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You may like to read our earlier articles on (a) testing for small intestine bacterial overgrowth, (b) the causes of SIBO and (c) the effects of SIBO.

This article ‘small intestine bacterial overgrowth: results’ discusses how to interpret the results. The first thing to appreciate is testing for SIBO has to include both measuring hydrogen production and methane production.

For those interested this is a great paper on interrupting results. It is also worth reminding readers that there are different solutions used for hydrogen breath testing and thus interpreting results depends on the solution used (for example glucose of lactulose). With glucose testing you have a greater risk of false negatives and with lactulose you have a greater risk of false positives.

A Negative Breath Test

If there is no malabsorption of the test substance, there is no significant increase in hydrogen levels, i.e. readings vary by less than 5 ppm above or below the basal value, and there are no symptoms. Only if both criteria apply (lack of H2 increase and lack of symptoms) normal findings should be diagnosed.

However as we discuss later on under hydrogen sulfide, we can’t always be 100% certain about the results. The key may be in placing the results in the context of the clients symptoms, health history and other test results.


Although methane is detected in 30%-50% of the healthy adult population worldwide, its production has been epidemiologically and clinically associated with constipation related diseases, like constipation predominant irritable bowel syndrome and chronic constipation (Triantafyllou et al., 2014)

We frequently find this to be the case and have yet to see one client who hasn’t had elevated methane levels who has struggled with chronic constipation. Methane acts like a neuromuscular transmitter resulting in reduced propagation of the peristaltic movement in the intestine. Mark Pimentel described the different methane profiles:

There are different methane profiles, and this gets a little complicated. There are methane profiles where methane is elevated throughout the test and hydrogen is zero all the way across the board. The only way that can happen is if the methanogen organisms are colocalized precisely where all the hydrogen organisms are. In other words, the methanogens are eating up every bit of hydrogen that you find on a breath test because they’re sitting beside each other throughout the gut (Mark Pimentel – from an interview with Chris Kresser, found here)

If the methane is present on the breath test at greater than 3 parts per million at any part during the test, the association to constipation is over 90% (Mark Pimentel)


SIBO is diagnosed on glucose if there is a rise in breath hydrogen by 12 ppm above the basal

Conventionally, a double peak when lactulose is used as the solution (one due to SIBO and the other from colon) is considered as diagnostic of SIBO.

What About Flat Lining – Hydrogen Sulfide

Too much H2S, produced by the overgrowth of harmful, pathogenic bacteria as occurs during inflammation causes the intestinal epithelial barrier to break down. Increased levels of bacterial H2S stimulate the production of destructive compounds called ROS, which inhibit mitochondrial function directly.

This can be sometimes be seen when both hydrogen and methane levels are flat lining. Often clients will be experiencing particularly smelly flatulence (think rotten egg smell!).

Chris Kresser explains this and mentions studies that show patients with lower methane production (zeros in report), can have higher concentrations of sulfate-reducing bacteria, and the product of sulfate reduction is hydrogen sulfide. Hydrogen sulfide should be removed by first-pass detoxification in the liver, but if detox mechanisms are impaired (and they often are when the digestive system is compromised due to the gut-liver axis), the hydrogen sulfide can accumulate in the small intestine and the colon.

In this sort of situation there is a test that can be considered to explore this further. There’s a lab called Protea Biopharma, available through Invivo Labs in the UK, that run the test. It’s actually marketed as a test for Th1/Th2 immune imbalance (which may be useful in it’s own right in patients with chronic fatigue syndrome – read our article here on CFS being a Th2 dominant condition), but it may be valid in a situation where the client has a flatline SIBO breath test result.

The test is a urine test. The general principle is that when sulfides combine with iron compounds, they produce iron sulfide, which is a dark color in the urine. So if the patient does this test and the urine turns dark, it suggests there are bacteria in the gut that are producing large amounts of hydrogen sulfide. Normally the enzyme sulfide oxidase, which is found in the wall of the gut, oxidises the hydrogen sulfide. However, if levels are elevated, the enzyme is not able to oxidise all of it, and it gets passed into the blood, and then filtered out by the kidneys, and excreted in the urine.

Small Intestine Bacterial Overgrowth: Results – Conclusions

As you can see diagnosis SIBO isn’t necessarily as simple as running a breath test. It requires blending the results with the symptoms and story of the client as well as the clinicians skill and experience in working in the field of digestive health.

The next article in this series on small intestine bacterial overgrowth is on treatment of SIBO.