Calprotectin is a protein that serves as a biomarker for inflammation, particularly within the gastrointestinal tract. It is a calcium-binding protein found predominantly inside specific immune cells throughout the body. When inflammation occurs in the lining of the digestive system, these immune cells rush to the site and release the protein.
Measuring the level of calprotectin in a stool sample is a non-invasive way to detect and quantify this internal inflammation. The protein’s stability as it passes through the intestines makes it an ideal marker for diagnosing and monitoring certain chronic bowel conditions. High levels of this protein indicate that an active inflammatory process is underway, which prompts further investigation by healthcare professionals.
Calprotectin’s Role in the Body: A Marker of Inflammation
Calprotectin is a complex of two smaller proteins, S100A8 and S100A9, which form a heterodimer. This structure gives the protein strong antimicrobial properties, allowing it to inhibit the growth of pathogens by binding and sequestering transition metals like zinc, iron, and manganese. This process is part of the body’s innate immune response, often referred to as nutritional immunity.
The protein is found in high concentration—up to 60% of the soluble content—within the cytoplasm of neutrophils, which are the most abundant type of white blood cell. When the body detects an injury or infection, neutrophils migrate to the inflamed tissue in large numbers. Once at the site of inflammation, these cells become activated and release their contents, including calprotectin, into the surrounding tissue.
If inflammation occurs in the intestine, the released calprotectin mixes with the bowel contents. Because the protein is resistant to degradation by digestive enzymes, it remains intact as it travels through the digestive tract. The amount of calprotectin in the stool is directly proportional to the number of neutrophils recruited to the intestinal wall, providing an indirect measure of inflammation severity.
The Purpose and Process of the Fecal Calprotectin Test
The Fecal Calprotectin Test is a non-invasive method used to screen for intestinal inflammation. Doctors primarily utilize this test to distinguish between disorders that cause inflammation and those that simply affect bowel function. This helps determine which patients require more intensive procedures, such as a colonoscopy.
The process begins with the patient collecting a small sample of their stool at home. The sample is then sent to a laboratory where specialized techniques, often using an immunoassay method, quantify the exact amount of calprotectin present. Results are typically reported in micrograms of calprotectin per gram of stool (\(\mu\)g/g).
The test is used for patients presenting with symptoms like chronic diarrhea, abdominal pain, or weight loss. A normal or low calprotectin level suggests that significant intestinal inflammation is unlikely. This result can save patients from undergoing unnecessary, costly, and resource-intensive endoscopic procedures.
Interpreting Test Results and Clinical Significance
Interpreting Fecal Calprotectin Test results relies on established numerical cut-off points to determine the likelihood of active intestinal inflammation. While specific ranges vary between laboratories, a level below 50 \(\mu\)g/g is considered normal and suggests the absence of active inflammation. Results in this range indicate that an inflammatory bowel condition is unlikely.
A high level of calprotectin indicates active mucosal inflammation and necessitates further diagnostic testing. Levels greater than 250 \(\mu\)g/g are associated with Inflammatory Bowel Disease (IBD), which includes Crohn’s disease and Ulcerative Colitis. Elevated results usually trigger a referral for an endoscopic examination to confirm the diagnosis and assess the disease extent.
The test’s primary clinical utility is differentiating between IBD and functional gastrointestinal disorders, such as Irritable Bowel Syndrome (IBS). Patients with IBS experience similar symptoms, but their condition lacks the chronic inflammation seen in IBD. Since IBS does not involve significant intestinal inflammation, calprotectin levels in these patients are typically normal or mildly elevated.
Levels between 50 \(\mu\)g/g and 250 \(\mu\)g/g are often considered borderline or inconclusive, requiring careful clinical judgment. Mild elevations can be caused by transient factors that are not chronic IBD, such as an acute gastrointestinal infection, the use of non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, or even colorectal polyps or cancer. Doctors may choose to retest the patient after a few weeks or investigate other possible causes before proceeding to more invasive tests.
For patients diagnosed with IBD, the calprotectin test monitors disease activity and mucosal healing. A rise in the protein level can signal an impending flare-up, while a drop indicates that treatment is successfully reducing inflammation. This helps manage the long-term course of the disease.