Answer: E. The carbohydrate breath test in combination with the presence of typical clinical features is the most common method used to diagnose bacterial overgrowth. The classic clinical features of small bowel bacterial overgrowth include bloating, diarrhea, abdominal pain, or weight loss, especially in those with predisposing conditions such as Roux-en-Y gastric bypass surgery.

Sensitivity and specificity of the breath test is lower than that of jejunal aspirate culture, and there is no standardized carbohydrate breath test; however, the combination of breath test and clinical features continues to be the preferred method of testing because it is easy to perform, noninvasive, and inexpensive. The breath test is based on the idea that carbohydrate metabolism by the overgrown bacteria leads to the production of hydrogen, methane, or radioactive carbon dioxide, which is then excreted in breath. The lactulose breath test is one of the most common breath tests used because it is nontoxic and safe to use in children and women of childbearing age. The results of the lactulose breath test are considered positive for small bowel bacterial overgrowth when any one of the following occurs: an absolute increase in hydrogen by ≥ 20 ppm above baseline in the first 90 minutes, a double peak pattern in hydrogen production (an early rise of hydrogen production within 1 hour of ingestion and then again for a colonic peak at 2 to 3 hours after ingestion), or methane ≥ 3 ppm at any time during the test.

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Jejunal aspirate cultures can also be used to diagnose small bowel bacterial overgrowth accurately, but this method is more expensive and more invasive than lactulose breath test. Jejunal aspirate cultures are difficult to perform and there are several limitations to the method. The patchy nature of bacterial overgrowth may cause it to be missed on sampling. In addition, culture requires careful technique and currently only approximately 40% of the total small bowel bacteria can be identified using conventional methods. Finally, contamination from oropharyngeal flora may occur during collection, causing a false positive. 

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