Approach

IBS is characterised by abdominal pain associated with altered bowel habits.[34] The pain is relieved by defecation or passing flatus. Other common accompanying symptoms include abdominal bloating, passage of mucus with stools, urgency of defecation, and the sensation of incomplete evacuation following a bowel movement.[10]

History

The patient's history may reveal exposure to several risk factors, such as physical or sexual abuse, previous enteric infection, and stress at home or at work.[20][21][22]​​[24][32]​​ Adult patients are twice as likely to be women.[14]

A family history of IBS may be present. Family history of inflammatory bowel disease, colorectal cancer, or coeliac disease should increase the index of suspicion for these conditions. A careful dietary history may reveal consumption of foods that exacerbate symptoms (e.g., caffeine, cow's milk, fructose-containing foods, artificial sweeteners, alcohol), irregular or inadequate meals, insufficient fluid intake, or excessive or low (particularly in those with constipation) fibre intake.[10]

Physical examination

The physical examination is usually normal. There may, however, be mild tenderness in the lower quadrants without a mass.

The Carnett test helps distinguish pain of abdominal wall origin from abdominal pain arising from inside the abdomen. In this test, if the pain is increased on tightening the abdomen, it arises from the abdominal wall (positive Carnett test). Intraperitoneal pain is lessened with abdominal wall contraction.[35] A negative Carnett test would be expected in patients with IBS.

Laboratory tests

There is no specific diagnostic test for IBS. The choice of tests for the initial work-up will depend upon factors such as symptoms and patient age.​[2]

Tests for non-IBS disease, including inflammatory bowel disease and colorectal cancer

Full blood count (FBC) should be done as part of the initial work-up.[34] If the patient is anaemic or if the white blood cell count is elevated, then a diagnosis other than IBS should be entertained.

Faecal occult blood testing may be considered. Faecal occult blood testing has a positive predictive value of 97% and a negative predictive value of 43% for distinguishing inflammatory bowel disease from IBS.[36] In the primary care setting, faecal occult blood testing may be used to inform the decision to refer a patient who has unexplained gastrointestinal symptoms, but who is at low risk for colorectal cancer, to a specialist.​​

Quantitative faecal immunochemical test (FIT) may be ordered when colorectal cancer is suspected. The UK guidelines recommend quantitative FIT to guide referral for suspected colorectal cancer in certain adults with unexplained abdominal pain or in those with a change in bowel habit.[37][38]​​​ If FIT value is ≥10 micrograms haemoglobin/g of faeces, urgent referral to secondary care is recommended. Based on FIT results, investigations such as colonoscopy can be avoided in people who are less likely to have colorectal cancer, thus making the resources available to those who need them the most.[37][38]

A faecal calprotectin test or a stool lactoferrin may be ordered to differentiate IBS from inflammatory bowel disease.[39][40][41][42][43] American College of Gastroenterology (ACG) guidelines favour calprotectin over lactoferrin because of its higher sensitivity and specificity for inflammatory bowel disease.[39]

C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) can also be used to rule out inflammatory bowel disease.[34][39][40] Although both are non-specific, ACG guidelines advise that CRP is the more useful of the two.[39] A comprehensive meta-analysis evaluated markers in 2145 patients with inflammatory bowel disease, IBS, or healthy controls and found that an elevated ESR or stool lactoferrin could not discriminate between patient groups, while a CRP ≤5 mg/L (≤0.5 mg/dL) or faecal calprotectin ≤40 micrograms/g reliably conferred a 1% or lower likelihood of inflammatory bowel disease, essentially excluding it as a diagnosis.[42]

Serological tests for coeliac disease

A tissue transglutaminase antibody test can help exclude coeliac disease.[34][39][40][44][45]

If the patient has diarrhoea and/or weight loss, coeliac disease should be suspected. The most reliable test is the immunoglobulin (Ig) A human antitissue transglutaminase (anti-tTG) antibody enzyme-linked immunosorbent assay. This test has a reported sensitivity of almost 100% and a specificity of 95% to 97% for coeliac disease.[44]

A positive result should be confirmed by duodenal biopsy.[40]

IgA endomysial antibodies (EMAs) may be tested for when anti-tTG is weakly positive or to confirm the diagnosis in children or adults for whom endoscopy is unsuitable.

Patients with IgA deficiency may have a false-negative anti-tTG result. Testing options for these patients include IgG tissue transglutaminase and IgG or IgA deamidated gliadin peptides.[40]

Tests to exclude bile acid malabsorption

Serum fibroblast growth factor 19 and 23‐seleno‐25‐homotaurocholic acid (SeHCAT) tests, if available, are recommended for patients presenting with chronic diarrhoea, to exclude bile acid malabsorption.[39][40][46]

Forty-eight hour stool collection for total bile acids may also be considered for the same indication.[40]

If other diagnostic tests are unavailable, an empiric trial of bile acid binder may be conducted in patients with chronic diarrhoea to exclude bile acid malabsorption.[39][40]

Hydrogen/methane breath test

In patients with diarrhoea or bloating, further investigation with a hydrogen breath test for bacterial overgrowth or lactase deficiency may be warranted. However, this test is not recommended to confirm diagnosis in patients who meet the IBS diagnostic criteria.[34]

Enteric pathogens test

Routine stool testing for enteric pathogens (i.e., faecal leukocytes, ova, and parasites) is not recommended for patients with suspected IBS.[39]

Faecal immunoassay or polymerase chain reaction is indicated for patients with risk factors for giardiasis.[39][40]

Imaging

Endoscopic assessment

Guidance recommends against routine colonoscopy for patients with IBS younger than 45 years without alarm features, which include:[39]

  • haematochezia

  • melaena

  • unintentional weight loss

  • older age of onset of symptoms (over 50 years)

  • family history of inflammatory bowel disease, colon cancer, or other significant gastrointestinal disease.

Colonoscopy should be considered for patients with alarm features.[39]

In patients with suspected IBS with diarrhoea who are at high risk of microscopic colitis, i.e., older age (over 60 years), female sex, and more intense diarrhoea, there is some evidence to support the use of colonoscopy.[39] Microscopic colitis should also be suspected if the patient does not have any abdominal pain.

Flexible sigmoidoscopy can detect abnormal mucosa, which could indicate inflammatory bowel disease and polyps or carcinoma; however, guidelines do not support its use to confirm diagnosis of IBS.[34]

Plain abdominal radiographs may be useful in the evaluation of patients with bloating.[47]

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