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Differential diagnosis and exclusion

TL;DR — IBS is diagnosed positively, but a small, non-zero set of organic conditions mimics it and each has a defined test with a known yield. Coeliac disease is the clearest case: pooled seroprevalence 6% (95% CI 5–8) and biopsy-proven prevalence 2% (2–3) in Rome III/IV IBS, with 4.42-fold higher odds of positive serology than controls (Shiha 2025, PMID 40493044) — so every patient gets coeliac serology regardless of subtype (Black 2020, PMID 32133113). Bile acid diarrhoea is the largest and most under-tested overlap: about a quarter to a third of IBS-D patients have an abnormal test (pooled SeHCAT <10% yield 30.8%, 24.7–37.7; Valentin 2016, PMID 26347530; pooled rate 28.1%, 22.6–34.0; Slattery 2015, PMID 25913530), and sequestrants beat placebo when the diagnosis is objectively made (RR 1.50, 1.14–1.96 for stool consistency; 2.80, 1.68–4.67 for frequency; Dilmaghani 2025, PMID 41090475). Faecal calprotectin separates IBD from IBS well (sensitivity 85.8%, specificity 91.7%), but at 1% IBD prevalence its positive predictive value is only 9% while its negative predictive value is 99.8% (Dajti 2023, PMID 37823411) — it is a rule-out test, not a rule-in test. Colonoscopy in patients without alarm features has essentially nil yield (0 of 11 in one prospective series; Asghar 2022, PMID 32882424), and the commonest genuinely missed diagnosis in the diarrhoea phenotype is microscopic colitis (5.7% of diarrhoeal functional bowel disorders vs 0% of constipation ones — with one in four chronic-diarrhoea patients not biopsied at all). This page is scoped to how organic disease is excluded and how often it is missed. IBD, coeliac disease, microscopic colitis and bile acid diarrhoea are not curated as conditions in this knowledge base.

The pre-test probabilities

Condition Prevalence in symptom-defined IBS Comparator Source
Coeliac disease (seropositive) 6% (95% CI 5–8) — Shiha 2025, PMID 40493044 (29 studies, 7,209 IBS patients)
Coeliac disease (biopsy-proven) 2% (2–3) OR 4.42 (2.82–6.92) for positive serology vs controls Shiha 2025, PMID 40493044
Coeliac disease (biopsy-proven, older pool) 4.1% (1.9–7.0) OR 4.34 (1.78–10.6) Ford 2009, PMID 19364994
Coeliac disease pooled OR vs controls: AGA 3.21, EMA/tTG 2.75, biopsy 4.48 — but no increase in North American studies — Irvine 2017, PMID 27753436
IBD, colorectal cancer, infectious diarrhoea pre-test probability <1% each general population Cash 2002, PMID 12425553
Bile acid malabsorption (SeHCAT <10%) 28.1% (22.6–34.0) in IBS-D — Slattery 2015, PMID 25913530 (6 studies, 908 patients)
Bile acid diarrhoea (any test) ~25% of functional bowel disorder with diarrhoea — Valentin 2016, PMID 26347530 (36 studies, 5,028 patients)
Exocrine pancreatic insufficiency (faecal elastase <100 µg/g) 6.1% (3.7–9.3) in Rome II IBS-D; 5% (2.2–10.4) in Rome IV IBS-D 0% in chronic diarrhoea and controls in the first study Leeds 2010, PMID 19835990; Olmos 2022, PMID 35704255
Microscopic colitis at colonoscopy 2.35% pooled; 2.9% vs 1.7% in matched controls (aOR 1.77, 1.61–1.95) — Wu 2023, PMID 36168183; Staller 2021, PMID 34420846
Colorectal cancer at colonoscopy 0.78% pooled; <0.1% without alarm symptoms or age <40 — Wu 2023, PMID 36168183

Two of those rows point in opposite directions and are shown unreconciled. The Sheffield/Pavia 2025 meta-analysis argues "a positive diagnosis of IBS should not be made without excluding CeD" (Shiha 2025, PMID 40493044), while Irvine 2017 found no increase in coeliac odds among IBS cases in North American studies and inconsistent results in population-based studies, concluding that the utility of screening "in North America or in the community is less clear" (PMID 27753436). The disagreement is geographic and design-driven, not statistical.

