Calprotectin is a protein released by white blood cells during inflammation. It makes up roughly 40 to 60 percent of the protein content inside neutrophils, the immune cells that rush to infection or injury sites first. When doctors order a “calprotectin test,” they’re almost always referring to a stool test that measures how much of this protein has migrated into your gut, which serves as a reliable marker of intestinal inflammation.
How Calprotectin Works in Your Body
Calprotectin is made of two smaller proteins (S100A8 and S100A9) that lock together to form a single unit. It lives inside neutrophils and, to a lesser extent, monocytes, another type of immune cell. When these cells activate in response to infection or tissue damage, they release calprotectin into the surrounding area.
Once released, calprotectin does two things. It amplifies the inflammatory response, helping recruit more immune cells to the site. And it fights microbes directly by binding to essential metals like zinc and manganese, effectively starving bacteria of the nutrients they need to grow. This metal-grabbing ability is one of the body’s frontline antimicrobial defenses.
Why It Shows Up in Stool
When the lining of your intestines becomes inflamed, its permeability increases. Neutrophils migrate through the gut wall toward whatever is causing the problem, whether that’s an infection, an autoimmune process, or another source of irritation. As those neutrophils accumulate and break down in the intestinal tissue, they dump calprotectin into the bowel. The protein then passes through the digestive tract and ends up in your stool, where it can be measured with a simple lab test.
The more inflammation in your gut, the more neutrophils migrate there, and the higher the calprotectin concentration in your stool. That direct relationship is what makes the test so useful.
What the Test Is Used For
The most common reason doctors order a fecal calprotectin test is to distinguish between inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Both conditions can cause abdominal pain, bloating, and changes in bowel habits, but they require very different management. IBD involves actual inflammation and tissue damage. IBS does not.
A normal calprotectin level makes IBD very unlikely, which means you can often avoid a colonoscopy. People with true IBS typically have normal calprotectin levels, while those with active Crohn’s disease or ulcerative colitis show elevated ones. A National Institutes of Health review found that false-negative results for IBD are unusual when using the standard cutoff of 50 mcg/g, making the test especially valuable for ruling out inflammatory disease in primary care.
For people already diagnosed with IBD, the test serves a different purpose: monitoring. Rising calprotectin levels in someone who feels fine may signal that a flare is building before symptoms appear. Research suggests that the direction and size of changes over time may be more predictive of relapse than any single number. A steady climb, even while you feel well, is a signal your doctor will take seriously.
Understanding Your Results
Most labs use the following ranges for adults:
- Below 50 mcg/g: Normal. No significant inflammation detected in the gut.
- 50 to 120 mcg/g: Borderline. Could indicate mild inflammation, such as well-controlled IBD, or be caused by medications like NSAIDs or aspirin.
- Above 120 mcg/g: Abnormal. Suggests active inflammation in the gastrointestinal tract.
For people with Crohn’s disease who are in remission, the American Gastroenterological Association uses a threshold of 150 mcg/g. A result below that, combined with a normal blood inflammation marker (CRP), is considered sufficient to rule out active disease without needing a colonoscopy. Results above that threshold typically prompt further investigation, often with endoscopy, before any treatment changes are made.
What Can Affect Your Results
Certain common medications can raise calprotectin levels even when no true gut disease is present. NSAIDs like ibuprofen, aspirin, and naproxen are the most well-known culprits. Proton pump inhibitors used for acid reflux, such as omeprazole and lansoprazole, can also elevate results. If you’re taking any of these, your doctor may ask you to stop them before retesting or will interpret a borderline result with that context in mind.
Gut infections, whether bacterial, viral, or parasitic, also raise calprotectin because they trigger the same neutrophil migration as chronic inflammatory disease. A single elevated result doesn’t automatically mean IBD. Your doctor will consider the full picture, including your symptoms, how long they’ve lasted, and whether an infection could be responsible.
Why Results Differ in Babies and Young Children
Infants naturally have much higher calprotectin levels than adults. In healthy babies under four months old, the median concentration is around 181 mcg/g, a number that would be flagged as abnormal on an adult reference range. By four to twelve months, the median drops to about 41 mcg/g, and by age one to three, it settles near 25 mcg/g.
This matters because applying adult cutoffs to a young child’s stool sample can lead to false alarms and unnecessary testing. Pediatric results need to be interpreted against age-specific ranges, something your child’s doctor should account for automatically.
How the Stool Sample Works
Collecting a stool sample for calprotectin testing is straightforward. You’ll typically receive a collection kit from your doctor or lab. Only a small amount of stool is needed, roughly the size of a walnut. One practical advantage of calprotectin is its stability: it remains reliable in a stool sample at room temperature for four to seven days, so you don’t need to rush the sample to a lab the same hour you collect it. Refrigerated samples last even longer, and frozen samples stay stable for months.
Some labs provide extraction kits that let you process the sample at home, which is more hygienic and convenient. The extracted sample stays stable at room temperature for three to five days depending on the kit. Your lab’s instructions will specify exactly what to do, but the key takeaway is that this test is forgiving in terms of timing and storage.