Investigations
1st investigations to order
FBC
Test
FBC should be done as part of the initial work-up.[34] If the patient is anaemic or if the FBC count is raised, then a diagnosis other than IBS should be entertained.
Result
normal; anaemia or raised WBC count suggests non-IBS disease
Investigations to consider
faecal occult blood test
Test
May be ordered when inflammatory bowel disease or colorectal cancer is suspected.[34] 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.
Result
normal; may be positive in inflammatory bowel disease or colorectal cancer
quantitative faecal immunochemical test (FIT)
Test
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]
Result
FIT value of ≥10 micrograms of haemoglobin/g of faeces indicates possible colorectal cancer
serologic tests for coeliac disease
Test
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]
UK guidelines recommend testing for coeliac disease in all patients with suspected IBS.[34]
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]
Result
negative
faecal calprotectin
Test
This may be ordered to differentiate IBS from inflammatory bowel disease (IBD).[34][39][40][41][42][43] It has greater clinical utility for this purpose than faecal lactoferrin. A comprehensive meta-analysis evaluated markers in 2145 patients with IBD, IBS, or healthy controls and found that an elevated stool lactoferrin could not reliably discriminate between patient groups, while a faecal calprotectin ≤40 micrograms/g conferred a 1% or lower likelihood of IBD, essentially excluding it as a diagnosis.[42]
Result
<50 micrograms/g makes IBD unlikely (and IBS more likely)
faecal lactoferrin
Test
This test may be ordered to differentiate IBS from inflammatory bowel disease (IBD).[39][40] It has inferior clinical utility for this purpose than faecal lactoferrin. A comprehensive meta-analysis evaluated markers in 2145 patients with IBD, IBS, or healthy controls and found that an elevated stool lactoferrin could not reliably discriminate between patient groups, while a faecal calprotectin ≤40 micrograms/g conferred a 1% or lower likelihood of IBD, essentially excluding it as a diagnosis.[42]
Result
<7.25 micrograms/g makes IBD unlikely
serum C-reactive protein (CRP)
Test
If testing for faecal lactoferrin or calprotectin are not available, serum CRP is a reasonable option to screen for inflammatory bowel disease (IBD).[34][39][40]
Serum CRP above 5-6 mg/L has a sensitivity of 0.73 (95% CI 0.64 to 0.80) and specificity of 0.78 (95% CI 0.58 to 0.91) for identifying IBD in patients with diarrhoea.[40] A comprehensive meta-analysis evaluated serological markers in 2145 patients with IBD, IBS, or healthy controls and found that a CRP ≤5 mg/L (≤0.5 mg/dL) reliably conferred a 1% or lower likelihood of IBD, essentially excluding it as a diagnosis.[42]
UK guidelines recommend serum CRP testing in all patients with suspected IBS.[34]
Result
normal
erythrocyte sedimentation rate (ESR)
Test
Erythrocyte sedimentation can be used to help rule out inflammatory bowel disease (IBD).[34][39][40] However, a comprehensive meta-analysis evaluated serological markers in 2145 patients with IBD, IBS, or healthy controls and found that an elevated ESR could not reliably discriminate between patient groups. C-reactive protein is therefore preferred.[42]
Result
normal
serum fibroblast growth factor 19
23‐seleno‐25‐homotaurocholic acid (SeHCAT) test
Test
Recommended for patients presenting with chronic diarrhoea, if available, to exclude bile acid malabsorption.[39][40]
The patient ingests a synthetic bile acid labelled with a radionuclide tracer atom (SeHCAT). Two whole-body scans using a gamma camera are conducted, 1 week apart, and the proportion of retained bile acid can be calculated.[34] The test is not available in North America.
Result
normal; <15% SeHCAT retention after 1 week indicates bile acid malabsorption
48-hour stool collection for total bile acids
Test
May be considered for patients presenting with chronic diarrhoea to exclude bile acid malabsorption.[40]
Result
normal; increased faecal bile acids suggests bile acid malabsorption
empiric trial of bile acid binder
hydrogen/methane breath test
Test
This may be ordered if the patient has diarrhoea and/or bloating. However, this test is not recommended to confirm diagnosis in patients who meet the IBS diagnostic criteria.[34]
Result
normal; abnormal if bacterial overgrowth or lactase deficiency
stool tests for Giardia lamblia
Test
Routine stool testing for enteric pathogens (i.e., faecal leukocytes, ova, and parasites) is not recommended for patients with suspected IBS, but these tests are commonly ordered by primary care physicians.[39]
Faecal immunoassay or polymerase chain reaction is indicated for patients with risk factors for giardiasis.[39][40]
Result
normal; WBCs in stool or presence of parasites suggests non-IBS disease
plain abdominal x-ray
Test
This test may be useful in the evaluation of a patient who has bloating.[47]
Result
normal; abnormal bowel pattern suggests obstruction
colonoscopy
Test
Guidance recommends against routine colonoscopy for patients with IBS younger than 45 years without alarm features, which include: haematochezia; melaena; unintentional weight loss; older age of onset of symptoms (over 50 years); or family history of inflammatory bowel disease, colon cancer, and other significant gastrointestinal disease.[39]
For patients with alarm features a colonoscopy should be considered.[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.
Result
normal; mucosal inflammation or ulceration suggests inflammatory bowel disease
flexible sigmoidoscopy
Test
Flexible sigmoidoscopy can detect abnormal mucosa, which could indicate inflammatory bowel disease, polyps, or carcinoma; however, guidelines do not support its use to confirm diagnosis of IBS.[34]
Result
normal; abnormal mucosa suggests inflammatory bowel disease
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