Diagnosis of IBS

Symptom-Based Diagnostic Criteria

Due to the lack of a diagnostic biologic marker for IBS, the diagnosis is made using symptom-based criteria. A number of symptom-based criteria have been used in the past; however, the currently accepted criteria are those that have been developed by the Rome II committee, a group comprised of experts specializing in functional gastrointestinal disorders.^[2] IBS is most recently defined as abdominal pain or discomfort not explained by biochemical or structural abnormalities that is present for at least 12 weeks (not necessarily consecutive) over the past 12 months and associated with at least 2 of the following features: (1) relief with defecation; (2) a change in stool consistency (eg, watery/loose or hard/lumpy); and/or (3) a change in stool frequency.

The Rome II criteria are a simplification of the Rome I Criteria.^[12] The Rome I criteria were similar to the above indicated features, but also stated that 2 or more of the following features must be present as least 25% of the time: (1) abnormal stool form; (2) passage of mucus; (3) bloating or distension; (4) abnormal stool passage (feeling of incomplete evacuation, straining, or urgency); and (5) altered stool frequency (> 3 bowel movements/day or < 3 bowel movements/week). These symptoms are presently used as supportive symptoms for the diagnosis of IBS.

Because patients may present with diarrhea and/or constipation, patients are often subgrouped by predominant bowel habit. However, GI symptoms may fluctuate over time and, therefore, a special task force of the American College of Gastroenterology has suggested that patients with IBS be identified using the following designations: IBS associated with abdominal pain, fecal urgency, and diarrhea; IBS associated with abdominal discomfort, bloating, and constipation; and IBS associated with alternating diarrhea and constipation.^[11]

Diagnostic Considerations

Once a dominant symptom complex is identified in the patient, it is also useful to exclude possible "red flags" that might be indicative of an organic disorder. Table 1 outlines the potential "alarm" symptoms that need to be considered when evaluating a patient with IBS.

Table 1. Alarm Symptoms Suggestive of Organic Disease

Once a thorough medical history and physical examination are performed, a variety of laboratory tests that have been advocated in the literature may be considered.^[1,13] These diagnostic tests include: (1) complete blood count (CBC); (2) thyroid-stimulating hormone (TSH) level; (3) erythrocyte sedimentation rate (ESR); (4) complete metabolic profile; (5) stool for ova and parasites (O&P); (6) stool culture and examination; (7) fecal occult blood testing; and (8) celiac sprue panel. Other diagnostic tests include flexible sigmoidoscopy, colonoscopy, or barium enema, and hydrogen breath tests, which should be considered on an individual basis. It is generally recommended that clinicians take an evidence-based medicine approach in their medical evaluation and consider the pretest probability of a test for diagnosing another medical condition (eg, colon cancer, inflammatory bowel disease, celiac sprue) before ordering it. The presence of alarm symptoms would suggest a higher pretest probability of an organic disorder that needs to be ruled out. The differential diagnosis of IBS often depends on the predominant symptom (eg, diarrhea or constipation), as outlined in Table 2.

Table 2. Differential Diagnosis of IBS*

*Consider dominant bowel symptom
†Not an all-inclusive list

Studies of reasonably good quality have suggested that the diagnostic yield associated with performing most of these tests is not very high -- for many, in fact, less than 2%.^[14,15] However, there are a few exceptions. One exception is lactose intolerance, which is present in approximately 25% of the population. However, lactose intolerance is often coexistent with IBS and, when treated, the patient still has symptoms of IBS. The other exception is celiac sprue. The pretest probability in the patient with IBS is significantly higher than that found in the general population (4.67% vs 0.25% to 0.50%).^[16,17] Therefore, screening with endomysial antibody, among other such screening tests, may be indicated, particularly if the patient has failed to respond to treatment. Seven percent of patients with celiac disease are IgA deficient and, therefore, the clinician may want to measure the IgA level as well when celiac disease is a diagnostic consideration.

Although the pretest probability of finding an etiology for symptoms by colonoscopy is not high, most gastroenterologists would agree that patients ≥ 50 years of age should undergo this examination (or, alternatively, a flexible sigmoidoscopy and barium enema) if a previous screening examination has not been done. This age threshold should be lowered to 40 years if there is a significant family history of colon cancer.

Another condition to consider excluding from the differential diagnosis when managing a patient with symptoms of IBS is bacterial overgrowth. Two studies from the same research group found that 78% to 84% of patients with IBS had bacterial overgrowth.^[18,19] In patients with evidence of bacterial overgrowth, those treated with neomycin had a ≥ 35% reduction in clinical response (ie, improvement) compared with an 11% reduction in patients on placebo.^[19] Although these data are extremely intriguing, there are some methodologic limitations in these studies and, therefore, the use of widespread hydrogen breath testing for bacterial overgrowth is still not generally advocated.

The diagnosis of IBS is primarily symptom-based, and the literature suggests that once made, the clinician can be confident of his/her diagnosis. Retrospective views of patients have suggested that the diagnosis is an enduring one, with 92% to 97% of the individuals maintaining the same diagnosis over 2-13 years.^[20,21]