The New Definition
Under Rome IV, irritable bowel syndrome is defined as recurrent abdominal pain occurring on average at least 1 day per week in the last 3 months, with symptom onset at least 6 months before diagnosis, and associated with two or more of: related to defecation, associated with a change in stool frequency, or associated with a change in stool form (appearance).
Two critical changes from Rome III stand out. First, "discomfort" was removed — only "pain" qualifies. This was controversial because many patients describe their primary symptom as discomfort or pressure rather than pain, potentially excluding them from diagnosis. Second, the frequency threshold increased from 3 days per month to 1 day per week, a more stringent bar that reduced global IBS prevalence estimates from approximately 11 percent to about 4 percent.
Subtyping by Stool Form
IBS subtypes are now classified exclusively by predominant stool form using the Bristol Stool Form Scale on days with abnormal bowel habits. IBS-C (constipation-predominant) features Bristol types 1-2 in more than 25 percent of abnormal bowel movements and types 6-7 in less than 25 percent. IBS-D (diarrhoea-predominant) is the reverse. IBS-M (mixed) has both types exceeding 25 percent. IBS-U (unsubtyped) fits none of these patterns.
Subtyping matters for treatment selection. IBS-D may respond to loperamide, rifaximin, eluxadoline, or bile acid sequestrants. IBS-C may benefit from linaclotide, lubiprostone, plecanatide, or PEG laxatives. IBS-M is the most challenging to treat and often requires combination approaches.
Positive Diagnosis
Rome IV emphasises that IBS is a positive diagnosis — made by matching symptom criteria — not a diagnosis of exclusion. Limited testing is recommended to exclude coeliac disease (tTG-IgA), inflammatory markers (CRP, faecal calprotectin), and, in IBS-D, bile acid malabsorption (SeHCAT or serum C4 where available). Routine colonoscopy is not recommended in patients under 45 without alarm features (rectal bleeding, unintentional weight loss, family history of colorectal cancer, anaemia, nocturnal symptoms).
This shift from exhaustive exclusion to targeted investigation saves healthcare resources and, crucially, gives patients a diagnosis sooner — reducing the anxiety of diagnostic uncertainty.
The Biomarker Gap
The absence of a diagnostic biomarker remains IBS's greatest limitation. Faecal calprotectin effectively distinguishes IBS from IBD but cannot confirm IBS. Research biomarkers (faecal volatile organic compounds, microbiome signatures, bile acid profiles, rectal mucosal mast cell counts) show promise but are not yet validated for clinical use. Until a biomarker is available, Rome IV criteria — however imperfect — remain the standard of care.