What is the best test for small intestinal bacterial overgrowth?

Small intestinal bacterial overgrowth (SIBO) can cause many IBS symptoms like gas, bloating, diarrhea, constipation and abdominal pain.  The best test for SIBO is a breath test, but there are two different types of breath tests.  Let’s discuss which one you should you get?

Dr. Michael Ruscio, DC: Hi, this is Dr. Ruscio. And if you’re not familiar with small intestinal bacterial overgrowth, or SIBO for short, this may be responsible for many of the symptoms of IBS — gas, bloating, abdominal pain, altered bowel frequency (meaning constipation or diarrhea). And so this is a condition. IBS affects about 10% of the population. So it’s a fairly common condition. And small intestinal bacterial overgrowth or SIBO may underlie many cases of IBS.

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DrMR: Now, this can be tested via a breath test. But there are two different types of breath tests that are typically used. There is a lactulose test. And there is a glucose test. And there is controversy as to which one is the best. And so I thought we could talk briefly about the pros and the cons to help you or your doctor determine what the best test might be for you.

Now, lactulose is kind of the favored test right now, you could say, in the SIBO community. And there’s something to be aware of with lactulose that I think is very important. Lactulose produces what’s called false positives, meaning the tests show SIBO, but the person doesn’t actually have SIBO. So they’re positive for the test, but they’re falsely positive. It’s a false positive test.

The reason for this is because lactulose makes its way all the way through the small intestine and then into the large intestine. Now, as that lactulose is traveling through the small intestine, the gas reading that we see as a byproduct of that should be low. But when it gets into the large intestine, the gas level should elevate.

However, if for some reason, that lactulose makes its way through the small intestine too quickly and gets into the large intestine during the time period when we think it should be in the small intestine, that will give us a false positive elevation of gas. And this has been consistently shown in many of the studies that the lactulose seems to produce what’s called false positives.

Now, the merit to the lactulose test is that because it travels all the way through the small intestine, it gives you a good indication of what’s happening at the end of the small intestine where small intestinal bacterial overgrowth may be more common. But we have to counterbalance that with the fact that false positives occur.

False Positives

What this means if you’re a patient being treated for SIBO or if you’re a doctor treating a patient for SIBO is that if someone tests positive with lactulose, you treat them, their symptoms improve, but their labs don’t, there’s a probability that that person may not be a good person to use lactulose with. And you may be seeing a false positive test. And we’ve actually talked with patients like that in our newsletter in the past. So it’s very important to keep that in mind.

Now, glucose does not produce the same false positives. So with glucose, if you see a positive, you can rest assured that’s a true positive. The negative aspect of glucose is that it does not make its way all the way to the end of the small intestine in most cases. And if there’s SIBO at the end of the small intestine, you may miss it on glucose testing.

So both these tests have merit. It’s just important that we understand the pros and the cons of each. Lactulose will overreport SIBO, or cause false positives. Glucose may underreport SIBO. However, when we look at the studies, the studies seem to favor glucose, in my opinion, in terms of giving us the most accurate gauge.

In any case, either test can be used. It’s just important you work with a doctor who’s well trained and knows how to interpret these tests and, more importantly, will interpret these tests cautiously.

There certainly seems to be this belief on the internet that you can never get over SIBO. And I think a lot of that has to do with the high use of lactulose testing and people not having the guidance of a good clinician. If someone has symptoms of SIBO, they’re treated for SIBO, and their symptoms get a lot better but their labs stay normal, and they’re using lactulose, I look at that like a false positive or that they don’t have SIBO. And now we can stop treating you because you feel better.

So again, both these tests have merit. If you look at our website, we recently did a podcast, which is fully transcribed, where we went through the studies on this issue point by point and really picked into the details.

But from a big picture perspective, both tests can be used. Lactulose will give false positives, so you have to be aware of that. Glucose seems to be more accurate, but it may miss some cases.

Additional Tips

One other tip I should mention — or two other tips — and one reiteration is that if you’re using lactulose with someone, you test them for SIBO, they’re positive, you treat SIBO, their symptoms get much better or go away, and then you retest them and they still have SIBO via lactulose testing, I would be very suspect of a false positive.

Also, with lactulose, if you only see positive SIBO at the very end of the test at maybe 100 or 120 minutes, if that’s the only positive point you see with lactulose, I’d be suspicious that that might be a false positive because if the lactulose makes its way through the small intestine quickly in that person because it may be acting as an irritant or a laxative for them or they have a congenitally shorter small intestine as some do — and many Asians actually do — then you may get a false positive.

With glucose, if you see positive SIBO, you can rest assured that that’s a true positive. However, there is a possibility that that test could be negative. And if the person still has symptoms, you may have missed a SIBO infection at the very end of the intestines. And that person, you may want to treat and then evaluate how they’re doing. If they respond well, then you’ve kind of affirmed that SIBO was actually there.

Both tests can be used. Both tests have merit. I personally use both. It’s just important to be aware of the pros and the cons.

So this is Dr. Ruscio. And I hope this information helps you get healthy and get back to your life. Thanks!

What do you think? I would like to hear your thoughts or experience with this.

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• Healthcare providers looking to sharpen their clinical skills, check out the Future of Functional Medicine Review Clinical Newsletter.

Dr. Ruscio is your leading functional and integrative doctor specializing in gut related disorders such as SIBO, leaky gut, Celiac, IBS and in thyroid disorders such as hypothyroid and hyperthyroid. For more information on how to become a patient, please contact our office. Serving the San Francisco bay area and distance patients via phone and Skype.