A formal diagnostic criteria for IBS has been developed in the form of the Rome II criteria (1):
At least 12 weeks, which need not be consecutive, in the preceeding 12 months of abdominal discomfort or pain that has two of the three features: 
 The following cumulatively support the diagnosis of IBS: 
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Red flag symptoms that indicate need for colonoscopy in patients with suspected irritable bowel syndrome (2): 
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Reference:
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