In contrast to the colon, which is colonized by many bacteria, the small intestine normally contains very few bacteria.  If the number of bacteria is abnormally high, they produce gas while the organisms digest nutrients, and one may experience symptoms of bloating, gas production, abdominal discomfort and diarrhea.  This condition is known as small intestinal bacterial overgrowth (SIBO). However, as many digestive diseases may present with the same symptoms, the challenge to patients and providers is to distinguish between symptoms of SIBO and irritable bowel syndrome (IBS) or other gastrointestinal disorders. IBS is a common disorder characterized by abdominal pain with diarrhea, constipation—or both diarrhea and constipation—and bloating.

Risk factors

Patients presented with this constellation of symptoms should be assessed for risk factors for development of SIBO.  Common risk factors for SIBO include decreased gastric acid from prior stomach resection or use of acid inhibiting medications, alterations in bowel anatomy (i.e., small bowel diverticulum) and motility disorders where movement through the intestines slow down (i.e., scleroderma). Other risk factors are the use of narcotics, which slow down gut movement, or an immune disorder.  If a patient has risk factors for SIBO, a provider may consider diagnostic testing to attempt to differentiate SIBO from IBS.  However, some patients have SIBO without a known predisposing condition.

IBS is diagnosed simply if a patient reports symptoms consistent with IBS. 

Testing the difference

There are two diagnostic tests available for SIBO, the first being a glucose-hydrogen/methane breath test in which lactulose is used instead of glucose for diabetic patients. The second is a small bowel aspirate performed during an upper endoscopy. Each of these tests has shortcomings and the utility of testing for SIBO is debated amongst gastroenterologists. Furthermore, these tests are not readily available through most gastroenterologists. Adding to the diagnostic dilemma, IBS is diagnosed simply if a patient reports symptoms consistent with IBS.  

Because of the similarity of symptoms between SIBO and IBS, the question of SIBO causing or mimicking IBS has been theorized.  Due to the shortcomings of SIBO testing and lack of diagnostic testing for IBS, differentiating the two disorders can be challenging.  While the answer is not clear at this time, hopefully advances in research will provide answers to this diagnostic dilemma.