The minimum test set, and its evidence

The pragmatic package proposed by Black and Ford (PMID 32133113) — endorsed in substance by ACG and BSG (guidelines) — is:

Test Who Why
Coeliac serology (tTG/EMA) all patients, any subtype Elevated pre-test probability (above); coeliac is treatable and commonly delayed
Full blood count, CRP all Cheap, low yield, "acceptable part of routine practice" (Black 2020, PMID 32133113)
Faecal calprotectin diarrhoea phenotype Rule out IBD; colonoscopy if positive
Colonoscopy alarm features, or clinical suspicion of microscopic colitis See yield data below
SeHCAT (or C4/FGF19 where available) IBS-D ~1 in 3 have bile acid diarrhoea and it is treatable
Hydrogen breath tests for lactose or SIBO no routine role Black 2020, PMID 32133113; see rifaximin-and-the-sibo-question

Stool testing beyond this is targeted: current guidance for IBS-D/functional diarrhoea without alarm symptoms is to test for Giardia, calprotectin, faecal immunochemical test and bile acid diarrhoea (Camilleri 2025, PMID 39480027).

Faecal calprotectin: excellent negative predictive value, poor positive

Seventeen studies, 1,956 patients, colonoscopy with histology or radiology as reference: summary sensitivity 85.8% (78.3–91), specificity 91.7% (84.5–95.7). At an IBD prevalence of 1%, NPV 99.8%, PPV 9% (Dajti 2023, PMID 37823411). Sensitivity was higher in Western than Eastern studies (88% vs 73%) and at a cut-off ≤50 µg/g than >50 µg/g (87% vs 79%), with similar specificity. All studies were at high or unclear risk of bias. The practical implication is asymmetric: a normal calprotectin in a low-prevalence setting effectively closes the IBD question; an abnormal one, taken alone, mostly generates colonoscopies that find nothing.

Colonoscopy: yield is concentrated in alarm features, age and diarrhoea

  • Pooled (12 studies, 28,630 IBS patients undergoing colonoscopy): CRC 0.78%, IBD 4.48%, microscopic colitis 2.35%, with no difference from non-IBS patients. CRC was <0.1% in IBS patients without alarm symptoms or younger than 40. With alarm symptoms, CRC rose to 2.47% vs 0.11% (risk difference 2.57%, 0.37–4.78) and IBD to 8.86% vs 4.25% (RD 10.75%, 4.81–16.68). IBD and microscopic colitis were commoner in IBS-D than IBS-C; CRC and microscopic colitis rose with age (Wu 2023, PMID 36168183).
  • Prospective Rome IV series (646 patients referred for colonoscopy): 98% had alarm features. Organic disease overall 12%, lowest in functional constipation and IBS-C (~6% each), IBS-M ~9%, highest in functional diarrhoea and IBS-D (~17% each; p=0.005). The excess in diarrhoeal phenotypes was entirely microscopic colitis (5.7% vs 0%, p<0.001), not IBD (7.2% vs 4.0%, p=0.2) or CRC (4.2% vs 2.3%, p=0.2). One in four chronic-diarrhoea patients had no colonic biopsies taken — the authors call this "a missed diagnostic opportunity." In the 11 patients (2%) without alarm features, colonoscopy was normal in every case (Asghar 2022, PMID 32882424).
  • National registry (Sweden, 21,944 IBS patients biopsied within 6 months of diagnosis vs 81,101 matched controls): biopsy-proven IBD 1.6% vs 5.9% (aOR 0.21, 0.19–0.24); precancerous polyps 4.1% vs 13.0% (aOR 0.28); CRC 0.8% vs 6.3% (aOR 0.17); coeliac 1.9% vs 3.4% on upper endoscopy (aOR 0.54). The one diagnosis enriched in IBS was microscopic colitis, 2.9% vs 1.7% (aOR 1.77, 1.61–1.95), especially in older patients and IBS-D. Yield rose with age and results held using unaffected siblings as comparators (Staller 2021, PMID 34420846).

