Getting the right diagnosis: Understanding how IBS is diagnosed

IBS can feel frustrating, especially if you’ve waited a long time for answers, plus it shouldn’t be self-diagnosed.

But the good news is that IBS can be diagnosed confidently and positively by your doctor, without needing endless tests.

This article explains what a diagnosis involves, what tests are usually done, what red flag symptoms need more investigation, and how you can work with your doctor to get a thorough, reassuring assessment.

What is IBS?

Irritable Bowel Syndrome (IBS) is a common, long-term condition that affects how the gut and brain communicate.

It causes symptoms such as:

• Abdominal pain or cramping
• Bloating or swelling
• Diarrhoea, constipation, or both
• Urgency
• Mucus in the stool
• Fatigue

IBS doesn’t cause structural damage or inflammation, meaning your bowel looks healthy on tests, but it doesn’t always function smoothly. The condition doesn’t result in long-term damage to your gut or raise your risk of cancer, but it can have a big impact on your daily life, wellbeing, and confidence.

The good thing is, with the right understanding and a personalised IBS Action Plan, symptoms can often significantly improve.

How is IBS diagnosed?

According to NICE guidelines (CG61), in adults, IBS should be diagnosed positively, not by ruling out every other possible cause.

This means your doctor will look for a pattern of symptoms that fit IBS, and carry out a few simple tests to rule out other key conditions.

A positive diagnosis is usually made when:

• You’ve had typical symptoms for at least six months, and
• You have abdominal pain linked with bowel changes (either relieved by going to the toilet or linked with changes in stool frequency or consistency),

and at least two of the following:

• Abdominal bloating or distension (outward swelling)
• Altered stool passage (straining, urgency, incomplete emptying)
• Symptoms made worse by eating
• Passage of mucus in your stool

IBS can only be confidently diagnosed when these symptoms are present without red flag symptoms (see below).

Steps to diagnose IBS accurately – step 1: check for “red flag” symptoms

Your GP will first make sure your symptoms don’t suggest something more serious.

If any of these are present, further tests or a referral may be needed:

• Unexplained or unintentional weight loss
• Blood in the stool (rectal bleeding)
• Nocturnal diarrhoea (waking from sleep to open bowels)
• Iron-deficiency anaemia
• Persistent fever or fatigue
• A lump in the abdomen or rectum
• Family history of bowel cancer, coeliac disease, or inflammatory bowel disease (IBD)
• New-onset symptoms after the age of 50

Step 2: routine tests that support a positive IBS diagnosis

If your symptoms fit the IBS symptom pattern described above and no red flags are present, your GP will arrange some basic investigations to rule out common alternatives.

These are quick, non-invasive blood or stool tests. The table below outlines what IBS tests involve.

Test	Purpose	What it involves and what to expect
Full Blood Count (FBC)	Checks for anaemia (low iron) or infection.	A small blood sample from your arm. No special preparation needed.
CRP or ESR (or Plasma Viscosity)	Measures inflammation in your body. Raised levels may suggest inflammatory bowel disease (IBD).	A simple blood test. Avoid infection at the time of the test, as that can temporarily raise CRP.
Coeliac Serology (tTG-IgA or EMA)	Screens for coeliac disease, an immune reaction to gluten.	A blood test. You must be eating gluten regularly (e.g., bread, pasta) for at least 6 weeks before testing, or results may be falsely negative.
Faecal Calprotectin (under age 60 with diarrhoea)	Checks for gut inflammation to help distinguish IBS from IBD.	A stool sample test done at home and returned to your doctor’s surgery.
Thyroid Function Tests (sometimes)	Checks for underactive thyroid, which can mimic IBS symptoms.	Blood test. Often done if other symptoms suggest thyroid imbalance (tiredness, weight change, cold intolerance).

If all these tests are normal and your symptoms match IBS criteria, your GP can make a positive diagnosis of IBS without referring you for further, more invasive testing.

Step 3: understanding key differentials

Doctors use the above tests to rule out certain conditions that can mimic IBS. This is called a differential diagnosis for IBS – that is the process of differentiating it from other conditions that have similar symptoms like abdominal pain, bloating, and changes in bowel habits.

Here’s what those similar conditions are, explained simply:

Coeliac disease

What is it? An autoimmune condition where the body reacts to gluten (a protein in wheat, barley, and rye). This reaction damages the small intestine and causes poor nutrient absorption.

