Irritable Bowel Syndrome (IBS) is a common functional gastrointestinal disorder. This means symptoms arise from a disturbance in how the gut works and communicates with the brain, rather than from visible damage or inflammation in the GI tract. IBS is characterized by recurrent abdominal pain linked to changes in bowel habits, including diarrhea, constipation, or both. Diagnosis relies on criteria like the Rome IV, which defines IBS based on symptom patterns and the exclusion of other diseases. Since symptoms like pain, bloating, and altered bowel movements are shared across many GI conditions, accurately distinguishing IBS from other serious disorders is necessary for proper care.
Inflammatory Bowel Disease and Other Structural Issues
The primary distinction is between IBS and Inflammatory Bowel Disease (IBD), which encompasses Crohn’s disease and Ulcerative Colitis (UC). Unlike functional IBS, IBD is a structural disease defined by chronic inflammation and physical damage to the gut lining. This inflammation is visible on imaging or endoscopy, often leading to ulcers, tissue damage, and permanent complications.
Crohn’s disease can affect any part of the GI tract, penetrating deep into the bowel wall, while Ulcerative Colitis is confined to the colon and rectum, affecting only the innermost lining. These are immune-mediated conditions where the immune system attacks the digestive tract, causing ongoing inflammation. IBD’s inflammatory nature causes systemic symptoms like fever, weight loss, and joint pain, which are not characteristic of IBS.
Microscopic colitis, including collagenous and lymphocytic colitis, also mimics IBS-related diarrhea. This inflammatory condition causes chronic watery diarrhea and abdominal pain, but is often missed on a standard colonoscopy because the bowel lining appears normal. Diagnosis requires a biopsy of the colon wall, revealing inflammatory cells or a thickened collagen layer under a microscope.
Conditions affecting nearby organs, such as the gallbladder or pancreas, can also present with generalized abdominal pain and altered bowel habits. Chronic pancreatitis involves persistent inflammation of the pancreas, leading to pain radiating to the back and changes in stool due to poor digestion. Gallstones or chronic gallbladder dysfunction may cause upper abdominal pain sometimes confused with the discomfort of IBS.
Conditions Related to Food Processing
A number of conditions that cause IBS-like symptoms are linked to the body’s reaction to or inability to process specific food components. Celiac disease is an autoimmune reaction to gluten, a protein found in wheat, barley, and rye. When a person with celiac disease consumes gluten, their immune system attacks and damages the small intestine’s villi, impairing nutrient absorption.
Celiac disease differs from IBS because it causes measurable intestinal damage, and diagnosis relies on specific blood tests followed by a biopsy. Non-celiac gluten sensitivity (NCGS) is a distinct condition where people experience similar digestive symptoms after consuming gluten, but without the autoimmune response or intestinal damage seen in celiac disease. NCGS is diagnosed by ruling out celiac disease and wheat allergy, and symptoms are usually limited to the GI tract.
Specific carbohydrate malabsorptions are common causes of IBS-like symptoms, especially the inability to digest sugars like lactose or fructose. Lactose malabsorption occurs when the small intestine does not produce enough lactase enzyme to break down lactose. Undigested sugars travel to the colon, where bacteria ferment them, producing excess gas, bloating, and diarrhea—symptoms nearly identical to diarrhea-predominant IBS. The low-FODMAP diet, which restricts these carbohydrates, is often used to manage IBS symptoms, highlighting the symptomatic overlap.
Bacterial Overgrowth and Infectious Causes
Microbial imbalances and persistent infections can result in symptoms that closely mimic IBS. Small Intestinal Bacterial Overgrowth (SIBO) occurs when an abnormally high number of bacteria colonize the small intestine, a region that normally maintains a low bacterial count. These bacteria ferment food prematurely, leading to severe bloating, abdominal pain, and gas after eating carbohydrate-rich foods.
SIBO is frequently misdiagnosed as IBS, or it may be an underlying cause for IBS symptoms in some individuals. The condition is typically diagnosed using a non-invasive breath test, which measures the hydrogen and methane gases produced by the bacteria. In contrast to IBS, SIBO symptoms often involve intense bloating that visibly worsens throughout the day.
Acute infections from parasites like Giardia or bacteria such as Clostridioides difficile (C. diff) can cause persistent, severe diarrhea mistaken for an IBS flare-up. In some cases, a person may develop Post-Infectious IBS (PI-IBS) following severe gastroenteritis, where the infection triggers long-term functional changes in the gut. This suggests a previous infection can permanently alter gut-brain communication, leaving the patient with chronic, IBS-like symptoms even after the pathogen is eliminated.
Diagnostic Red Flags
The process of distinguishing IBS from other conditions begins by screening for specific “red flag” symptoms that suggest a structural disease or a more serious underlying issue. The presence of these markers warrants immediate diagnostic investigation beyond a simple IBS diagnosis. One significant red flag is unexplained weight loss, as IBS itself does not typically cause a drop in body mass.
Other concerning signs include blood in the stool, especially if not linked to a local issue like hemorrhoids, and anemia, which may indicate chronic blood loss or malabsorption. Symptoms that wake a patient from sleep, such as abdominal pain or the need to have a bowel movement, are also considered red flags because functional disorders like IBS rarely interrupt sleep. A new onset of symptoms after the age of 50 prompts doctors to prioritize ruling out structural diseases, including colorectal cancer.
To exclude these conditions, a physician often begins with baseline blood work to check for inflammatory markers, such as C-reactive protein, and to assess for anemia. Stool tests look for evidence of infection, occult blood, and specific markers of inflammation like fecal calprotectin. If red flags are present, diagnostic imaging like a colonoscopy is recommended to visually inspect the bowel lining and take biopsies to rule out IBD or microscopic colitis. The absence of these red flags and the fulfillment of the Rome IV criteria ultimately support an IBS diagnosis.