SIBO is largely under-diagnosed because most doctors aren't aware of it. Many people don’t seek medical care for their SIBO symptoms, and testing for SIBO has proven to be difficult. Even the go-to breath test which measure levels of hydrogen and methane gas often gives false negatives.
"Unfortunately there is no perfect test. The small intestine (SI) is a hard place to get to. If we want to see or sample the SI, endoscopy only reaches into the top portion, and colonoscopy only reaches into the end portion. The middle portion, which is substantial (about 17 feet) is not accessible, other than by surgery. And stool testing predominantly reflects the large intestine (LI). Luckily, there is a non-invasive test which is commonly used in SIBO research; the Hydrogen & Methane Breath Test. Additionally there is a blood test validated to diagnose SIBO from food poisoning, otherwise known as Post-Infectious IBS." - Dr. Allison Seibecker