The consistent signal across three independent designs: an IBS label does not raise the risk of IBD or cancer above that of other patients referred for colonoscopy — it lowers it — and the only condition genuinely over-represented is microscopic colitis in the diarrhoea phenotype, which requires biopsies that are frequently not taken.

Bile acid diarrhoea: the largest treatable overlap

Metric Value Source
SeHCAT <10% retention (severe/moderate BAM) in IBS-D 28.1% (22.6–34.0), range 16.9–35.3% across 6 studies Slattery 2015, PMID 25913530
SeHCAT <5% 10% (7–13) Wedlake 2009, PMID 19570102
SeHCAT <10% 32% (29–35) Wedlake 2009, PMID 19570102
SeHCAT <15% 26% (23–30) Wedlake 2009, PMID 19570102
Diagnostic yield, SeHCAT <10% (24 studies) 0.308 (0.247–0.377) Valentin 2016, PMID 26347530
Diagnostic yield, fasting serum C4 (6 studies) 0.171 (0.134–0.217) Valentin 2016, PMID 26347530
Diagnostic yield, fasting serum FGF19 (3 studies) 0.248 (0.147–0.385) Valentin 2016, PMID 26347530
Diagnostic yield, 48-h faecal bile acids (2 studies) 0.255 (0.071–0.606) Valentin 2016, PMID 26347530
Response to colestyramine by SeHCAT severity 96% at <5%, 80% at <10%, 70% at <15% retention Wedlake 2009, PMID 19570102
Sequestrants vs placebo (6 RCTs, 182 participants, 1–8 weeks) Stool consistency RR 1.50 (1.14–1.96); frequency RR 2.80 (1.68–4.67); I²=0%; moderate certainty; no significant excess adverse effects Dilmaghani 2025, PMID 41090475

There is no universally available reference test — Valentin and colleagues could estimate only diagnostic yield, not sensitivity or specificity, "relative to a gold standard test" (PMID 26347530) — and SeHCAT itself "is not widely available in many countries outside Europe and Canada." So the commonest treatable organic mimic of IBS-D is defined by a test that most of the world cannot run. Wedlake and colleagues concluded in 2009 that "international guidelines for the management of irritable bowel syndrome need to be revised so that clinicians become more aware of this possibility" (PMID 19570102); seventeen years later BSG-aligned guidance recommends SeHCAT in IBS-D (Black 2020, PMID 32133113) while ACG does not make it a routine step (guidelines).

Smaller mimics

  • Exocrine pancreatic insufficiency. Faecal elastase-1 <100 µg/g in 6.1% (3.7–9.3) of 314 Rome II IBS-D patients versus 0% of 105 chronic-diarrhoea patients and 0% of 95 controls (p<0.001); enzyme supplementation improved stool frequency, consistency (both p<0.001) and abdominal pain (p=0.003) in the deficient group only (Leeds 2010, PMID 19835990). Replicated with Rome IV: 5% (2.2–10.4) of 140 IBS-D patients, with pancreatic steatosis the main EUS finding (71%) and dyspepsia the strongest predictor (OR 34.7, 4.95–366; Olmos 2022, PMID 35704255).
  • Giardiasis and its aftermath. Six years after a waterborne Giardia outbreak, Rome III IBS prevalence was 39.4% in exposed versus controls (adjusted RR 3.4, 2.9–3.9) alongside chronic fatigue at 30.8% (RR 2.9, 2.3–3.4), both declining slowly over time (Hanevik 2014, PMID 25115874). Giardiasis is therefore both a differential (active infection) and a cause (post-infectious-ibs); post-infectious sequelae after Giardia include IBS, chronic fatigue, arthritis and fibromyalgia (Miko 2025, PMID 41570191).
  • Lactose malabsorption. Self-reported lactose intolerance has only moderate accuracy against hydrogen breath testing across six studies (845 participants), with frequent false positives; the review concludes neither routine lactose-free diets nor routine breath testing is warranted in IBS (Pop 2024, PMID 39255349).
  • Non-coeliac gluten/wheat sensitivity. A self-reported entity with its own prevalence literature (Shiha 2025, PMID 41151790); relevant to dietary-therapy rather than to organic exclusion.
  • Ovarian cancer. Ovarian cancer incidence was elevated in the 3 months (HR 1.71, 1.08–2.70) and 6 months (HR 1.43, 1.06–1.93) after an index IBS diagnosis but not beyond 8 months — a pattern the authors interpret as misdiagnosis of ovarian cancer symptoms as IBS, not causation (Shin 2025, PMID 41081454).
  • Crohn's disease presenting as IBS. In 83 consecutively diagnosed Crohn's patients, median diagnostic delay was 8 months (range 0–324) (Maconi 2015, PMID 26004215).

