1. improvement of discomfort by defecation

2. onset associated with a change in the frequency of stool

3. onset associated with a change in the form (appearance) of stool

* Criteria should be fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis.

Irritable bowel syndrome patients frequently report symptoms referable to other organs. Large subsets have associated heartburn, early satiety, nausea, vomiting, and dyspepsia. High incidences of genitourinary dysfunction (dysmenorrhea, dyspareunia, impotence, urinary frequency, and incomplete urinary evacuation), fibromyalgia, low back pain, headaches, fatigue, insomnia, and impaired concentration have been observed in individuals with IBS.

Physical examination of the person with IBS usually is unimpressive. The patient may appear anxious and have cold, clammy hands. Diffuse tenderness or a palpable bowel loop may be evident on abdominal examination. Organomegaly, adenopathy, or occult fecal blood is not consistent with a diagnosis of IBS and warrants a search for organic disease.

Differential diagnosis

While the diagnosis of IBS is based upon identifying symptoms that are consistent with the condition (see Table 28.1), many other conditions may present in a similar manner and need to be excluded in a cost-effective manner. Patients with inflammatory bowel disease, microscopic colitis, celiac disease, thyroid dysfunction, colorectal neoplasia, and infectious diarrhea can have symptoms that mimic IBS. The presence of “alarm features” such as weight loss or gastrointestinal bleeding, refractory diarrhea or a family history of colorectal cancer should be used to help direct the evaluation. In the absence of “alarm features” the Rome criteria for diagnosing IBS are very specific.

Diagnostic investigation

Diagnosing IBS confidently involves a directed evaluation to confirm that organic disease is not present. The extent of the diagnostic investigation depends on patient age and the predominant symptoms.

Laboratory studies

Normal values of selected laboratory tests help to confirm a diagnosis of IBS. In contrast, anemia, leukocytosis, leukopenia, or elevations of the sedimentation rate suggest organic disease. Thyroid chemistries are performed in some cases of diarrhea-predominant or constipation-predominant disease to exclude hyperthyroidism or hypothyroidism, respectively. Celiac disease serologies, including endomysial and tissue transglutaminase antibodies, are obtained in individuals with possible celiac disease. Stool samples may be obtained to exclude giardiasis in some patients with diarrhea-predominant disease.

Structural studies

Structural testing is recommended for many patients with suspected IBS. In patients older than age 45–50, colonoscopy is recommended to screen for colorectal cancer. Sigmoidoscopy or colonoscopy may be performed in younger individuals, especially if the diagnosis is uncertain. Biopsy of the colon during lower endoscopy is indicated in some patients with prominent diarrhea to rule out microscopic colitis as a cause of symptoms. Upper endoscopy may be performed for reflux or dyspeptic symptoms. Endoscopic small intestinal biopsy is indicated if serological testing suggests celiac disease.