Patient guide · Gastroenterology
Hydrogen breath test, investigation of lactose / fructose intolerance and small intestinal bacterial overgrowth (SIBO).
The hydrogen (and methane) breath test uses lactulose, glucose, lactose or fructose as a substrate and measures exhaled hydrogen and methane over 2–3 hours. Diagnoses lactose / fructose intolerance and small intestinal bacterial overgrowth (SIBO / IMO).
Reviewed by Pulse Atlas Editorial Board, · Published 2026-07-30 · Next review 2027-07-30 · 6-minute read
Why patients choose us
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The right hands
We route you to a consultant gastroenterologist and an accredited breath-test lab that follows the North American Consensus protocol.
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A single, clean answer
Baseline plus every 15–30 minutes for 2–3 hours - hydrogen and methane both measured, so lactose, fructose, SIBO and IMO are separable.
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Independent, and free
We are paid by no clinic, so the recommendation is impartial and costs you nothing.
Key facts
The hydrogen breath test, in six lines.
Everything a patient needs to hold in their head before the appointment.
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What it is
A breath test measuring exhaled hydrogen and methane after you drink a sugar substrate.
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Substrates
Lactulose, glucose, lactose or fructose - the choice depends on the clinical question.
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Duration
2–3 hours in the clinic, with breath samples every 15–30 minutes.
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During the test
No smoking, no exercise, no eating between samples.
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Antibiotics
Withhold antibiotics for four weeks before the test - they suppress the gut flora that produce the gas.
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Reporting standard
Interpreted against the North American Consensus thresholds for hydrogen and methane rise.
Indicative pricing
What a private hydrogen breath test costs in London.
Indicative ranges across our partner clinics. Send the details and we quote firm figures across two or three options.
In short
A hydrogen breath test in our network: £220–£450, with a written report typically within 48 hours.
| Test type | Indicative range | Typical duration | Report turnaround |
|---|---|---|---|
| Lactose hydrogen breath test | £220–£380 | 2.5 hr | 48 hours |
| Fructose hydrogen breath test | £220–£380 | 2.5 hr | 48 hours |
| Glucose SIBO breath test (H₂ + CH₄) | £260–£420 | 2 hr | 48 hours |
| Lactulose SIBO / IMO breath test (H₂ + CH₄) | £260–£450 | 3 hr | 48 hours |
| Breath test + gastroenterology consultation | £500–£900 | Half-day | Same visit |
| Full functional gut work-up (breath + stool + bloods) | £700–£1,400 | Half-day | Same week |
Prices vary by clinic, whether hydrogen and methane are both measured, and whether a same-visit gastroenterology consultation is included. We come back with a firm quote within one working day.
The problem
A breath test is only as good as its preparation and interpretation.
The North American Consensus thresholds and the choice of substrate decide whether the test answers the right question. We route you to a lab that runs the protocol properly, and a gastroenterologist who reads it.
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Bloating and diarrhoea after dairy?
A lactose hydrogen breath test confirms - or rules out - lactase deficiency.
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IBS-style symptoms not responding?
A SIBO or IMO breath test can uncover an overgrowth that changes the treatment plan.
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Failed low-FODMAP trials?
A fructose breath test separates fructose malabsorption from a broader FODMAP intolerance.
Preparation and the test day
From referral to report - what happens, in order.
Preparation is doing most of the work. The clinic day is straightforward once the four-week and 24-hour rules are respected.
Phase 1 · Before your test
Antibiotics, diet, fasting
Phase 2 · On the day
2–3 hours at the clinic
Phase 3 · After
Report within 48 hours
- 01
Before
Referral or self-book
A short, confidential form. Symptoms, prior tests, medications, insurer if you have one.
- 02
Before
No antibiotics for four weeks
Antibiotics wipe out the bacteria that produce hydrogen and methane - the test cannot be interpreted without them.
- 03
Before
Low-fibre diet, 24 hours pre-test
White rice, chicken, fish, eggs, clear broth. Avoid pulses, wholegrains, dairy and fermentable vegetables.
- 04
Before
Fast for 12 hours
Water only from the evening before. No chewing gum, no mints, no smoking on the morning of the test.