Historical baseline

The 2002 systematic review that first quantified this problem found the pre-test probability of IBD, colorectal cancer or infectious diarrhoea to be under 1% in patients meeting symptom-based criteria, that recommended tests "rarely identify organic GI disease" in them, and that the single exception was coeliac disease, ten times more likely than in the general population (Cash 2002, PMID 12425553). Twenty-four years of subsequent data have not overturned that structure; they have quantified it and added bile acid diarrhoea and microscopic colitis to the exception list.

Open questions

  • Should coeliac serology be universal? Two 2016–2025 meta-analyses reach opposite operational conclusions for North America and community settings (Irvine 2017, PMID 27753436 vs Shiha 2025, PMID 40493044).
  • Is bile acid diarrhoea a differential or a mechanistic subtype of IBS-D? A quarter to a third of IBS-D patients test positive (Valentin 2016, PMID 26347530) and respond to sequestrants (Dilmaghani 2025, PMID 41090475), yet the entity has no reference standard and no globally available test.
  • What is the true miss rate of microscopic colitis? One in four chronic-diarrhoea patients undergoing colonoscopy were not biopsied (Asghar 2022, PMID 32882424); a targeted PubMed search on 2026-09-02 retrieved no study quantifying the downstream outcomes of those missed biopsies.
  • Does the near-zero colonoscopy yield in alarm-free patients (0/11; Asghar 2022, PMID 32882424) hold in a larger prospective series? The number is too small to be decisive and no larger alarm-free cohort was retrievable in this session.
  • Should faecal elastase be part of the IBS-D panel? Two independent series put EPI at 5–6.1% with symptomatic response to enzyme replacement (Leeds 2010, PMID 19835990; Olmos 2022, PMID 35704255), but no guideline includes it.
  • How long is the window in which an IBS label delays an organic diagnosis? Only fragmentary data exist (Maconi 2015, PMID 26004215; Shin 2025, PMID 41081454).