Common symptoms: bloating, diarrhoea, constipation, fatigue, weight loss, or anaemia.

Diagnosis: a blood test for coeliac antibodies (tTG-IgA or EMA), followed by a biopsy (an examination of removed tissue) of the small intestine if positive.

Important: keep eating gluten before testing as stopping early can hide the problem.

More info: NHS – Coeliac Disease

Inflammatory Bowel Disease (IBD)

What is it? An umbrella term for Crohn’s disease and ulcerative colitis, which cause chronic inflammation and damage to the gut lining.

Common symptoms: Persistent diarrhoea (often with blood), abdominal pain, weight loss, fatigue, and fever.

Diagnosis: IBD is confirmed through blood/stool tests (including faecal calprotectin), imaging, and colonoscopy with biopsy.

Important: IBD needs specialist care, sometimes long-term medication, and shouldn’t be confused with IBS.

More info: Crohn’s & Colitis UK

When diarrhoea might be due to bile acid malabsorption (BAM)

Some people diagnosed with IBS with diarrhoea (IBS-D) actually have a condition called Bile Acid Malabsorption (BAM).

BAM happens when the body doesn’t properly reabsorb bile acids (natural substances made by the liver that help digest fats).

Too many bile acids then reach the large bowel, which can irritate the gut and cause chronic watery diarrhoea, urgency, bloating, or wind.

Why it matters

BAM can mimic IBS-D, so identifying it helps tailor treatment. If BAM is confirmed, specific medicines called bile acid sequestrants (such as colestyramine or colesevelam) can bind excess bile acids and often reduce symptoms.

The SeHCAT scan

A SeHCAT scan is a simple, painless test used to diagnose BAM:

• You swallow a small capsule containing a harmless radioactive tracer that behaves like bile acid.
• A hospital scanner measures how much tracer remains in your body after one week.
• If very little remains, it suggests bile acids are being lost too quickly, therefore confirming BAM.

The SeHCAT scan is available on the NHS in many (but not all) areas. It’s usually arranged by a hospital specialist, typically a gastroenterologist, after referral from your GP, if your diarrhoea hasn’t improved with standard IBS treatment or other causes need to be ruled out.

When to ask about BAM

You could ask your doctor about testing for BAM if you have:

• Ongoing diarrhoea that hasn’t improved with standard IBS advice or medicines
• Diarrhoea that started after gallbladder surgery or a severe gut infection
• Loose stools with urgency or accidents despite dietary changes

Step 4: making a positive diagnosis

NICE guidelines aim to help GPs make a positive diagnosis of IBS, not to use it as a “last resort” label.

Once your symptoms clearly meet the IBS criteria, and the above tests exclude key alternatives, your GP can confirm an IBS diagnosis and start discussing management options.

This approach avoids unnecessary investigations, helps reduce anxiety, and allows earlier focus on practical strategies such as diet, lifestyle, and stress support.

Step 5: advocating for a thorough assessment

If you suspect you have IBS, here’s how to make sure your assessment is complete and collaborative:

• Ask your GP:

💬 “Have all the standard IBS tests been done – including blood tests for coeliac disease and inflammation markers?”

• Mention any family history of bowel disease or cancer.
• Tell your doctor if symptoms started after infection, antibiotic use, or stress, as this can suggest post-infectious IBS.
• If symptoms affect your quality of life, ask:

💬 “Could I be referred to a dietitian or gastroenterologist to explore tailored management?”

• Keep a symptom diary or use your IBS tracker to bring to appointments as it helps your clinician see your real-life experience.

Summary: what to know about diagnosing IBS

✅ IBS is diagnosed positively based on symptoms and simple tests.

❌ You don’t need every possible test to rule out other conditions.

🚩 Red flag symptoms mean further tests may be needed to stay safe.

🧠 A positive diagnosis allows you and your clinician to focus on improving your wellbeing, not just finding what’s “wrong.”

📚 Useful links and resources

• NICE (CG61): Irritable Bowel Syndrome in Adults – Diagnosis and Management
• NHS: Irritable Bowel Syndrome (IBS)
• Guts UK Charity
• British Dietetic Association: Find a Dietitian
• Crohn’s & Colitis UK

References

1. Crohn’s & Colitis UK (n.d.) Making a difference.
2. NHS (2023) Coeliac disease – NHS.
3. NICE (2008, updated 2017) Irritable bowel syndrome in adults: diagnosis and management.