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On the day
Baseline breath sample
A single exhaled breath into the analyser establishes your starting hydrogen and methane.
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On the day
Drink the test substrate
Lactulose, glucose, lactose or fructose in water - the substrate is chosen for the clinical question.
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On the day
Breath samples every 15–30 minutes
For 2–3 hours. You sit quietly - no exercise, no snacks, no smoking, no sleeping.
Typical end-to-end: 4 weeks preparation, one clinic morning. Report: within 48 hours.
What it shows
The patterns a hydrogen and methane breath test picks up.
Different curves point to different diagnoses. These are the patterns the report describes, and what each means for the next step.
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Lactose intolerance
A rise in hydrogen after a lactose challenge confirms lactase deficiency.
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Fructose intolerance
A hydrogen rise after fructose points to fructose malabsorption.
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SIBO - hydrogen-producing
An early hydrogen rise on lactulose or glucose meets the criterion for small intestinal bacterial overgrowth.
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IMO - methane-producing
A sustained methane elevation is diagnostic of intestinal methanogen overgrowth.
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Rapid orocaecal transit
A very early hydrogen peak can reflect fast small-bowel transit rather than overgrowth.
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Post-glucose SIBO diagnosis
Glucose is absorbed proximally - a hydrogen rise implies bacteria in the upper small bowel.
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Non-responder pattern
A flat curve with typical symptoms suggests a non-hydrogen, non-methane producer or a non-fermentative cause.
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Red flag: extreme methane + severe constipation - MDT gastroenterology review
Marked methane elevation with refractory constipation warrants combined gastroenterology and dietetic review.
Next steps
What a positive result changes.
The treatment routes a positive breath test opens - from diet to targeted antibiotics to repeat testing.
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Dietary adjustment
Structured lactose or fructose avoidance under a registered dietitian - the mainstay of intolerance management.
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Lactase supplementation
Oral lactase tablets with dairy-containing meals for confirmed lactose intolerance.
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Rifaximin for SIBO
A gut-selective, poorly absorbed antibiotic - the first-line evidence-based therapy for hydrogen-positive SIBO.
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Rifaximin + neomycin for IMO
Combination therapy targets methanogenic archaea in intestinal methanogen overgrowth.
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Low-FODMAP dietary trial
A structured elimination and reintroduction protocol, led by a FODMAP-trained dietitian.
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Probiotic / prebiotic trial
Targeted strains and fibres, chosen against the phenotype - not a blanket recommendation.
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Neurogastroenterology follow-up
For overlapping motility and visceral hypersensitivity, a subspecialty opinion sharpens the plan.
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Repeat testing to confirm eradication
Repeat hydrogen and methane breath testing after treatment to confirm normalisation.
Our vetted London network
A small panel of clinics, we picked them.
Partners across central, north, west and south London. Not listed publicly - introductions are made privately, once we understand your case.
Selection criteria
How we choose every clinic in our network.
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Consultant gastroenterologists or accredited breath-test scientists
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North American Consensus protocol for hydrogen and methane interpretation
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Full written report within 48 hours, with images and raw curves available
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Onward gastroenterology, dietetics or neurogastroenterology pathway if abnormal
Differentials and red flags
When another pathway is a better use of your time.
A hydrogen breath test answers a specific question. These conditions can mimic or complicate the picture - and change what should be done first.
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Coeliac disease
Symptoms overlap - a coeliac serology should be considered before or alongside breath testing.
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Inflammatory bowel disease
Faecal calprotectin and endoscopy remain the pathway for suspected IBD.
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Chronic pancreatitis
Exocrine insufficiency mimics carbohydrate malabsorption - faecal elastase is the first test.
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Post-cholecystectomy diarrhoea
A well-recognised cause of loose stool after gallbladder removal - a different pathway.
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Bile acid diarrhoea
SeHCAT or a therapeutic trial of a bile-acid sequestrant is the right work-up.
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Rome IV IBS with alarm features
Weight loss, bleeding, nocturnal symptoms or a family history of cancer warrant urgent structural assessment first.
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Structural gut disease
Strictures, diverticula and adhesions can mimic SIBO - cross-sectional imaging may be needed.
