End-expiratory hydrogen and methane are measured by gas chromatography at baseline and hourly for 3 hours after 25 grams of fructose are ingested. The diagnostic criteria for fructose malabsorption is a rise above baseline of 20 parts per million (hydrogen and methane) for any post-dosing breath sample.
Causes of False-Negative Results
False-negative results are reported in 5-18% of all fructose malabsorbers. There are various reasons for false-negative results. Some patients possess bacteria that produce neither hydrogen nor methane. However, the major cause for false-negative results is poor patient preparation. Administration of antibiotics within 4 weeks of testing sterilizes the colon to reduce bacterial colony counts. Laxatives and enemas can also decrease hydrogen and methane responses. Hydrogen and methane production can decrease with severe diarrhea that changes colonic pH.
Causes of False-Positive Results
False-positive results are almost always the result of improper test preparation or conduct. Patients need to avoid foods with high fiber content 24 hours prior to the test. If patients consume foods with fiber, they will have high hydrogen or methane levels being produced during the test. Cigarette smoke contains hydrogen gas. If patients smoke prior to the test, a false test result may result. Sleeping during the test can delay gastric emptying and slow down the removal of hydrogen from the blood.