References

  1. Shiha MG, Schiepatti A, Manza F, Maimaris S, Aziz I, Sanders DS. Global Prevalence of Celiac Disease in Patients With Rome III and Rome IV Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis. Am J Gastroenterol. 2025;120(12):2776-2787. PMID 40493044
  2. Irvine AJ, Chey WD, Ford AC. Screening for Celiac Disease in Irritable Bowel Syndrome: An Updated Systematic Review and Meta-analysis. Am J Gastroenterol. 2017;112(1):65-76. PMID 27753436
  3. Ford AC, Chey WD, Talley NJ, Malhotra A, Spiegel BM, Moayyedi P. Yield of diagnostic tests for celiac disease in individuals with symptoms suggestive of irritable bowel syndrome. Arch Intern Med. 2009;169(7):651-8. PMID 19364994
  4. Cash BD, Schoenfeld P, Chey WD. The utility of diagnostic tests in irritable bowel syndrome patients: a systematic review. Am J Gastroenterol. 2002;97(11):2812-9. PMID 12425553
  5. Black CJ, Ford AC. Rational investigations in irritable bowel syndrome. Frontline Gastroenterol. 2020;11(2):140-147. PMID 32133113
  6. Dajti E, et al. Systematic review with meta-analysis: Diagnostic performance of faecal calprotectin in distinguishing inflammatory bowel disease from irritable bowel syndrome in adults. Aliment Pharmacol Ther. 2023;58(11-12):1120-1131. PMID 37823411
  7. Wu J, Wang C, Lv L. Diagnostic yield of colonoscopy for organic disease in irritable bowel syndrome and its risk factors: A meta-analysis. Neurogastroenterol Motil. 2023;35(2):e14481. PMID 36168183
  8. Asghar Z, et al. Diagnostic Yield of Colonoscopy in Patients With Symptoms Compatible With Rome IV Functional Bowel Disorders. Clin Gastroenterol Hepatol. 2022;20(2):334-341.e3. PMID 32882424
  9. Staller K, et al. Diagnostic yield of endoscopy in irritable bowel syndrome: A nationwide prevalence study 1987-2016. Eur J Intern Med. 2021;94:85-92. PMID 34420846
  10. Slattery SA, Niaz O, Aziz Q, Ford AC, Farmer AD. Systematic review with meta-analysis: the prevalence of bile acid malabsorption in the irritable bowel syndrome with diarrhoea. Aliment Pharmacol Ther. 2015;42(1):3-11. PMID 25913530
  11. Wedlake L, A'Hern R, Russell D, Thomas K, Walters JR, Andreyev HJ. Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome. Aliment Pharmacol Ther. 2009;30(7):707-17. PMID 19570102
  12. Valentin N, Camilleri M, Altayar O, Vijayvargiya P, Acosta A, Nelson AD, Murad MH. Biomarkers for bile acid diarrhoea in functional bowel disorder with diarrhoea: a systematic review and meta-analysis. Gut. 2016;65(12):1951-1959. PMID 26347530
  13. Dilmaghani S, BouSaba J, Lupianez-Merly C, Prokop L, Murad MH, Camilleri M. Meta-Analysis: Efficacy and Safety of Sequestrants for Bile Acid Diarrhoea. Aliment Pharmacol Ther. 2025;62(11-12):1054-1065. PMID 41090475
  14. Camilleri M. When and What to Test for Diarrhea: Focus on Stool Testing. Am J Gastroenterol. 2025;120(4):778-784. PMID 39480027
  15. Leeds JS, et al. Some patients with irritable bowel syndrome may have exocrine pancreatic insufficiency. Clin Gastroenterol Hepatol. 2010;8(5):433-8. PMID 19835990
  16. Olmos JI, et al. Exocrine Pancreatic Insufficiency is Undiagnosed in Some Patients with Diarrhea-Predominant Irritable Bowel Syndrome Using the Rome IV Criteria. Dig Dis Sci. 2022;67(12):5666-5675. PMID 35704255
  17. Hanevik K, Wensaas KA, Rortveit G, Eide GE, Mørch K, Langeland N. Irritable bowel syndrome and chronic fatigue 6 years after giardia infection: a controlled prospective cohort study. Clin Infect Dis. 2014;59(10):1394-400. PMID 25115874
  18. Miko S, Kache PA, Imada E, Freeland AL, Haston JC. Postinfectious Syndromes and Long-Term Sequelae after Giardia Infections. Emerg Infect Dis. 2025;31(14):45-52. PMID 41570191
  19. Pop A, Popa SL, Pop DD, Ismaiel A, Nechita VI, Dumitrascu DL. Self-Perceived Lactose Intolerance Versus Confirmed Lactose Intolerance in Irritable Bowel Syndrome: A Systematic Review. J Gastrointestin Liver Dis. 2024 Sep 9 (online ahead of print). PMID 39255349
  20. Shiha MG, et al. Global prevalence of self-reported non-coeliac gluten and wheat sensitivity: a systematic review and meta-analysis. Gut. 2025 Oct 28 (online ahead of print). PMID 41151790
  21. Shin A, Xu H, Sarnoff R, Church A, Chang L. Research Communication: An Observational Cohort Study on Risk of Ovarian Cancer in Women with Irritable Bowel Syndrome. Aliment Pharmacol Ther. 2025;62(9):944-947. PMID 41081454
  22. Maconi G, et al. The impact of symptoms, irritable bowel syndrome pattern and diagnostic investigations on the diagnostic delay of Crohn's disease: A prospective study. Dig Liver Dis. 2015;47(8):646-51. PMID 26004215