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Make the Diagnosis: Irritable Bowel Syndrome

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Prior Probability

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During a lifetime, everyone experiences lower gastrointestinal tract symptoms though few people need to visit their physician for evaluation. When patients with these symptoms do go to their physician for an evaluation, irritable bowel syndrome (IBS) is the most likely diagnosis. The community-based prevalence is estimated to be between 5%-20% of adults.1-3 In patients referred to gastroenterologists for further evaluation, IBS is still likely, with 57% of referred patients diagnosed ultimately with irritable bowel syndrome.4-13

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Population for Whom Irritable Bowel Syndrome Should Be Considered

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Lower abdominal discomfort, changes in bowel habit or the nature of the passed stool, flatulence, passage, or a sensation of incomplete rectal emptying all prompt consideration of IBS.

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Assessing the Likelihood That a Patient Has Irritable Bowel Syndrome

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Among individual findings, only the presence of looser stools at the onset of abdominal discomfort has an LR > 2.0. (Table 55-1) Most patients with IBS have some abdominal discomfort (high sensitivity and low LR-), but the presence of abdominal discomfort has a myriad of causes. The lack of utility for individual findings requires the clinician to assess combinations of findings. All patients should have a clinical examination to confirm the absence of physical findings that suggest alternative diagnoses.

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Table Graphic Jump Location
Table 55-1Likelihood Ratios That a Patient With Lower Abdominal Symptoms Has Irritable Bowel Syndrome
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Symptom duration in the Rome criteria and Kruis model is important, as the longer a patient experiences symptoms without an alternative diagnosis, the higher the likelihood of IBS. The Rome I criteria have been updated to the, as yet, unvalidated Rome III criteria. The Rome III criteria require that the onset of symptoms be at least 6 months prior to the diagnosis of IBS. The duration of symptoms is not part of the Manning criteria.

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Any of 3 abnormal basic laboratory tests (Kruis model) make an alternative diagnosis to IBS more likely: anemia (hemoglobin < 12 g/dl for females and < 14 g/dl for males), leukocytosis (white blood cell count > 10,000 cells/μl), or elevated erythrocyte sedimentation rate (> 20mm/2 hours).

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Reference Standard Tests

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Irritable bowel syndrome is considered a functional, rather than organic, diagnosis as no structural lesions account for the ...

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