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Post-radiotherapy
Pelvic or abdominal radiotherapy predisposes to SIBO - a low threshold to retest.
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Post-surgical anatomy (blind loop)
Roux-en-Y, resections or blind loops materially raise SIBO risk and change the interpretation.
Reading your report
A breath-test report can look intimidating. It isn’t.
Whatever the finding, the report keeps to the same four parts.
A quiet reminder
The report is written for your doctor, not for you - and that’s normal.
If you would like us to talk you through it before your follow-up, just ask.
- 01 Header
Indication, substrate and preparation
Your details, the reason for the test, the substrate used and confirmation that preparation criteria were met.
- 02 Technique
Sampling protocol and analyser
Baseline plus interval breath samples every 15–30 minutes over 2–3 hours, on a validated hydrogen and methane analyser.
- 03 Findings
Hydrogen and methane curves, in ppm
The full curves for hydrogen and methane, with peak rise above baseline and time to peak - the raw data behind the impression.
- 04 Impression
The conclusion: read this first
Positive or negative against North American Consensus thresholds, the likely phenotype (lactose intolerance, fructose malabsorption, SIBO, IMO), and the concrete next step.
Recognised by major UK insurers
Cover depends on your policy and clinic; we confirm with your insurer before booking.
Frequently asked
Everything we get asked about the hydrogen breath test.
Quick answers on preparation, timing, substrate choice, SIBO vs IMO, and how long results take.
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What does a hydrogen breath test show?
It measures hydrogen and methane in your exhaled breath after you drink a sugar substrate - lactulose, glucose, lactose or fructose. A rise above threshold indicates lactose or fructose intolerance, small intestinal bacterial overgrowth (SIBO), or intestinal methanogen overgrowth (IMO).
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How do I prepare for the test?
Withhold antibiotics for four weeks, follow a low-fibre diet for 24 hours, fast for 12 hours (water only), and avoid smoking, chewing gum and exercise on the morning of the test.
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How long does the test take?
Between 2 and 3 hours, depending on the substrate. You sit quietly in the clinic while breath samples are collected every 15–30 minutes.
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Which substrate should I have?
Lactose for suspected lactose intolerance, fructose for fructose malabsorption, glucose for proximal SIBO, and lactulose for a wider small-bowel and IMO assessment. The gastroenterologist chooses on your history.
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What is the difference between SIBO and IMO?
SIBO is small intestinal bacterial overgrowth - hydrogen-producing bacteria. IMO is intestinal methanogen overgrowth - methane-producing archaea, more often associated with constipation. Treatment differs, which is why both gases are measured.
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How quickly will I get results?
The written report against the North American Consensus thresholds is typically issued within 48 hours, with an onward gastroenterology or dietetic pathway if the test is positive.
Sources
What this guide is based on.
- British Society of Gastroenterology. Guidance on breath testing and functional gut disorders.
- Rezaie A et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American Consensus.
- European Society of Neurogastroenterology and Motility. Position statements on SIBO and functional gut disorders.
- Pimentel M et al. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American College of Gastroenterology.
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In practice, in London
How hydrogen breath test tends to unfold when you go private
With hydrogen breath test, the London question is usually about report turnaround and the radiologist reading it - not whether the scan is available. Public provision for hydrogen breath test is competent but constrained by capacity. Private London clinics tend to have shorter diaries and longer appointment slots, so you get the same specialists with more time. For people who’ve been going round in circles with primary care, that first proper conversation is often what shifts things.
A private hydrogen breath test pathway in London usually looks like this: an initial consultation, any diagnostics booked at a nearby facility (most within Zone 1 or 2), and a written report sent to you and your GP within a few days. The consultants we work with hold NHS posts alongside their private lists, which keeps the standards consistent across both settings. For hydrogen breath test specifically, the difference between a routine report and a sub-speciality read is where private care earns its keep.
The value of going through a concierge for hydrogen breath test isn’t access - anyone with an insurer or a credit card can get a private appointment in London. The value is knowing which consultant reads this particular presentation best, which unit turns reports around fastest, and which pathway won’t hit a dead end if the findings point somewhere unexpected.