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Digestive Disorders > IBS

How is Irritable Bowel Syndrome Diagnosed? The Complete Guide

Dr. Sarah Toler, CNM, DNP

Dr. Sarah Toler, CNM, DNP

Irritable bowel syndrome (IBS) is a syndrome of symptoms without a specific physical cause. Unfortunately, this means that there is no one test or clear-cut way to diagnose IBS. Instead, an IBS diagnosis is a “diagnosis of exclusion,” which means that many other gastrointestinal disorders must be ruled out first before IBS can be diagnosed.

IBS is called a functional bowel disorder(FBD). This means that the disorder impacts the way the intestine functions, but evidence of the disease can’t be visualized when looking closely at the intestine. The diagnosis of IBS can be complicated because symptoms vary between people. IBS symptoms even vary in the same person over time.

After ruling out other gastrointestinal conditions that could cause symptoms similar to those of IBS, a healthcare provider may focus on treating the symptoms of IBS rather than the disease itself. Sometimes lifestyle changes may relieve some IBS symptoms enough to make a person with IBS comfortable.

What is the definition of irritable bowel syndrome?

The diagnostic criteria used for a healthcare provider to diagnose IBS are called the Rome IV criteria. According to this guideline, the following criteria should be met to diagnose IBS:

  • Recurrent abdominal pain present at least 1 day per week in the last 3 months
  • Two or more of the following signs must be associated:

    • The pain is related to bowel movements
    • The pain is associated with a change in how often a person has bowel movements
    • The pain is associated with a change in appearance of stool

The abdominal pain and associated symptoms must be present for the last 3 months and their onset should be at least 6 months before diagnosis.

How is irritable bowel syndrome diagnosed?

Two things are crucial to diagnose irritable bowel syndrome:

The first crucial element to diagnosing IBS is the medical history. Together, a person experiencing symptoms and their healthcare provider can clarify the pattern and extent of the symptoms. The triggers, duration, and timing of irritable bowel symptoms are recorded. Symptom, trigger, nutrition, and sleep diaries, combined with the medical history can help give the healthcare provider the overall picture of what influences IBS symptoms.

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The second critical factor in an IBS diagnosis is the exclusion of other possible causes of IBS symptoms. Sometimes, physical changes can be found in people with irritable bowel syndrome. In most cases, no physical manifestations of IBS can be identified. Therefore, the irritable bowel syndrome diagnosis is a diagnosis of exclusion. This means that other diseases with similar symptoms have to be ruled out before IBS can be diagnosed.

A gastrointestinal specialist will want to perform some tests including a general physical, a colonoscopy, an ultrasound, and blood, stool, and urine tests.

Which diseases should be excluded before diagnosing irritable bowel syndrome?

Irritable bowel syndrome has four subtypes:

  1. Diarrhea type (IBS-D)
  2. Constipation type (IBS-C)
  3. Mixed type (IBS-M), in which both diarrhea and constipation occur
  4. Unclassified type (IBS-U), in which bowel movements cannot be accurately classified in one of the 3 subtypes mentioned above

The following table shows a selection of diseases that should be excluded before the diagnosis of irritable bowel syndrome can be made:

Symptom Important conditions to exclude
Diarrhea Intestinal infection with bacteria, viruses, fungi or parasites
Small Intestinal Bacterial Overgrowth (SIBO) colonization
Chronic inflammatory bowel diseases such as Crohn's disease, ulcerative colitis, celiac disease
Carbohydrate malabsorption (e.g. lactose, fructose and sorbitol)
Diabetes mellitus / exocrine pancreatic insufficiency
Food allergy
Side effects of medication or laxative abuse (laxative abuse)
Hyperactive thyroid (hyperthyroidism)
Colon cancer
Constipation Side effects of medication (e.g. opiates, anticholinergics, antihypertensives, antidepressants)
Underactive thyroid (hypothyroidism) or underactive parathyroid gland (hypoparathyroidism)
Colon cancer
Bulges in the intestine (diverticular disease)
Pain Crohn's disease

Ulcerative colitis

Colon cancer
Ovarian cancer (ovarian tumor)
Narrowing of the small intestine (stenosis)

Pain disorder after abdominal surgery

Bloating Improper colonization of the small intestine (SIBO)
Carbohydrate malabsorption
Pain disorder after abdominal surgery
Intolerance to fermentable carbohydrates (FODMAP intolerance)

Nerve or muscle diseases can also be responsible for symptoms similar to those of IBS. Some of these include:

  • Hirschsprung's disease
  • Chronic intestinal pseudo-obstruction (CIP or CIPO)
  • Acute colonicl pseudo-obstruction (ACPO, “Ogilvie syndrome”)
  • Idiopathic megacolon or megarectum (IMC and IMR)
  • Slow transit constipation
  • Anorectal dysfunction (e.g. pelvic floor dyssynergya, anismus, spastic pelvic floor syndrome)

What tests are used to diagnose irritable bowel syndrome?

