IBS is meant to be diagnosed positively, not by ruling out everything else

Irritable bowel syndrome is supposed to be diagnosed by recognising a pattern, not by exhausting every other possibility first. The American College of Gastroenterology's clinical guideline suggests using a positive diagnostic strategy rather than a diagnostic strategy of exclusion, on the grounds that it shortens the time to starting appropriate treatment [s1]. That is a different model of care from the one many people with IBS actually experience, and it is the single most useful thing to know about the condition.

The guideline, published in December 2020, was the ACG's first on IBS and was built with GRADE methodology — the standard framework for rating how certain the evidence behind a recommendation is [s1]. Twenty-five clinically important questions were assessed after a literature search: nine on diagnostic testing and sixteen on treatment [s1]. Consensus among the panel was reached using a modified Delphi approach [s1].

What the condition is now called

The British Society of Gastroenterology's 2021 guideline describes a change that has happened quietly in the specialist literature and barely at all in public: IBS has been reclassified as a disorder of gut-brain interaction rather than a functional gastrointestinal disorder [s3]. The BSG attributes the reclassification to substantial advances in understanding the condition's complex pathophysiology since its previous guideline in 2007 [s3].

The wording matters because "functional" has long been heard by patients as a polite way of saying nothing is wrong. The replacement term is a positive claim about mechanism — that the communication between gut and brain is where the disorder sits — rather than a statement about what could not be found on a test.

How common it is depends entirely on which definition you use

The Rome Foundation Global Study surveyed 73,076 adults across 33 countries on six continents, using internet surveys in 24 countries, personal household interviews in seven, and both in two [s2]. Diagnostic criteria for at least one disorder of gut-brain interaction were met by 40.3% of internet respondents (95% CI 39.9 to 40.7) and 20.7% of household respondents (95% CI 20.2 to 21.3) [s2].

For IBS specifically, the study produced a result that should make anyone quoting an IBS prevalence figure check which criteria it came from. Under the Rome IV criteria, 4.1% of internet respondents met the definition; under the older Rome III criteria, 10.1% did [s2]. In the household surveys the same comparison ran 1.5% against 3.5% [s2]. The condition did not change between those two numbers. The definition did.

Across all the disorders studied, prevalence was higher in women than men — an odds ratio of 1.7 (95% CI 1.6 to 1.7) in the internet survey and 1.3 (95% CI 1.3 to 1.4) in the household survey — and the disorders were associated with lower quality of life and more frequent doctor visits [s2].

The tests the guideline actually endorses

A positive diagnostic strategy is not the same as no testing at all. The ACG guideline makes two specific suggestions about ruling things out, both narrow.

The first is serologic testing to rule out coeliac disease in patients with IBS and diarrhoea symptoms [s1]. The second is checking faecal calprotectin — a stool marker of intestinal inflammation — in patients with suspected IBS and diarrhoea symptoms, to rule out inflammatory bowel disease [s1]. Both are cheap, both are aimed at conditions that are genuinely mistakable for IBS, and both are targeted at the diarrhoea-predominant presentation rather than applied to everyone.

What the guideline does not suggest is a general sweep of imaging and endoscopy in someone whose history fits and who has no warning signs. That is the practical content of "positive diagnostic strategy": the pattern does the work, and the tests are used to exclude the two conditions most likely to be hiding inside it.

What the guideline endorses as treatment

The ACG's therapeutic recommendations span sixteen of the twenty-five questions and are graded individually [s1]. Among the ones the panel endorsed: a limited trial of a diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols — the low-FODMAP diet — to improve global symptoms; chloride channel activators and guanylate cyclase activators for global symptoms in constipation-predominant IBS; rifaximin for global symptoms in diarrhoea-predominant IBS; and gut-directed psychotherapy for global IBS symptoms [s1].

Two features of that list are worth noticing. It is short, for a condition affecting several percent of the world's adults. And it is heterogeneous — a diet, two drug classes acting on intestinal secretion, a poorly absorbed antibiotic and a psychological therapy — which is what a treatment list looks like when the underlying mechanism is genuinely multiple rather than single.

The BSG guideline reached its recommendations by a different route, updating a series of trial-based and network meta-analyses of dietary, pharmacological and psychological therapies and grading each recommendation with GRADE [s3]. It also does something guidelines rarely do prominently: it identifies areas of unmet need for future research and lists treatments still in development [s3].

What this leaves unsettled

Neither guideline claims the problem is solved. The BSG frames its own document as a practical framework for evidence-based management rather than a set of answers, and explicitly flags unmet need [s3]. The ACG notes that additional detail on specific drugs, doses and duration of therapy sits inside the full guideline rather than in any summary of it [s1].

For a reader, the useful takeaways are narrow and concrete. A diagnosis of IBS made on pattern recognition plus two targeted tests is what the guideline suggests, not a sign that the investigation was cut short. A prevalence figure for IBS is meaningless without the criteria attached. And a change in bowel habit is not the same thing as IBS: the guideline's own logic depends on someone with clinical training deciding that the pattern fits and that the exclusions have been covered.

This article is informational and is not medical advice. Persistent or changing bowel symptoms, and any symptom that is new, warrant assessment by a clinician rather than self-diagnosis.

Sources

• ACG Clinical Guideline: Management of Irritable Bowel Syndrome — American Journal of Gastroenterology, 2020-12-14
• Worldwide Prevalence and Burden of Functional Gastrointestinal Disorders, Results of Rome Foundation Global Study — Gastroenterology, 2020-04-12
• British Society of Gastroenterology guidelines on the management of irritable bowel syndrome — Gut, 2021-04-26