The diagnosis of bacterial overgrowth can be made by physicians in various ways. Malabsorption can be detected by a test called the D-xylose test. Xylose is a sugar that does not require enzymes to be digested. The D-xylose test involves having a patient drink a certain quantity of D-xylose, and measuring levels in the urine and blood; if there is no evidence of D-xylose in the urine and blood, it suggests that the small bowel is not absorbing properly (as opposed to problems with enzymes required for digestion).
The gold standard for detection of bacterial overgrowth is the aspiration of more than 105 bacteria per millilitre from the small bowel. The normal small bowel has less than 104 bacteria per millilitre.
Breath tests have been developed to test for bacterial overgrowth, based on bacterial metabolism of carbohydrates to hydrogen, or based on the detection of by-products of digestion of carbohydrates that are not usually metabolized. The hydrogen breath test involves giving patients a load of carbohydrate (usually in the form of rice) and measuring expired hydrogen concentrations after a certain time. It compares well to jejunal aspirates in making the diagnosis of bacterial overgrowth. 13C and 14C based tests have also been developed based on the bacterial metabolism of D-xylose. Increased bacterial concentrations are also involved in the deconjugation of bile acids. The glycocholic acid breath test involves the administration of the bile acid 14C glychocholic acid, and the detection of 14CO2, which would be elevated in bacterial overgrowth.
Some patients with symptoms of bacterial overgrowth will undergo gastroscopy, or visualization of the stomach and duodenum with an endoscopic camera. Biopsies of the small bowel in bacterial overgrowth can mimic those of celiac disease, making the diagnosis more challenging. Findings include blunting of villi, hyperplasia of crypts and an increased number of lymphocytes in the lamina propria.
However, some physicians suggest that if the suspicion of bacterial overgrowth is high enough, the best diagnostic test is a trial of treatment. If the symptoms improve, an empiric diagnosis of bacterial overgrowth can be made.
Read more about this topic: Small Bowel Bacterial Overgrowth Syndrome