Calprotectin is a protein released by certain immune cells. The Fecal Calprotectin (FCP) test is a non-invasive way to measure this protein in a stool sample, acting as a reliable indicator of inflammation within the gastrointestinal tract. Doctors use the FCP test to differentiate between inflammatory conditions and functional bowel disorders that do not involve inflammation. This test offers an early assessment of whether a patient’s symptoms are caused by active intestinal inflammation.
The Function of Calprotectin
Calprotectin is a complex of two proteins, S100A8 and S100A9, found in high concentrations within neutrophils, a type of white blood cell involved in the innate immune response. When the intestinal lining is injured or inflamed, neutrophils migrate rapidly to the affected area and release calprotectin into the gut lumen.
The protein performs defensive functions, including binding essential metal ions like zinc and manganese. By sequestering these nutrients, calprotectin restricts bacterial growth, participating in the body’s natural antimicrobial defense. Because the protein is highly resistant to degradation by digestive enzymes, it remains stable. This stability allows the amount of calprotectin in a stool sample to correlate directly with the degree of intestinal inflammation.
Defining Fecal Calprotectin Levels
Fecal calprotectin results are typically reported in micrograms per gram (\(\mu\)g/g) of stool, and these values are categorized into three clinical ranges. These thresholds are guidelines and can vary slightly between laboratories.
Low Range
A result considered low is usually less than 50 \(\mu\)g/g, suggesting that intestinal inflammation is unlikely. This low threshold is often used to rule out significant inflammatory conditions, such as Inflammatory Bowel Disease (IBD).
Intermediate Range
The intermediate range falls between 50 \(\mu\)g/g and approximately 200 \(\mu\)g/g. Results in this range can be caused by mild, transient inflammation from factors like NSAIDs or a recent gastrointestinal infection. Doctors frequently recommend repeating the FCP test after several weeks to see if the level has normalized.
High Range
A high result is generally defined as a level above 200 \(\mu\)g/g, though some clinical guidelines may use 250 \(\mu\)g/g as a cutoff. A result exceeding this threshold indicates the presence of significant active intestinal inflammation. Patients with highly active inflammation may have calprotectin levels reaching several hundred or over 1,000 \(\mu\)g/g. The greater the concentration measured, the more severe the inflammation is likely to be.
What Elevated Results Indicate
A significantly elevated Fecal Calprotectin level (above 200 \(\mu\)g/g) is strongly associated with Inflammatory Bowel Disease (IBD), which includes Crohn’s disease and Ulcerative Colitis. These chronic conditions involve persistent inflammation of the digestive tract lining, leading to the sustained recruitment and activation of neutrophils. The degree of calprotectin elevation tends to correlate with the severity of the disease activity observed during examination or endoscopy.
In contrast, a normal or low calprotectin level makes a diagnosis of active IBD highly improbable. Low results are commonly seen in functional gastrointestinal disorders, such as Irritable Bowel Syndrome (IBS). IBS presents similar symptoms like abdominal pain and diarrhea but does not involve destructive intestinal inflammation. Using the FCP test to distinguish between IBD and IBS helps doctors avoid unnecessary invasive procedures.
Moderately elevated calprotectin levels (below 200 \(\mu\)g/g) can be caused by other factors that induce temporary inflammation. These include acute bacterial or viral gastroenteritis, diverticulitis, or inflammation resulting from the use of certain medications like NSAIDs. Because calprotectin is a marker of inflammation and not a disease-specific marker, these intermediate results require careful interpretation alongside a patient’s symptoms and medical history.
When a result is markedly high, it prompts further diagnostic investigation to determine the exact cause and extent of the inflammation. This typically involves a colonoscopy, which allows a direct visual inspection of the intestinal lining and the collection of tissue samples for biopsy. The FCP test serves as an effective screening tool, helping to identify which patients require a more invasive diagnosis.