Before any diagnostic tests are done, a healthcare practitioner will gather information on a person's medical and family history as well as signs and symptoms, including the frequency and/or duration of diarrhea. The healthcare practitioner will be considering things that are out of the ordinary and that might be associated with conditions that cause diarrhea and other symptoms. A series of questions may be asked to help make informed choices about which tests will be helpful in establishing a diagnosis.
There is no single laboratory test that can definitively diagnose inflammatory bowel disease (IBD). However, laboratory testing is an important tool for evaluating a person who may have IBD. If a person has persistent diarrhea and abdominal pain, an initial set of tests are done to help evaluate the person's condition. These may be done in conjunction with imaging tests, such as an X-ray or CT scan.
Examples of some common initial tests include:
- CBC (complete blood count) to check for anemia; bleeding caused by IBD and similar conditions can lead to anemia
- CMP (comprehensive metabolic panel) to help evaluate the person's general health
- Fecal occult blood test or fecal immunochemical test to look for blood in the stool
- CRP (C-reactive protein) to look for inflammation; this test may also be used later to help distinguish IBD from irritable bowel syndrome (IBS) and may be used after diagnosis to monitor the course of the disease
- ESR (erythrocyte sedimentation rate) to detect inflammation, if CRP is not available
At the same time or in follow up, a healthcare practitioner may order several tests to rule out other causes of diarrhea, abdominal pain, and colitis. Other possible causes include viral or bacterial infections, parasites, colon cancer, and other chronic conditions such as celiac disease. Some examples of tests that may be done to help rule out these other conditions include:
- Stool culture or GI pathogens panel to look for infection of the digestive tract
- O&P (Ova and parasite) to detect parasites
- Cytomegalovirus testing to detect CMV infection
- Clostridium difficile tests to detect toxin produced by the bacteria C. difficile, which may overgrow the digestive tract; may be seen following antibiotic therapy
- Anti-tissue transglutaminase (anti-tTG) and other tests for celiac disease (see Celiac Disease Tests)
If it is suspected that the person is unable to digest or absorb nutrients properly (malabsorption), then tests may be done to detect nutritional deficiencies. Examples include:
As infections and other causes such as celiac disease are ruled out, other laboratory tests may be done to help pinpoint a cause:
- Stool WBC to detect white blood cells in the stool, an indication of inflammation in the digestive tract; only certain diseases result in the presence of WBCs in the stool, so this test may help narrow the possibilities.
- Calprotectin and lactoferrin are two stool (fecal) tests that detect substances released by white blood cells. These substances are associated with inflammation and with disease activity, severity, and relapse. They may be used to help distinguish between IBD and non-inflammatory disorders and also to monitor IBD. These tests are more sensitive than the test for stool WBC.
If initial tests rule out other causes and it is suspected that a person has IBD, then a healthcare practitioner may order tests that detect antibodies frequently present in the blood of people who have IBD. One or more of these may be ordered to help distinguish between the two most common types of IBD, ulcerative colitis (UC) and Crohn disease (CD). There is some overlap in the presence of these antibodies in UC and CD, and they are not sensitive or specific enough to diagnose either condition, but they may give the healthcare practitioner additional useful information.
Examples of antibody tests that may be done include:
- pANCA (Perinuclear anti-neutrophil cytoplasmic antibody). More common with UC, it is found in about 60% to 80% of people with UC but only about 5% to 15% of people with CD.
- ASCA (Saccharomyces cerevisiae antibodies), IgG and IgA. ASCA is more common with CD. ASCA IgG is found in about 60% to 70% of people with CD and in 10% to 15% of people with UC. ASCA IgA is found in about 35% of people with CD and less than 1% of those with UC.
- Anti-CBir1 (Clostridium species antibodies). Found to in about 55% of people with CD and 10% of people with UC.
- Anti-Omp C (Escherichia coli antibodies). Associated with rapidly progressing Crohn disease and found in 55% of people with CD and 5% to 10% of people with UC.
- Anti-I-2 (Pseudomonas fluorescens antibodies) This is positive in 55% of CD cases and 10% of UC cases.
Ultimately, a biopsy may be done to establish a diagnosis of IBD. A biopsy is considered the gold standard for the diagnosis of inflammatory bowel disease (and for distinguishing between ulcerative colitis and Crohn disease. A sample of tissue is obtained from the digestive tract during endoscopy or colonoscopy (see below). The tissue is evaluated by a pathologist for inflammation and abnormal changes in cell structures that are characteristic for each type of IBD. (For more about biopsies, read Anatomic Pathology.)
One or more imaging tests may be done at the same time as laboratory testing to help diagnose and/or monitor UC and CD. Typically, the less invasive ones, such as an X-ray, are done earlier in the diagnostic process, while more invasive tests such as colonoscopy are done only as necessary and when a diagnosis is difficult. These imaging tests can be used to look for characteristic changes in the structure and tissues of the intestinal tract and to detect blockages.
- X-ray (abdominal). This may be one of the initial imaging test done to view the digestive tract.
- Computed tomography (CT). Also may be done early in the process to view the digestive tract; these typically provide greater detail than a standard X-ray.
- X-ray with barium. Barium contrast dye allows an evaluation of the intestines. This is more commonly used in severe IBD to rule out serious complications like a perforated colon.
- Sigmoidoscopy. A slender tube is used to examine the last 2 feet of the colon.
- Colonoscopy. A slender tube is used to examine the entire colon; it includes a light and camera and can be used to take biopsies.