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.

Which sugar will I actually be given?

All five tests we track offer both, so it is usually decided by whoever orders the test rather than fixed by the product you buy. That makes it a question to ask before paying rather than after. Which sugar was used should also appear on the report, and a result quoted without it is missing the part that decides what it measured.

Does the length of the test matter?

It does, and these are not all run over the same window. A shorter run can end before a later rise appears. A longer one is more likely to capture fermentation that began once the sugar reached the colon, which is the specific way a lactulose test can look positive for the wrong reason. The window is part of the method, not an administrative detail.

Lactulose or glucose breath test

By BeatSIBO editorial team · Updated August 6, 2026

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

What separates the two tests

The short answer

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.

Glucose
Absorbed by you?:
Yes, early
How far it travels:
Mostly the upper small intestine
The error it invites:
Missing overgrowth further down
Lactulose
Absorbed by you?:
No, not at all
How far it travels:
The full length, and can reach the colon
The error it invites:
A colonic rise that looks like a small intestine result

What each sugar can and cannot reach
Substrate	Absorbed by you?	How far it travels	The error it invites
Glucose	Yes, early	Mostly the upper small intestine	Missing overgrowth further down
Lactulose	No, not at all	The full length, and can reach the colon	A colonic rise that looks like a small intestine result

Choosing between them is choosing which error you would rather make. That is a conversation with a clinician rather than a fact this page can settle.

How often they agree, stated carefully

The short answer

In one 2025 study, 47 patients took both tests. Of the overgrowth diagnoses made in that group, 74% came from the lactulose test alone and 13% from glucose alone. Only 8.7% were identified by both. The tests were mostly finding different people, which is a fact about what they measure rather than about how carefully they were run.

Scalese G et al., Journal of Clinical Medicine 2025 Peer-reviewed study

Read the denominator, because it does real work here. The 8.7% is a share of the diagnoses made within that subgroup, not a share of all patients, and the concordant cases were specifically those positive for methanogen overgrowth on both tests. The authors add the caveat themselves: the subgroup was too small for a concordance index to be calculated at all.

We stated this figure wrongly for two days, as a flat agreement rate between the two tests. It is logged at our corrections page, because a site that publishes other people's measurement errors has no business quietly fixing its own.

The same study looked at whether symptoms could sort people into subtypes and found they could not: clinical symptoms were not predictive of subtype. 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

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.
Which sugar will I actually be given?: All five tests we track offer both, so it is usually decided by whoever orders the test rather than fixed by the product you buy. That makes it a question to ask before paying rather than after. Which sugar was used should also appear on the report, and a result quoted without it is missing the part that decides what it measured.
Does the length of the test matter?: It does, and these are not all run over the same window. A shorter run can end before a later rise appears. A longer one is more likely to capture fermentation that began once the sugar reached the colon, which is the specific way a lactulose test can look positive for the wrong reason. The window is part of the method, not an administrative detail.