How Is IBS Diagnosed?

Irritable Bowel Syndrome (IBS) affects up to 15% of the global population, yet its diagnosis often feels elusive and frustrating. Statistics say that it often takes up to 6 years and visits with three or more healthcare providers to receive a diagnosis. Don’t become one of these statistics. Advocate for yourself. This article explains what you should be asking for from your medical doctor. 

The Summary: How is IBS Diagnosed? It starts with several tests, then the Rome IV criteria is considered, and working through the low FODMAP diet aided by a Monash trained Registered Dietitian—all of these used together—lead to an accurate diagnosis. Read on for particulars.

There are many medical issues that mimic IBS, and some of them are deadly. You have to go through a correct diagnosis protocol in order to know how to address your symptoms.

Never Self-Diagnose

We wish we didn’t have to say this, but DO NOT SELF-DIAGNOSE. We see it happen all the time and it can lead to even more dire problems. There are many medical issues that mimic IBS, and some of them are deadly. You have to go through a correct diagnosis protocol in order to know how to address your symptoms. It might be IBS; it might not. About 36% of you will have IBS along with additional digestive issues, and treatment is going to be unique to your medical presentation (we all have unique gastrointestinal tracts). 

Still Not Convinced? Have Any Red Flag Symptoms?

As mentioned, there are medical diseases and syndromes that can mimic IBS. If you have any of these red flag symptoms, please do not wait. Get a medical diagnosis now.

• Unintentional weight loss
• Rectal bleeding or blood in the stool
• Anemia (especially iron-deficiency anemia)
• Fever
• Nocturnal symptoms (diarrhea waking you from sleep)
• Family history of colorectal cancer, IBD, or celiac disease
• Onset after age 50 (in someone previously symptom-free)
• Persistent severe pain that’s not relieved by passing gas or a bowel movement

Diagnosis Through Exclusion, Rome IV Criteria, Plus The Low FODMAP Diet

The answer lies in ruling out other conditions, applying standardized symptom criteria like the Rome IV guidelines, and in some cases, observing patient response to the low FODMAP diet. Importantly, IBS is now classified as a Disorder of Gut-Brain Interaction (DGBI)—a term that reflects its complex neurological and gastrointestinal roots.

First: Rule Out Other Conditions

Before diagnosing IBS, doctors must rule out several conditions that mimic its symptoms but require very different treatments. This process typically includes:

• Complete Blood Count (CBC): Checks for anemia or infection
• C-reactive Protein (CRP) or Fecal Calprotectin: To assess inflammation, especially to screen for Inflammatory Bowel Disease (IBD) like Crohn’s disease or ulcerative colitis
• Celiac Disease Blood Panel: Screens for autoimmune reaction to gluten
• Stool Studies: To rule out parasites, infections, or malabsorption
• Colonoscopy or Sigmoidoscopy: Especially for adults over 45 or those with red flag symptoms

Conditions like diverticulitis, which causes localized pain and fever, and endometriosis, which may cause GI symptoms tied to menstrual cycles, should also be considered.

Only after these are excluded does IBS begin to come into focus. But you aren’t done yet.

How is IBS Diagnosed? It starts with several tests, then the Rome IV criteria is considered, and the low FODMAP diet conducted with a Monash trained Registered Dietitian—all of these used together—lead to an accurate diagnosis.

What Is a DGBI?

IBS is categorized as a Disorder of Gut-Brain Interaction, or DGBI—a term introduced by the Rome Foundation to reflect dysfunction in the way the gut and brain communicate. These are not psychological conditions, but they are influenced by how the brain processes pain, stress, and motility signals from the gastrointestinal tract.

DGBIs can involve gut hypersensitivity, motility issues, and microbial imbalances. This modern understanding has helped reduce the stigma of IBS as “all in your head” and supports a more holistic treatment approach.

Applying the Rome IV Criteria

To formally diagnose IBS, doctors use the Rome IV diagnostic criteria, which were updated in 2016 by an international team of GI experts. According to these criteria, IBS is defined by:

Recurrent abdominal pain at least 1 day per week in the last 3 months, associated with two or more of the following:

• Related to defecation
• Associated with a change in stool frequency
• Associated with a change in stool form (appearance)

Symptoms must have started at least 6 months before diagnosis.
IBS is further subtyped based on stool patterns:

• IBS-D (diarrhea-predominant)
• IBS-C (constipation-predominant)
• IBS-M (mixed)
• IBS-U (unclassified)

These criteria are essential in moving IBS from a “diagnosis of exclusion” to a positive diagnosis based on symptoms. Read more in our article on IBS Sub-Types.

Enter the Low FODMAP Diet

The low FODMAP diet, developed by researchers at Monash University, has become a powerful management tool—and in some cases, an indirect diagnostic aid. FODMAPs are fermentable carbohydrates that can be poorly absorbed in the small intestine. In people with IBS, they lead to excessive gas, bloating, diarrhea, and pain.

The diet includes three stages:

1. Elimination Phase (2–6 weeks): Remove all high FODMAP foods
2. Challenge Phase: Gradually test individual FODMAP groups
3. Integration Phase: Create a personalized long-term plan

Examples of high FODMAP foods include garlic, onions, apples, milk, legumes, and wheat. Response to the low FODMAP diet can help reinforce an IBS diagnosis, especially when symptoms improve dramatically and return with reintroduction.

Studies show that up to 75% of people with IBS experience relief on a low FODMAP diet, although it must be implemented under the supervision of a trained dietitian, for this high percentage result.

Other Helpful Tests

In select cases, additional tests may be warranted:

• Hydrogen and Methane Breath Tests: To screen for SIBO (small intestinal bacterial overgrowth) or sugar malabsorption, such as lactose or fructose intolerance. SIBO is controversial and difficult to diagnose accurately, but may coexist with IBS.
• Anorectal Manometry: Used for chronic constipation to assess muscle coordination in the pelvic floor
• Gynecological Evaluation: For women with possible endometriosis, especially when symptoms worsen with menstrual cycles

Final Thoughts

IBS is real. It’s not something that shows up on an x-ray, but that doesn’t mean it’s imaginary. It’s a neurogastrointestinal disorder—a DGBI—where symptom patterns, rather than lab results, guide diagnosis.

Proper evaluation starts by ruling out serious conditions, applying Rome IV criteria, and often trying a low FODMAP diet under expert supervision. For many people, that dietary response becomes part of the diagnostic puzzle and a life-changing first step toward symptom control.

If you suspect you have IBS, speak with a gastroenterologist and consider working with a Monash-trained dietitian to try the low FODMAP diet appropriately and effectively. Don’t self-diagnose—and don’t give up. Relief is possible, and the right diagnosis makes all the difference.

---

Additional Sources and References:

From FODMAP Everyday®:

• The Role Of The Dietitian vs. The Gastroenterologist
• What Is IBS?
• Deadly Diseases That Mimic IBS
• Understanding the Relationship Between Endometriosis & IBS
• Understanding the Overlap Between IBS & Anxiety
• Have IBS? Top Reasons To Work With A Dietitian
• How To Choose A Dietitian
• Dietitians & Nutritionists: What’s The Difference?
• Patient Dos And Don’ts For A Great Telehealth Appointment 

Other:

• Rome IV Diagnostic Criteria
• IBS Overview – National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
• Celiac Disease Diagnosis – NIDDK
• ACG IBS Clinical Guidelines (2021) – American Journal of Gastroenterology
• IBD and IBS Symptom Comparison