06/11/2026 - Gosia
SIBO and IMO, Part 2: the breath test
If you think a breath test is just blowing into a few tubes, I admire your optimism. The test measures gas produced by microbes in your gut, so anything that changes those microbes or how fast your gut moves can skew the result. That's why the preparation starts weeks before the test itself.
In the UK, the reference point is a best practice protocol from the gastrointestinal physiology group of the British Society of Gastroenterology. According to it, you should avoid antibiotics for four weeks before testing. Laxatives and medication that speeds up the gut should ideally be stopped for a week, and probiotics for at least a week. But only stop prescribed medication if your doctor agrees it's safe. A breath test isn't worth that.
For at least 24 hours before the test, you avoid fermentable foods. That leaves you with plain protein such as chicken, fish and eggs, plain white rice, and plain tea, coffee or water. No dairy, no fruit or vegetables, no beans, no wheat or other grains, no alcohol. Not exactly a culinary highlight, but it's one day. After that comes a 12-hour fast, so most people test first thing in the morning.
On the day itself, no smoking, and that includes e-cigarettes. During the test, no physical activity and no napping either. You can brush your teeth beforehand, as long as you don't swallow the toothpaste. You'll mostly be sitting, so bring a book.
Each hospital or lab will give you its own instructions, and they won't always match the above exactly. So read them properly.
How long does it take? It depends on the test.
You give a baseline breath sample, drink the sugar solution, and then keep giving samples at set intervals. How often and for how long depends on the protocol, so it really does vary. UK guidance actually prefers lactulose as the first choice, with glucose as a back-up, but hospitals do things differently. Mine used glucose, and the test was done in a little over an hour. My private home test with Healthpath used lactulose and took over three hours, with a sample every 30 minutes.
Now the bit everyone asks about. NHS or private?
On the NHS, you start with your GP. For IBS-type symptoms, NICE guidance says your GP should first rule out other conditions with a few basic blood tests, including inflammation markers and coeliac antibodies. If SIBO is still on the table, you'd usually be referred to gastroenterology, and the breath test is done in hospital. Which sugar is used, and whether breath testing is offered at all, varies between hospitals.
The NHS route costs nothing. The price is time. My wait for gastroenterology was long, and I know I'm not the only one.
One tip from my own experience: ask whether methane is being measured. Not every hospital tests for it by default. My NHS test was originally planned as hydrogen only. Because I'd tested positive for methane before, I asked for it to be included, and the consultant agreed. If constipation is one of your main symptoms, that question matters, because IMO is diagnosed on methane, not hydrogen.
The private route is quicker. Companies such as Healthpath (not sponsored, lol) send a kit to your home, you do the test yourself and post the samples to a lab. With Healthpath, they go to a lab in Germany, and my results came back in about two weeks. You pay for it, of course.
But here's the catch. A private result doesn't come with a prescription. You still need a doctor to look at it and decide on treatment. Some GPs are happy to act on a private result. Others will say it needs to go through gastroenterology. I met both kinds, a few days apart.
So if you're going private, it's worth asking your GP before you order the kit whether they'd act on the result. It might save you money, time and a slightly awkward appointment.
And whichever route you take, remember what I said in Part 1. A breath test isn't perfect. A glucose test can miss overgrowth further down the small intestine, and a lactulose test can come back positive when it isn't SIBO at all. The result should always be read by a doctor, together with your symptoms and history.