Understanding the Tests: Fecal Calprotectin vs. FIT
While both the Fecal Calprotectin (FC) and the Fecal Immunochemical Test (FIT) are non-invasive and require a stool sample, their underlying mechanisms and diagnostic applications are fundamentally different. Recognizing this distinction is vital for both patients and healthcare providers when investigating symptoms like abdominal pain, diarrhea, or changes in bowel habits.
Fecal Calprotectin: The Inflammation Detector
What it is and what it detects
Fecal calprotectin is a protein released by a type of white blood cell called a neutrophil. When there is inflammation in the intestinal tract, these neutrophils migrate to the site of inflammation and release calprotectin into the stool. The test measures the concentration of this protein, with high levels indicating intestinal inflammation.
Primary Uses
The FC test is a key diagnostic tool used to differentiate between Inflammatory Bowel Disease (IBD) and Irritable Bowel Syndrome (IBS). IBD, which includes Crohn's disease and ulcerative colitis, involves chronic inflammation, whereas IBS does not. Because the symptoms of these conditions can overlap, the FC test helps clinicians determine the next steps in a patient's care. Elevated calprotectin suggests IBD or another inflammatory condition, potentially requiring a colonoscopy for further investigation. If the levels are low, it points toward a non-inflammatory cause, such as IBS, which can help a patient avoid an invasive procedure.
Other Indications for Fecal Calprotectin Testing
- Monitoring disease activity and treatment effectiveness in patients with a confirmed IBD diagnosis.
- Predicting the risk of relapse in IBD patients who are in clinical remission.
- Investigating persistent diarrhea, abdominal pain, or unexplained weight loss.
Fecal Immunochemical Test (FIT): The Occult Blood Detector
What it is and what it detects
The FIT test uses antibodies specific to human hemoglobin to detect tiny, hidden amounts of blood in the stool. This is often called fecal occult blood. Unlike older tests that could react to blood from red meat, the FIT test is highly specific to blood from the lower gastrointestinal tract.
Primary Uses
The primary use of the FIT test is as a screening tool for colorectal cancer and precancerous polyps. Certain polyps and tumors can bleed intermittently, and the FIT test can detect this bleeding, even if it's not visible to the naked eye. A positive result on a FIT test does not mean a person has cancer, but it does indicate that further investigation, usually a colonoscopy, is needed to find the source of the bleeding.
Other conditions that might cause a positive FIT
- Benign polyps that bleed.
- Hemorrhoids.
- Inflammatory bowel disease, especially when accompanied by bleeding.
- Diverticular disease.
Comparison: Fecal Calprotectin vs. FIT
The following table summarizes the key differences between the two stool tests:
| Feature | Fecal Calprotectin (FC) Test | Fecal Immunochemical Test (FIT) |
|---|---|---|
| Primary Biomarker | A protein released by neutrophils (Calprotectin). | Human hemoglobin (blood). |
| Primary Purpose | To detect intestinal inflammation. | To detect occult blood for colorectal cancer screening. |
| Common Use Case | Differentiating IBD from IBS; monitoring IBD activity. | Annual screening for colorectal cancer in average-risk individuals. |
| Indications for Test | Persistent watery or bloody diarrhea, abdominal pain, unexplained weight loss. | Screening adults typically aged 45-75; initial work-up for lower GI symptoms where cancer is a concern. |
| Test Prep Requirements | Can be affected by NSAIDs; typically no other restrictions. | No dietary or medication restrictions. |
| Interpretation | High Level: Likely inflammation (e.g., IBD, infection). Low/Normal Level: Non-inflammatory cause (e.g., IBS). | Positive: Bleeding detected, requires colonoscopy. Negative: No blood detected, but doesn't rule out all issues. |
The Clinical Decision: Which Test to Use?
The choice between a fecal calprotectin and a FIT test depends entirely on the clinical context. For a gastroenterologist, the decision hinges on the patient's symptoms, age, and medical history. As demonstrated by one study, for a younger person with symptoms suggesting inflammation but with no strong cancer risk factors, a calprotectin test may be the initial step. The goal is often to avoid unnecessary, invasive procedures like a colonoscopy when inflammation is not the root cause. Conversely, for an older individual with concerning lower GI symptoms, a FIT test may be used first to prioritize ruling out colorectal cancer. Some advanced guidelines even propose algorithms for incorporating both tests for a more comprehensive assessment.
Interpreting Results and Next Steps
- High Fecal Calprotectin: A high result typically leads to further investigation, such as a colonoscopy, to confirm an inflammatory condition like IBD. The higher the level, the more severe the inflammation is likely to be.
- Low Fecal Calprotectin: A low or normal result, especially in the context of symptoms like bloating and discomfort, makes a diagnosis of IBS more likely than IBD, and may help rule out the need for an endoscopy.
- Positive FIT Test: A positive FIT test is a call to action. It indicates bleeding that must be investigated to determine its cause, with a colonoscopy being the standard follow-up procedure.
- Negative FIT Test: A negative FIT test makes it very unlikely that an average-risk patient has a significant colorectal cancer, but it doesn't rule out other non-bleeding conditions, including some types of IBD.
Conclusion: Two Different Tools for Different Jobs
In conclusion, a fecal calprotectin test is not the same as a FIT test. While both are convenient, non-invasive stool tests, they are designed to detect different biomarkers for different clinical purposes. The FC test is a marker for intestinal inflammation, particularly useful for distinguishing IBD from IBS and monitoring disease activity. The FIT test, meanwhile, is a marker for occult blood, primarily used for colorectal cancer screening. By understanding what each test measures and why it is used, healthcare providers can make more informed decisions and patients can better understand their diagnostic journey.
For more detailed information on fecal testing and its application in inflammatory bowel disease, resources like the National Institutes of Health can provide valuable insights.
