Dear Doctor: How accurate is the blood test for irritable bowel syndrome? My gastroenterologist says that one exists but that he doesn't feel it has been proved effective.
Dear Reader: Irritable bowel syndrome (IBS) is a disorder characterized by abdominal pain in association with altered bowel movements. Sufferers can have diarrhea or constipation, or they can alternate between the two. IBS affects about 11 percent of the population and accounts for anywhere from 25 to 50 percent of all visits to gastroenterologists. It affects women more than men, with 14 percent of women affected and 9 percent of men. IBS is what's called a "functional bowel disorder," meaning that it's not related to inflammation but to how the bowels contract and relax.
To diagnose IBS, a doctor must rule out other causes for a patient's symptoms. But newer blood tests can help. One assesses levels of an antibody to the cell membrane protein vinculin; and the other assesses levels of an antibody to a toxin produced by the gut (anti-CdtB). A 2015 study looked at 2,375 people with diarrhea-associated IBS and compared their blood levels of these two antibodies to the levels in people with inflammatory bowel disease (IBD), celiac disease or no bowel issues. In those with IBS, the authors found elevated anti-CdtB and anti-vinculin levels in 43.7 and 32.6 percent of patients respectively. In other words, these are not very sensitive tests for detecting IBS.
Still, a positive test can point doctors in the right direction. In this study, 91.6 percent of those who tested positive for anti-CdtB had diarrhea-associated IBS, while 83.8 percent of those who tested positive for anti-vinculin had the condition. People with inflammatory bowel disease had similar antibody levels as healthy adults. This means that, if someone does test positive for the high antibody levels, they likely have diarrhea-associated IBS.
A 2017 study additionally evaluated the antibody levels in people with constipation-associated IBS and people with IBS that fluctuated between constipation and diarrhea. In this study, a positive result for high levels of either anti-CdtB or anti-vinculin was found in 58.1 percent of people with diarrhea-associated IBS; 44 percent of those with fluctuating IBS; 26.7 percent of those with constipation-associated IBS; and 16.3 percent of healthy controls. The authors concluded that the results of antibody-level testing in people with constipation-associated IBS was not much different than it was for healthy people.
It's still important to rule out other potential causes for IBS symptoms of abdominal bloating and altered bowel movements. This can be done with blood tests for C-reactive protein and celiac-disease antibodies; a stool test assessing potential bacterial causes; or a stool calprotectin test, which is positive in inflammatory bowel disease.
Nonetheless, the antibody tests for CdtB and vinculin can help solidify the diagnosis of diarrhea-associated IBS or of IBS with mixed diarrhea and constipation.
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