Gas molecules expanding inside a SIBO test bag

The Complications with Testing for SIBO

Testing for small intestine bacterial overgrowth isn’t necessarily straightforward. Before you consider ordering an at home test, it’s helpful to understand the limitations and nuances of testing so you can get the most accurate diagnosis possible.

The most common test currently being used by SIBO doctors and specialists is a breath test, which tests for both hydrogen and methane type SIBO. This test is able to identify the presence of SIBO by registering the amount of these gases being produced in the small intestine.

The test measures these gases in parts per million, and the amount of gas present indicates the severity of the SIBO. So for example, a reading of 30ppm would be mild, whereas 100ppm would be high/severe.

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Here’s how a test works:

  • The patient would do a prep diet the day before a test.
  • They would then do an overnight fast.
  • The following morning, the patient would breathe into a test tube to create a baseline reading.
  • They would then drink a liquid solution made with water and either glucose or lactulose.
  • The patient would then breathe into a different test tube every 20 minutes for three hours.

This is a basic outline, but it can vary depending on the lab center and this is where testing can get a little tricky.

Lactulose or glucose?

Firstly, the glucose or lactulose solution is what the SIBO would be reacting to. If SIBO were present, the bacteria/archaea would ferment the solution and give off gases. There are pros and cons of both, but I tend to prefer lactulose.

The reason being is that the glucose solution is absorbed very quickly in the small intestine, usually in the first one-three feet; this means that if the SIBO is present further down the small intestine we would miss it. This is clearly a problem, when the average small intestine is 22 feet long! As a result, testing with glucose can cause a negative test when in fact SIBO is just further into the gut.

Lactulose, on the other hand, lasts the duration of the intestine and so is much more helpful in identifying SIBO further down. However, it also makes its way into the large intestine too, which means false positives are possible, though working with a specialized practitioner and taking into account your symptoms and history, makes this much less likely. Sometimes, there’s also a dip in the results graph at the time that the solution leaves the small intestine and enters the large intestine, and that is wonderfully helpful for showing the difference between the two, though isn’t necessary.

Two or three hours?

The other challenge is whether the test is a two-hour or three-hour test. In my training with Dr. Siebecker, we’re advised to favor the three-hour test and I couldn’t agree more. The reason for the varying times is down to the assumption that it would take the solution two hours to travel through the small intestine. While this may be true for ‘healthy’ people, those with SIBO often have obstructions in their small intestine or slow gut motility, which slows down the transit time. As a result, the solution may not reach the large intestine in those two hours or even in the three hours, meaning that the fermentation may not begin until that third hour. By ending the test at two hours or even 2.5 hours, we could end up with a false negative.

What about hydrogen sulfide?

The final complication? The breath test cannot measure hydrogen sulfide SIBO, only methane and hydrogen. This doesn’t mean all is lost for those with H2S type SIBO, there are ways to identify it which I’ll dive into in another column.

To summarize, these tests aren’t without their complications and diagnosis should always be made with consideration of the your symptoms and medical history, rather than the test alone. In another column, I’ll explore how to interpret your results.

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