lunes, 15 de octubre de 2018

Diagnosis of IBS: Breath Tests

One of the many frustrating aspects of irritable bowel syndrome (IBS) is it can appear quite vague and “un-scientific” when it comes to getting a diagnosis. In today’s modern world we expect diseases to be diagnosed rapidly and efficiently, we go to the Drs, they take some blood and a few days later we have our results. With IBS on the other hand, its lots of questionnaires, symptom diaries and trial and error.
One area that does appear, on the face of it, to offer a clear yes/no answer are breath tests. But how useful are breath tests when it comes to an IBS diagnosis?

What are breath tests?

Breath tests are used to determine whether you absorb or malabsorb a particular sugar. The tests offered are usually for lactose, fructose, sorbitol and mannitol. The patient will be given a dose of one the above sugars and then the amount of hydrogen and/or methane is measured in the patient’s breath. The understanding is that any of the sugars that are not absorbed are fermented by intestinal bacteria which produce the gases hydrogen and methane. The gases are carried in the bloodstream to the lungs where they are exhaled. A cut off point is established and if the amount of exhaled gas is above that point then the patient is diagnosed as “intolerant” to that sugar.
For an individual that suspects they may have IBS these tests appear very attractive because all of the mentioned sugars are associated with the condition plus the tests promise a quick and clear result.


Sadly here comes the but. While lactose intolerance is a recognised condition and the breath test to determine it well established, the same cannot be said for fructose, sorbitol and mannitol. One issue is the lack of standardisation of the test, different centres have different cut-off points so your diagnosis may be different depending on the centre’s cut-off point not the amount of hydrogen you produced in your test.
Focussing on fructose for a second, we all have a limited capacity to absorb fructose. That means that at a large enough dose every single one of us will malabsorb fructose. A study from way back in 1986 found that 8 out 10 healthy subjects malabsrobed a 50g dose of fructose, whereas only 1 out of the 10 malabsorbed the 15g dose (1). Another study in 2014 found similar results, in a group of 16 healthy (non IBS) participants, a 40g dose of fructose was shown to distend the small bowel with water and cause IBS type symptoms even though they were not IBS sufferers (2).
This means that depending on what dose they give you could be wrongly labelled as intolerant to fructose or even given a false IBS diagnosis. What has been shown is that some people are more sensitive than others to a single dose of fructose but to label them intolerant is probably an exaggeration.
In conclusion, while the idea of a quick test and diagnosis is very attractive, especially in a condition such as IBS where patients are usually desperate for a straight answer. The lack of standardisation of the tests and the fact that we all could be diagnosed as fructose “intolerant” at the right dose means that sadly breath tests are not a reliable way to get an IBS diagnosis.


1.  J J Rumessen and E Gudmand-Høyer, 1986. Absorption capacity of fructose in healthy adults. Comparison with sucrose and its constituent monosaccharides.Gut. 27 (10) 1161-1168

2.  Murray, K et al. 2014. Differential effects of FODMAPs (fermentable oligo-, di-, mono-saccharides and polyols) on small and large intestinal contents in healthy subjects shown by MRI. American Journal of Gastroenterology. 109 (1) 109-110