Calprotectin is a protein found in the neutrophils (a type of white blood cell) that is released into the gastrointestinal tract during inflammation. The level of calprotectin in stool can indicate the level of inflammation in the digestive tract. A fecal calprotectin test measures the level of this protein in a stool sample to help physicians determine if a patient has inflammatory bowel disease (IBD) such as Crohn's disease or ulcerative colitis, or another condition that causes intestinal inflammation.
What is the calprotectin protein?
Calprotectin is a calcium and zinc-binding protein found in the cytosol of neutrophils. It represents up to 60% of the total cytoplasmic proteins in these immune cells. Neutrophils are normally present in the intestinal tract in very small numbers. However, when the intestinal lining becomes inflamed due to IBD, foreign bodies, infections or cancer, the number of neutrophils in the affected intestinal tissue increases dramatically.
As the neutrophils die at the inflamed site, calprotectin leaks from the cytosol into the intestinal lumen and eventually into the feces. Since calprotectin is very stable and resistant to breakdown by intestinal bacteria, its level in stool directly reflects the level of intestinal inflammation. That is why fecal calprotectin testing has become an effective non-invasive way to identify intestinal inflammation.
How is the test performed?
The fecal calprotectin test is a simple and easy test to perform. To collect the stool sample, patients are provided with a plastic collection container with a tight-fitting lid to collect a small sample of their stool. The sample should represent the entire bowel movement and not just soft or loose pieces.
Once collected, patients return the stool sample to their doctor or a laboratory. The laboratory then performs various types of immunoassays such as enzyme-linked immunosorbent assay (ELISA) to accurately quantify the level of calprotectin in the stool sample. The entire testing process takes only a few hours to complete after receiving the stool sample from the patient.
Interpreting fecal calprotectin test results
Calprotectin levels are reported either in micrograms of calprotectin per 1 gram of stool (μg/g) or parts per million (ppm). Most laboratories consider a level below 50 μg/g as normal with no signs of intestinal inflammation. Levels between 50-100 μg/g are considered marginally elevated while levels above 100 μg/g strongly suggest active intestinal inflammation from IBD, infections or other causes.
The higher the calprotectin level, the more severe is likely to be the intestinal inflammation. Levels above 250-300 μg/g are generally seen in patients with confirmed active Crohn's disease or ulcerative colitis. Levels may also rise above normal during intestinal infections caused by bacteria, viruses, or parasites though usually not as high as with IBD. A normal or negative calprotectin test helps rule out active inflammation.
Utility of fecal calprotectin testing
Some of the key utilities of fecal calprotectin testing in clinical practice are:
- Screening and assessment of inflammatory bowel disease (IBD): Fecal calprotectin levels correlate well with endoscopic assessment of inflammation in IBD. A high level suggests active disease while a normal level rules out active inflammation.
- Monitoring response to treatment in IBD: Serial Fecal Calprotectin Test helps monitor a patient's response to medications or surgery and detect any signs of relapse early. Treated IBD patients should have calprotectin levels below 100 μg/g.
- Differential diagnosis of IBS: It helps rule out IBD in patients with suspected irritable bowel syndrome (IBS) symptoms by detecting or ruling out underlying intestinal inflammation.
- Assessment of intestinal infections: It aids in identifying intestinal infections versus IBS when patients present with abdominal pain, diarrhea, or bloody stools.
- Guiding need for colonoscopy: Colonoscopy is generally not needed when calprotectin levels are normal but is indicated when levels are high to assess inflammation and severity.
Advantages of fecal calprotectin testing
Some key advantages of this non-invasive diagnostic test include:
- It measures intestinal inflammation directly from the stool sample without any need for an endoscopy.
- Results are available faster than imaging tests or endoscopy, usually within a day.
- It detects even mild inflammation better than other blood tests and allows early treatment.
- Serial monitoring helps gauge treatment response without repeated endoscopies.
- More accurate than symptoms alone in detecting intestinal inflammation from IBD or other diseases.
- Better tolerated than endoscopy, imaging tests involving radiations or bowel preparations.
- Cost-effective initial screening method to identify patients who need endoscopic evaluation.
Limitations of the test
While fecal calprotectin testing is very useful, it has some limitations:
- Cannot identify the specific cause or location of intestinal inflammation.
- Normal levels do not rule out IBD completely as levels may be normal during remission.
- Other intestinal or systemic non-gastrointestinal conditions can also mildly raise levels.
- Diet, medications, bowel transit time can slightly influence calprotectin levels.
- Levels may not correlate well with histological severity seen during endoscopy.
- Cannot replace endoscopy or imaging in evaluating complications like strictures or abnormalities.
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