There is no gold standard for diagnosing IBS.1 In diagnosing IBS, it’s important to identify all the symptoms present, particularly any “red flags”. Doctors will also need to consider alternative diagnoses for IBS, especially for symptoms that may be shared with other diseases.
Medical history and physical examination
Diagnosis may begin with a complete medical history and physical examination. Due to the nature of the symptoms, findings from the physical examination are typically normal.2 A physical examination may include examining the abdominal area and the perianal region. A medical history may include information about a family history of IBS, which may suggest a more likely diagnosis, and a psychological assessment about stress, depression, and anxiety.3
For more detailed guidelines on diagnosis, doctors may refer to the Rome III criteria and Manning criteria. The Rome III criteria are more commonly used in research than in clinical practice. The criteria is based on pain symptoms, particularly abdominal pain.2 The Rome III criteria can be used to determine IBS subtype.3
The Manning criteria is based on 6 of the primary symptoms of IBS and is more widely used for clinical diagnosis. Unlike the Rome III criteria, the Manning criteria does not provide a method to differentiate diarrhea-predominant IBS from constipation-predominant IBS.4
Some tests may be used to ensure a more accurate diagnosis. For example, the Bristol Stool Scale may be used to describe stool consistency. Stool consistency helps to differentiate between constipation and diarrhea, which helps to determine the best treatment option and monitor treatment response.2
A complete blood count, serum laboratory tests, stool analysis for parasites, and abdominal imaging will likely yield minimal useful diagnostic information and are not typically recommended for routine diagnostic evaluation.2 However, these tests may be ordered if supported by a person’s medical history and any “red flag” symptoms.3 Test results also may be used to rule out some diseases that have similar symptoms.
Lactose intolerance is best tested using the lactose hydrogen breath test. However, lactose intolerance is not a clear indicator of IBS.3 Another type of breath test is the routine hydrogen breath test, which is not typically recommended for analysis, because of the conflicting evidence regarding the association between IBS and small intestinal bacterial overgrowth.2 Therefore, results from these two types of breath tests may not conclusively show that a person has IBS, but it may be used to show that the symptoms are not due to lactose intolerance or small intestinal bacterial overgrowth.
There is limited evidence to suggest the usefulness of radiological imaging in patients with IBS-like symptoms.3 Additional tests, such as a colonoscopy with biopsy may be used if certain features are present that warrant such an invasive test.2