What is the actual difference between the two sugars?

Glucose is absorbed early in the small intestine, so it mostly probes the upper portion. Lactulose is not absorbed by humans and travels the full length, so it can reach the colon. They sample different territory, which is why they disagree.

How often do they agree?

In one 2025 study, on 8.7% of cases. That is not a measurement error. Two tests probing different regions with different sugars are answering related but distinct questions, so disagreement is the expected outcome rather than a fault.

Does a positive on one and a negative on the other mean anything?

It means the two tests disagreed, which happens most of the time. It is a reason to discuss the result with a clinician rather than to average them, and it is a reason to be suspicious of any confident interpretation of a single test.

That is a clinical decision and it is not obviously twice as useful. Given the concordance figure, two tests are quite likely to give you two different answers and no principled way to choose between them.

Lactulose or glucose breath test

By B. Gray Randolph · Updated August 5, 2026

If you have had two breath tests that disagreed, that is the normal outcome, not a laboratory failure.

What separates the two tests

Glucose is absorbed early in the small intestine, so it mostly probes the upper portion. Lactulose is not absorbed by humans at all and travels the full length, which means it can reach the colon. The two sugars sample different territory, so they are answering related but distinct questions and their results are not interchangeable.

How often they agree

On 8.7% of cases in one 2025 study. Nine times out of ten, two tests both sold as SIBO breath tests, given to the same people, reached different conclusions. That is a fact about what the tests measure rather than about how carefully they were run.

Scalese G et al., Journal of Clinical Medicine 2025 Peer-reviewed studyWe have not yet confirmed a direct link to this document, so we are not giving you one. The citation above is enough to find it, and we would rather say this than send you to a journal front page.

The same study looked at whether symptoms could sort people into subtypes and found they could not: clinical symptoms did not significantly differ between test-positive and test-negative groups. So symptom reasoning does not rescue you when the tests disagree, which is the situation most people are actually in.

Why neither is simply better

Each buys something and costs something. Glucose is more specific for the upper small intestine and can miss overgrowth further down. Lactulose covers more ground and can produce a rise originating in the colon that looks like a small-intestine result. Choosing between them is choosing which error you would rather make, and that is a conversation with a clinician rather than a fact we can settle here.

What is not settled here

There is a live argument in the literature that breath testing should not be used for this purpose at all.

Kashyap P et al., Neurogastroenterology & Motility 2024 Peer-reviewed studyWe have not yet confirmed a direct link to this document, so we are not giving you one. The citation above is enough to find it, and we would rather say this than send you to a journal front page.

We are not in a position to settle that and we are not going to pretend the disagreement is smaller than it is. What we can say is that a test whose validity is actively disputed among specialists should not be read by a patient as a verdict, and that the confidence of a website explaining it is not evidence about the test.

What the guideline says

Pimentel M et al., American Journal of Gastroenterology 2020 Clinical guideline

All six recommendations conditional, five of six resting on evidence the authors themselves grade as very low quality. That is the state of the best-documented part of this field, and it is the context in which any single result should be read.

What to do with a disagreement

Do not average them and do not pick the one you prefer. Take both results, with their dates and substrates, to a clinician. The disagreement is information about how hard the question is, and it is worth more in that conversation than either number alone.

Common questions

What is the actual difference between the two sugars?: Glucose is absorbed early in the small intestine, so it mostly probes the upper portion. Lactulose is not absorbed by humans and travels the full length, so it can reach the colon. They sample different territory, which is why they disagree.
How often do they agree?: In one 2025 study, on 8.7% of cases. That is not a measurement error. Two tests probing different regions with different sugars are answering related but distinct questions, so disagreement is the expected outcome rather than a fault.
Which one is better?: Neither, and the framing is the problem. Glucose is more specific for the upper small intestine and can miss overgrowth further down; lactulose covers more ground and can produce a rise from the colon that looks like a small-intestine result. Each buys something and costs something.
Does a positive on one and a negative on the other mean anything?: It means the two tests disagreed, which happens most of the time. It is a reason to discuss the result with a clinician rather than to average them, and it is a reason to be suspicious of any confident interpretation of a single test.
Should I get both?: That is a clinical decision and it is not obviously twice as useful. Given the concordance figure, two tests are quite likely to give you two different answers and no principled way to choose between them.