Calprotectin is a protein that serves as a marker for inflammation, particularly within the gastrointestinal tract. It is released by immune cells migrating to an inflamed area of the gut lining. Measuring its concentration is performed through a simple, non-invasive fecal test. This measurement provides healthcare providers with a reliable way to detect the presence and relative severity of intestinal inflammation. The test helps guide further diagnostic steps without immediately requiring invasive procedures like a colonoscopy.
How Calprotectin Indicates Gastrointestinal Inflammation
Calprotectin is primarily contained within neutrophils, a type of white blood cell. When the lining of the intestine becomes inflamed due to a disease process, these neutrophils travel to the site of damage within the gut wall. There, the neutrophils release calprotectin into the intestinal lumen.
This protein is resistant to degradation by digestive enzymes, allowing it to be easily measured in a stool sample. The amount of calprotectin in the feces directly correlates with the number of neutrophils that have migrated into the bowel, making it a quantitative marker for inflammation.
The test is particularly useful for distinguishing between inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Conditions like Crohn’s disease and ulcerative colitis (IBD) involve chronic inflammation, resulting in high calprotectin levels. In contrast, IBS is a functional disorder that presents with normal or slightly elevated calprotectin levels because it does not involve significant intestinal inflammation.
Understanding the Numerical Thresholds
The interpretation of a calprotectin result is based on specific numerical thresholds, measured in micrograms of protein per gram of stool (\(\mu\text{g}/\text{g}\)). A result below \(50\ \mu\text{g}/\text{g}\) is considered a normal level, indicating that significant intestinal inflammation is unlikely. A normal result has a high negative predictive value, meaning it can reliably rule out conditions like IBD in most patients.
Results that fall into a borderline or “gray zone,” between \(50\) and \(150\ \mu\text{g}/\text{g}\), suggest the possibility of mild or localized inflammation. This intermediate range can be caused by various factors, including mild infectious gastroenteritis, use of non-steroidal anti-inflammatory drugs (NSAIDs), or even early-stage IBD. For these borderline results, a healthcare provider may recommend retesting the level after a few weeks or investigating other non-IBD causes.
A calprotectin level exceeding \(150\ \mu\text{g}/\text{g}\) is considered significantly elevated and strongly suggests active intestinal inflammation. When the level rises above \(250\ \mu\text{g}/\text{g}\), it becomes a stronger indicator of active mucosal disease, such as untreated or flaring inflammatory bowel disease. Levels in this range signal a high likelihood of active tissue damage that requires prompt medical attention.
Values that are extremely high, for example, over \(500\ \mu\text{g}/\text{g}\), are highly predictive of an active inflammatory condition like severe IBD or a serious infectious colitis. The severity of the elevation mirrors the severity of the mucosal inflammation seen during an endoscopic examination. These very high results are used for initial diagnostic workup and for monitoring known IBD, where they can predict an increased risk of disease relapse.
Next Steps Following a High Calprotectin Result
An elevated calprotectin result, particularly one above \(250\ \mu\text{g}/\text{g}\), requires clinical follow-up to determine the source of the inflammation. The high level signals that the gut lining is actively inflamed, but it does not specify the exact cause or location of the problem. Therefore, a referral to a gastroenterologist is a standard next step to manage the investigation.
The diagnostic process involves an endoscopic assessment, such as a colonoscopy or sigmoidoscopy. This invasive procedure allows the specialist to directly visualize the intestinal mucosa to confirm the presence of inflammation, assess its extent, and collect tissue samples (biopsies) for a definitive diagnosis. The endoscopic findings and biopsy results are necessary to confirm whether the inflammation is due to IBD, an infection, or another cause.
Beyond initial diagnosis, calprotectin testing is an important tool for managing patients already diagnosed with IBD. Serial measurements are used to monitor the effectiveness of prescribed treatments, with the goal being to achieve a significant reduction in the calprotectin level. A consistently high level suggests that the disease remains active or that the current treatment regimen is not adequately controlling the inflammation. Regular monitoring can help predict a potential disease flare-up, allowing for therapeutic adjustments before symptoms become severe.