A gastrointestinal specialist may recommend some tests to help determine a diagnosis of IBS. Diagnostic guidelines generally recommend a complete blood count (CBC), the determination of the inflammatory marker C-reactive protein (CRP), and the erythrocyte sedimentation rate(ESR). A urine test (urinalysis) should also be performed. Depending on the main symptom, the following tests might also be recommended:

  • Ultrasound
  • Concentrations of blood salts and minerals (electrolyte concentrations)
  • Kidney (renal) function parameters
  • Liver and pancreatic enzymes
  • Thyroid stimulating hormone (TSH)
  • Blood sugar (glucose) and HbA1c (Glycated hemoglobin))
  • Stool test
  • Celiac disease antibodies
  • Calprotectin / lactoferrin in stool
  • Hydrogen breath test (H2 breath test or HBT)

How accurate is the diagnosis of irritable bowel syndrome?

About 5 out of 100 people are misdiagnosed with irritable bowel syndrome. This group of people are able to find an organic cause for symptoms within six years of the IBS diagnosis. An organic cause means that the condition shows detectable physical or biochemical changes within cells, tissues or organs. As a result, patients with irritable bowel syndrome misdiagnosis are sometimes treated incorrectly for years. With the right examinations and tests, this risk can be greatly minimized.

Is irritable bowel syndrome mistaken for inflammatory bowel disease (IBD)?

The symptoms of inflammatory bowel diseases like Crohn's disease and ulcerative colitis and those of irritable bowel syndrome are similar in many cases. As a result, some patients with inflammatory bowel disease are wrongly diagnosed with irritable bowel syndrome.

A colonoscopy in combination with a stool and blood test can rule out chronic inflammatory bowel disease in almost all cases. However some signs like blood in your stool are considered warning signs (“red flags”) and should not be ignored, since they are not typical for IBS.

How many people with celiac disease are wrongly diagnosed with irritable bowel syndrome?

Even people with celiac disease (an intolerance to the wheat protein gluten) are sometimes first diagnosed with irritable bowel syndrome. This is due to the fact that about 3 out of 4 people with celiac disease, like the majority of people with irritable bowel syndrome, experience abdominal pain and flatulence. About half of these people may be misdiagnosed with irritable bowel syndrome and never receive the correct diagnosis.

About a third of people with celiac disease are misdiagnosed with a mental health condition. One in five people will spend ten years in the healthcare system before they receive an accurate celiac disease diagnosis.

Can colon cancer be misdiagnosed as irritable bowel syndrome?

Colon cancer, also called colorectal cancer, is one of the most common types of cancer. In about one in 100 cases, colon cancer is diagnosed within one year following an irritable bowel syndrome diagnosis. This rate is higher than that of the general population and suggests that in rare cases, cancer can actually be hidden behind the diagnosis of irritable bowel syndrome.

It is important to note that in the first year after the IBS diagnosis, the increased risk for colorectal cancer is likely related to diagnostic confusion because of the overlapping symptoms in both conditions. For this reason, correct irritable bowel diagnosis includes the exclusion of colon cancer by colonoscopy. However, everyday clinical practice shows that healthcare providers do not always adhere to the recognized diagnostic guidelines and sometimes prematurely diagnose irritable bowel syndrome.

Can the symptoms of irritable bowel syndrome be confused with those of ovarian cancer?

Almost nine out of ten women with ovarian cancer have symptoms that can also occur with irritable bowel syndrome. When a person with ovaries experiences symptoms of irritable bowel syndrome, it is recommended that they receive a gynecological examination to rule out any ovarian pathology.

Can the diagnosis of irritable bowel syndrome hide a mental illness?

Mental health conditions like anxiety and depression can often cause or worsen gastrointestinal symptoms. There is often a link between mental health conditions and irritable bowel syndrome, and often one condition may worsen the other. Many people with irritable bowel syndrome find their mental health and gastrointestinal health are interrelated.

What to do after an irritable bowel syndrome diagnosis

Several effective treatment options for irritable bowel syndrome exist. The choice of the right therapy depends on which problem is the most bothersome for the person experiencing the symptoms (abdominal pain, constipation, diarrhea, flatulence). Solutions can range from changes in diet to alternative therapies, supplements, and medications.

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Dr. Sarah Toler, CNM, DNP

Dr. Sarah Toler, CNM, DNP

Sarah Toler is a Certified Nurse Midwife, Doctor of Nursing Practice, and science writer. She focuses on improving women's health and access to health care by working with digital health platforms. Her area of expertise is mental health, particularly the physical manifestations of stress and anxiety.

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