After fasting overnight, a breath test begins with the ingestion of a carbohydrate — most often glucose or lactulose when testing for SIBO — and the hydrogen and methane content of the person’s exhaled breath is measured at specific intervals over the next few hours. However, a number of factors can reduce the accuracy of breath tests and lead to a false positive or false negative.[1]
The person being tested could reduce the accuracy of the test in the following ways:
- Using antibiotics within 4 weeks of testing
- Using laxatives or gut-motility drugs within a week of testing
- Not fasting long enough (8 to 12 hours) before the test
- Eating fermentable foods, such as complex carbohydrates, the day before the test
- Smoking before the test
The carbohydrate used during the test also affects the results. Glucose is more likely to lead to a false negative than lactulose, but glucose is regarded as a more accurate diagnostic tool. Lactulose is more likely to lead to a false positive by quickly reaching the large intestine, where hydrogen will be produced. Using too large a dose can speed up intestinal transit even more, leading to an early peak in gas production that could lead to a false positive.[2][1]
Clinicians and researchers may also set their own cutoffs for the level of gas production required to diagnose SIBO, which would obviously have an impact on results.[1]