Breath hydrogen testing for assessing the presence of carbohydrate malabsorption is frequently applied to refine dietary restrictions on a low fermentable carbohydrate (FODMAP) diet. Its application has also been extended for the detection of small intestinal bacterial overgrowth. Recently, several caveats of its methodology and interpretation have emerged. A review of the evidence surrounding its application in the management of patients with a functional bowel disorder was performed. Studies were examined to assess limitations of testing methodology, interpretation of results, reproducibility, and how this relates to clinical symptoms. A wide heterogeneity in testing parameters, definition of positive/negative response, and the use of clinically irrelevant doses of test carbohydrate were common methodological limitations. These factors can subsequently impact the sensitivity, specificity, and false positive or negative detection rates. Evidence is also increasing on the poor intra‐individual reproducibility in breath responses with repeated testing for fructose and lactulose. On the basis of these limitations, it is not surprising that the diagnosis of small intestinal bacterial overgrowth based on a lactulose breath test yields a wide prevalence rate and is unreliable. Finally, symptom induction during a breath test has been found to correlate poorly with the presence of carbohydrate malabsorption. The evidence suggests that breath hydrogen tests have limited clinical value in guiding clinical decision for the patient with a functional bowel disorder.

Bacterial fermentation of carbohydrates in the colon produces the gases hydrogen, methane, and carbon dioxide. Excessive and rapid production of these gases can distend the colonic lumen resulting in the genesis of symptoms in patients with functional bowel disorders (FBD). Therefore, the assessment for the presence of carbohydrate malabsorption via the use of breath hydrogen tests have been widely utilized to identify specific triggers of functional bowel symptoms in these patients.

Utility in refining dietary triggers on a low Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols diet A low Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols (FODMAP) diet is commonly used to treat symptoms in patients with FBD. Traditionally, breath hydrogen tests for lactose, fructose, and sorbitol are undertaken prior to implementation of a low FODMAP diet to simplify dietary restrictions for patients with FBD. A negative breath test suggests that the tested FODMAP does not play a role in symptom induction and foods containing the specific FODMAP can be included in the patient's diet while FODMAPs that are universally unabsorbed are restricted. The use of this algorithm for instituting a low FODMAP diet is thought to improve patient compliance; an important predictor of success for this treatment, by reducing the number of dietary restrictions for the patient. Gearry et al.1 found that a positive breath test to fructose and lactose were contributing factors to improvements in abdominal pain and flatulence following a 4‐week trial of low FODMAP diet in quiescent inflammatory bowel disease patients with co‐existing irritable bowel syndrome (IBS). However, such correlations have not been reproduced by more recent studies in children on a fructose‐restricted diet.2, 3

Limitations of methodology and interpretation of hydrogen breath testing The prevalence of sorbitol, fructose, and lactose malabsorption in FBD patients are similar to healthy controls. 4

The sensitivity, specificity and false positive/negative rates for accurate detection of carbohydrate malabsorption are dependent on the testing methodology used. A significant heterogeneity in the definition of a positive test, frequency of breath sampling, testing duration, and the dosage of test sugar have been observed in the literature. A false negative outcome is likely to occur if the testing duration is < 4 h or sampling frequency is > 15 min. 5 4

Changes in breath methane concentrations after consumption of a test carbohydrate have been utilized to indicate carbohydrate malabsorption in non‐hydrogen producing individuals. Whilst carbohydrate malabsorption has been well studied in patients who produce breath hydrogen, breath methane is less predictable, and baseline levels are difficult to control, even with dietary changes. 6

Considerable intra‐individual variability in breath hydrogen responses exists with repeated testing. Komericki and colleagues found that 29% of patients with IBS having an initial positive fructose breath test lost this response upon repeat testing at 1 year despite no intervention being administered during the interim period. 7 8 9 In the recent years, evidence is accumulating for significant limitations in the interpretation and methodology of breath hydrogen test and its application in FBD patients following a low FODMAP diet.

Clinical relevance of symptom response on breath testing Apart from breath hydrogen responses, the development of symptoms during a test have also been used to guide dietary restriction in these patients despite evidence opposing this strategy. Increased symptoms have been reported in 20 and 25% of patients with negative breath tests to lactose and fructose, respectively. In a cohort of FBD patients,10 the use of symptoms during testing to predict fructose malabsorption was shown to have poor sensitivity (21%) and moderate specificity (75%). A total of 55% of patients reported the presence of symptoms up to 12 h after ingestion of test carbohydrate, but this was poorly correlated to the presence of malabsorption.10 Malabsorption of polyols are also poor predictors of symptom induction as overall symptoms after ingestion of sorbitol and mannitol did not correlate with the degree of malabsorption.11 This may indicate that symptom induction for osmotically active sugars such as polyols and fructose can occur independently of the presence of malabsorption. Indeed, the osmotic effects of these monosaccharides promote small intestinal distension.12

Detection of small intestinal bacterial overgrowth Breath hydrogen tests following lactulose and glucose ingestion are also commonly used in the detection of small intestinal bacterial overgrowth (SIBO) in patients with FBD. This concept was first demonstrated by symptomatic improvement and normalization of breath hydrogen responses following interventions that modify the intestinal microbiota.13 The existence of SIBO in IBS is controversial as diagnostic tests have inconsistent and poorly validated methodology. Although the glucose breath test has a higher sensitivity and positive predictive value for SIBO compared with lactulose,14 it is absorbed efficiently and proximally in the small bowel, which can result in a false‐negative results in patients with distal SIBO or slow transit. This is, however, not seen after lactulose ingestion as it travels throughout the entire gastrointestinal tract, although there are other limitations using lactulose to define SIBO. These include inconsistent definitions and intra‐individual variability. Table 1 summarizes the various cut‐off values used to define SIBO using lactulose breath hydrogen test. Based on these cut‐off values, the prevalence of SIBO can vary drastically between 4 and 78%.13 In IBS patients demonstrating SIBO based on an early rise (< 90 min) in breath hydrogen after the ingestion of lactulose, 30% lost this response upon repeat testing.11 Simultaneous transit and breath hydrogen assessment has also shown that such early rise nearly always represents rapid transit rather than SIBO.15 Few data exist regarding the reliability of other cut‐off values. This suggests that lactulose breath hydrogen measurements are poorly reliable to diagnose SIBO in patients with FBD and therapeutic strategies such as antibiotic therapy to eradicate SIBO as a means of achieving symptomatic relief should not be based on such results. Table 1. Commonly used criteria to diagnose small intestinal bacterial overgrowth (SIBO) using the lactulose breath hydrogen test • “Double peaks”—the presence of two distinct hydrogen peaks • “ERBHAL” or “Early Rise in Breath Hydrogen After Lactulose”—a significant hydrogen rise < 90 min after lactulose • Elevated hydrogen > 20 ppm at baseline • Peak hydrogen concentrations • > 5 ppm rise in hydrogen concentrations prior to caecal contrast combined with scintigraphic measurements

Conclusions Recent evidence suggests that breath hydrogen tests contribute little value to the clinical management of patients with FBD, whether for guidance on instituting a low FODMAP diet or prescription of antibiotics for eradication of SIBO. Symptoms produced during a breath test are also inaccurate for predicting a true intolerance to individual FODMAPs. Instead, following a diagnosis of IBS, restriction of all dietary FODMAPs should be undertaken followed by food rechallenge to identify responsible triggers of symptoms by an experienced dietitian. At this stage, the utility of breath testing is limited to research studies for assessment of mechanisms of intestinal gas production or as a marker of dietary compliance following dietary interventions with fermentable carbohydrates.