Diagnosis of IBS - tests & investigations

medical testing

Dr. Jane Varney - Research Dietitian, 16 July 2018

Irritable bowel syndrome (IBS) was once considered a diagnosis of exclusion - patients were required to have multiple (often very invasive) tests to rule out other causes of their symptoms before a diagnosis of IBS was finally made. However, this approach is no longer recommended. This is partly because excessive tests are expensive, time consuming and often a source of anxiety for patients (the latter of which may trigger symptoms in some). Also, without any alarm features (such as unexplained weight loss, age over 50 years, blood present in bowel movements/stools or a family history of IBD, bowel cancer or coeliac disease etc.), the chance of these tests coming back positive is very low. 

However, a limited number of tests are still required before a diagnosis of IBS can be made. Screening for coeliac disease is generally considered essential, mainly due to the large overlap in symptoms between the two conditions. Additional tests may also be needed if you have any alarm features or other symptoms that are not typical of IBS. 

Screening and diagnostic tests for coeliac disease: 

Anti endomysial antibody (IgA) and tissue transglutaminase antibody (IgA) 

Sometimes called coeliac serology tests, these blood tests are used to screen for coeliac disease. It is important that you are eating a gluten containing diet when you have this test. If you receive a negative result, this accurately indicates that you do not have coeliac disease. However, if you receive a positive result, you will need further investigations (a gastroscopy and small intestinal biopsies) to determine whether or not you have coeliac disease. 

It is important to note that these tests are only accurate if you are including gluten in your diet. If you have this test while following a gluten free diet, you may be wrongly told that you do not have coeliac disease. 

HLA DQ2/DQ8 genetic test 

This blood test looks for genes associated with coeliac disease. The test is useful to screen for and exclude coeliac disease. If a negative result is reached, this indicates that you do not have coeliac disease. By contrast, if a positive result is reached, this indicates that coeliac disease may be present, and that further testing is required to diagnose or rule out this condition. This test is particularly useful in people following a gluten free diet as results are not affected gluten intake. 

Small intestinal biopsy 

This is the gold-standard test used to and diagnose (or exclude) coeliac disease. Small intestinal biopsies require you to undergo a gastroscopy (a simple day procedure done under light sedation). During the gastroscopy, several samples (biopsies) are taken from the small bowel. These are examined under a microscope to look for changes consistent with coeliac disease (known as villous atrophy). 

It is important to note that this test is only accurate if you are including gluten in your diet. If you have this test while following a gluten free diet, you may be wrongly told that you do not have coeliac disease. 

Screening & diagnostic tests for Inflammatory bowel disease (IBD) & bowel cancer: 

Faecal calprotectin 

Calprotectin is a protein that, when present in high levels, indicates inflammation in the bowel. This test may be used to rule out inflammatory bowel disease (IBD) and requires you to provide a small faecal (poo) sample for testing. This test may be recommended if you have noticed blood in your stools or other symptoms that might indicate IBD. 

Faecal occult blood analysis 

This test requires you to take a small sample of your faeces (poo) from two separate bowel motions using a test kit. The test detects tiny amounts of blood originating from pre-cancerous polyps or early stages of bowel cancer. This test may be recommended as part of routine screening for bowel cancer, especially if you have a family history of the disease or are aged over 50 years when your gastrointestinal symptoms begin. 

Colonoscopy and colonic biopsies 

These tests examine the large intestine and may be used to look for diverticular disease, inflammatory bowel disease and some cancers. The procedure is usually performed under sedation. During the procedure, a doctor will insert a flexible tube with a camera on the end into your rectum (bottom) and up into your colon. This allows your doctor to look for abnormal tissue and to take a sample (biopsy) for further examination. The process usually takes around 20–30 minutes.

Other investigations: 


This is a surgical procedure (key hole surgery) used to diagnose (and sometimes treat) endometriosis, which can have some similar symptoms to IBS (such as bloating, constipation and chronic pelvic pain). Laparoscopy may be recommended when other symptoms of endometriosis are present such as irregular, painful and/or heavy periods, infertility etc. 

Thyroxine and thyroid stimulating hormone

These blood tests are used to evaluate thyroid function and detect thyroid conditions such as hyper- or hypothyroidism. Both hyper- and hypothyroidism can affect bowel function, leading to gastrointestinal symptoms, so these tests may be included routinely as part of the diagnostic process for IBS. 


1. Lacy BE, Mearin F, Chang L, Chey WD, Lembo AJ, Simren M, et al. Bowel Disorders. Gastroenterology. 2016;150(6):1393-407.e5.

2. Issa B, Ormesher L, Whorwell P, Shah M, Hamdy S. Endometriosis and irritable bowel syndrome: a dilemma for the gynaecologist and gastroenterologist. The Obstetrician and Gynaecologist. 2016 Jan 27;18(1):9-16. Available from, DOI: 10.1111/tog.12241. 

3. Moayyedi P, Mearin F, Azpiroz F, Andresen V, Barbara G, Corsetti M, et al. Irritable bowel syndrome diagnosis and management: A simplified algorithm for clinical practice. United European Gastroenterology Journal. 2017;5(6):773-88.

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