Is your diarrhoea definitely caused by IBS?

13th March 2023

Figures vary widely, but it is thought that Irritable Bowel Syndrome (IBS) affects approximately 1 in 20 of us in the UK. A third of people with IBS are classed as having IBS-D. IBS-D is IBS with predominately diarrhoea symptoms. It’s important that you’re given the correct diagnosis so you can start the right treatment path.

How is IBS diagnosed?

The GP will want to rule out other diseases but will probably be able to make a diagnosis based on the described symptoms.

Further tests may include blood tests, which will be used to assess the following:

  • Whether anaemia is present.
  • Liver and kidney function.
  • Any signs of inflammation in the bowel.
  • Whether coeliac disease may be the issue.
  • Faecal Calprotectin: this is a stool test increasingly used to look for inflammation so the doctor is sure that symptoms are not caused by other bowel disease. The doctor can then be confident that the diagnosis is IBS.

What do these tests look for?

Coeliac disease

Coeliac disease is an autoimmune condition, occurring in people who become sensitive to a protein called gluten in their diet. The first step at the GPs for a coeliac disease diagnosis is a blood test. This looks for antibodies indicative of coeliac disease.

To diagnose coeliac disease you need to continue to eat gluten whilst under investigation.

Crohn's disease

Crohn’s disease is a condition in which inflammation develops in various parts of the gut and is part of the Inflammatory Bowel Disease (IBD) group. A blood test and a faecal calprotectin test will help the GP to see if there is any inflammation which could indicate crohn’s disease.

Ulcerative colitis

Ulcerative colitis is a disease of the rectum and the large bowel. It is also a part of the IBD group. Similarly to crohn’s disease a blood test and faecal calprotectin test will help the GP to see if there is any inflammation which could indicate ulcerative colitis.

Bowel cancer

Bowel cancer is one of the most common cancers in the UK with almost 9 out of 10 people diagnosed over the age of 60. If it is caught in the early stages, there is a greater than 90% chance of a cure.

Because polyps may bleed, one of the screening methods involves testing the poo chemically for traces of blood, this is known as a FIT test. Further investigations of the bowel are carried out if the test is positive.

If you’re aged between 56 to 74 in England and registered with a GP, you are automatically sent a bowel screening test every 2 years. It is important to complete this test and send it back.


doctor chatting with patient

To come to an IBS diagnosis your doctor should help you rule out other possible conditions or causes of your diarrhoea first.

What other digestive conditions cause diarrhoea?

If your symptoms continue after being tested for the above, it may be worth learning more about other digestive conditions that can cause diarrhoea too.

Bile Acid Diarrhoea / Malabsorption (BAD / BAM)

The main symptom for BAM is diarrhoea. When bile acids are not properly re-absorbed from the end of the small bowel, they pass instead into the large bowel. This irritates the lining of the bowel and stimulates salt and water secretion.

BAM is diagnosed via a test called the SeHCAT scan. You cannot diagnose BAM through a blood or poo test. 

Pancreatic Exocrine Insufficiency (PEI)

Any disease that affects pancreatic tissue can cause PEI. The most common causes in the UK are cystic fibrosis, acute and chronic pancreatitis and pancreatic cancer.

If PEI is suspected, diagnostic tests will look at the structure of the pancreas, usually by an ultrasound, CT, MRI scan or Endoscopic Ultrasound (EUS). They will investigate how well the pancreas is working, by a faecal elastase-1 poo test.

Microscopic colitis

Microscopic colitis is also a part of the IBD group. However, it cannot be seen on an endoscopy, blood test or faecal calprotectin test like crohn’s disease and ulcerative colitis. Rather, a small sample of tissue has to be taken (during an endoscopy) of the bowel and examined under a microscope.

One study showed that 1 in 3 people with microscopic colitis were initially incorrectly diagnosed with IBS. 

Other digestive conditions that can cause diarrhoea may be not show up on the initial tests your GP asks for. It's important, if symptoms continue, to know about these other digestive conditions and to ask for the relevant tests for them.


If simple treatments for IBS have not worked and your diarrhoea symptoms continue, or if your symptoms change for an unknown reason – contact your doctor.

Also remember the red flag symptoms:

  • Unintentional weight loss.
  • Bleeding from your bottom.
  • Diarrhoea that wakes you up at night.
  • Unrelenting watery diarrhoea that is green, yellow or pale and difficult to flush.
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