That’s a great question. SIBO, as you might imagine is always in flux. Your body’s control mechanisms (stomach acid, efficient digestion of food, bile, immunity, motility, etc) are always working to keep intestinal bacteria in check, especially in the small intestine. And bacteria are often trying to work their way back into the small intestine where more nutrients are available.
I believe you can test negative and still have periodic bouts or blooms of SIBO.
Other possible contributors to false negative results:
1. Eliminating fiber and high residue foods from your diet either the day before or the night before. By doing this, you are actually beginning an intervention diet that helps control SIBO. I would like to see this practice adjusted to only limiting fiber the night before.
2. Using glucose instead of lactulose. Glucose is absorbed very quickly and may not detect SIBO, but lactulose is not digested at all so can detect SIBO throughout the length of the small intestine.