinal enzymes, its concentration in a stool sample reliably reflects the level of active intestinal inflammation. This measurement helps clinicians distinguish between inflammatory and non-inflammatory conditions, often guiding the need for more invasive diagnostic procedures like colonoscopy.
Understanding Fecal Calprotectin
Calprotectin is a complex of two proteins, S100A8 and S100A9, which together account for a large portion of the soluble protein content within neutrophils. When the lining of the intestine becomes inflamed, these neutrophils are recruited to the site of injury as part of the body’s immune response. The subsequent activation and breakdown of these white blood cells release their contents, including calprotectin, into the intestinal lumen.
Because the protein passes through the digestive tract largely intact, the amount measured in a stool sample is directly proportional to the number of neutrophils recruited to the gut lining. The test is particularly helpful in differentiating between functional bowel disorders, such as Irritable Bowel Syndrome (IBS), and organic diseases like Inflammatory Bowel Disease (IBD). IBS typically does not involve significant inflammation, while IBD does.
Establishing Normal Ranges Across the Lifespan
The concentration of fecal calprotectin considered normal varies significantly depending on a person’s age, reflecting the developmental stage of the gastrointestinal system. Establishing a single, universal cut-off value is inaccurate because the gut immune system matures over time. In healthy adults, a normal result is typically considered to be below 50 micrograms per gram (\(\mu\)g/g).
Newborns and infants exhibit the highest calprotectin levels, reflecting the extensive remodeling and immunological changes occurring in the gut during the first year of life. Median concentrations in the first one to three months can be quite high, sometimes exceeding 350 \(\mu\)g/g. These levels gradually decrease as the infant ages, stabilizing considerably by the end of the first year.
For children between the ages of one and four years, normal ranges remain higher than adult values but are significantly lower than those seen in infants. By the age of four, levels generally begin to stabilize and approximate the adult range. For children older than four, a cut-off of 50 \(\mu\)g/g is often used as the standard upper limit for a healthy result.
Calprotectin levels can increase again in older adults, even without a diagnosed inflammatory disease. Studies show that healthy individuals over the age of 70 may have an average concentration several times higher than those under 50, sometimes reaching median values around 160 \(\mu\)g/g. This rise suggests a low-grade, age-related inflammation that must be considered when interpreting results in the elderly population.
Interpreting Elevated Results
When a fecal calprotectin result exceeds the normal, age-appropriate range, it indicates the presence of active intestinal inflammation. The degree of elevation is used by clinicians to assess the likelihood of a serious organic disease, most notably Inflammatory Bowel Disease (IBD), which includes Crohn’s disease and Ulcerative Colitis. Results between 50 \(\mu\)g/g and 250 \(\mu\)g/g are often considered a “gray area” or mildly elevated, suggesting inflammation that may require further investigation or repeat testing.
A concentration consistently above 250 \(\mu\)g/g strongly suggests the presence of significant, active inflammation, making IBD or an infectious colitis highly probable. For patients already diagnosed with IBD, measuring this protein is a valuable tool for monitoring disease activity. High calprotectin levels correlate well with the severity of the disease, helping clinicians assess treatment effectiveness and predict the risk of relapse.
Conversely, a normal or low calprotectin result in a symptomatic patient makes a diagnosis of IBD highly unlikely. This low result suggests that symptoms, such as abdominal pain or altered bowel habits, are more likely due to a non-inflammatory functional disorder like IBS. This distinction helps physicians avoid unnecessary and invasive procedures.
Other Variables Affecting Calprotectin Levels
While fecal calprotectin is an excellent marker for chronic intestinal inflammation, several temporary factors can cause a non-disease related elevation, potentially leading to a misleading result. The use of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), such as ibuprofen or naproxen, can cause irritation and low-level inflammation of the gut lining. This irritation can lead to a false-positive or mildly elevated calprotectin reading.
Acute gastrointestinal infections, whether bacterial, viral, or parasitic, also cause a temporary influx of neutrophils to the gut, resulting in significantly elevated calprotectin levels. The level will typically normalize once the infection has resolved. Other conditions like colorectal cancer, diverticulitis, and celiac disease can also cause elevations due to localized inflammatory responses. Clinicians must consider these variables and the patient’s full medical history when interpreting test results.