Due to the absence of pathognomonic biological or physical markers, diagnosis relies on symptom-based criteria and the exclusion of identifiable mechanical, biochemical, or overt inflammatory conditions. To this end, a detailed clinical history and careful physical examination are essential. The chronicity of a disordered bowel habit and abdominal pain/discomfort in the absence of other diseases is indicative of IBS. Patients suffering of IBS with diarrhoea predominance will have more frequent diarrhoeal symptoms than constipation-related symptoms, and it is the relative frequency of diarrhoeal versus constipation-related symptoms that allows categorisation of patients into those suffering from IBS-D or IBS-C.

In 1978 the Manning criteria of IBS were published and they were and still are used to diagnose IBS:

If the first four symptoms are present, it is highly likely that the patient suffers from IBS. The last two symptoms also often occur in IBS patients, but are not as distinctive for IBS as the first four.

The manning criteria

1. visible abdominal distension
2. relief of pain with/after bowel movement
3. more frequent bowel movements with the onset of pain
4. loose stools at onset of pain
5. passage of mucus via the rectum
6. feeling of incomplete evacuation

In 1988 an international consensus meeting was held by gastroenterologists in Rome and they further specified the criteria for IBS. These criteria were revised in 1999 and they now form the standard for the IBS diagnosis:

The Rome II criteria for IBS (Irritable Bowel Syndrome)

At least twelve weeks or more, which need not be consecutive, in the preceding 12 months, of abdominal discomfort or pain, that has two out of the three features:

1. relieved with defecation and/or
2. onset associated with a change in frequency of stool and/or
3. onset associated with a change in form(appearance)of stool

Symptoms that cumulatively support the diagnosis of Irritable Bowel Syndrome:

• abnormal stool frequency (> 3/day or <3/week)
• *abnormal stool form (lumpy/hard or loose/watery)
• abnormal stool passage (straining, urgency, or feeling of incomplete evacuation)
• passage of mucus
• bloating or feeling of abdominal distension

*Bristol stool form scale (BSF-Scale) is a diagnostic aid to classify the stool form into 7 groups. Because the form of the stool depends on the time it spends in the colon, there is a correlation between the colonic transit time and the BSF type.

Additional laboratory investigations of blood and stools, and colonic visualization examinations may be useful; however, clinical studies do not generally support a role for these tests without supportive clinical features being present. (Drossman, Camilleri et al. 2002; Olden 2003) The presence of so-called alarm signs (Table 1) should prompt a more comprehensive evaluation of the patient.

Table 1. Alarm signs that should prompt a comprehensive clinical investigation to rule out other diagnoses.

Differentiating IBS from other gastrointestinal disorders

Certain diseases or conditions of the gastrointestinal tract have symptoms that may be mistaken for IBS with diarrhoea predominance due to the similarity of presenting symptoms. Among these are celiac disease, inflammatory bowel disorders, and faecal incontinence.

Celiac disease, also known as celia sprue or gluten-sensitive enteropathy, may present with symptoms similar to those of IBS with diarrhoea predominance, namely diarrhoea, abdominal cramps, improvement of symptoms with defecation, and abdominal distension. In contrast to IBS, patients with celiac disease improve when treated with a gluten-free diet and relapses when gluten is reintroduced. Non-invasive serologic tests followed by biopsy of the small intestine allow clear differentiation between IBS and celiac disease.(Ciclitira and Ellis 2003)

Inflammatory bowel disease (Crohn’s disease and ulcerative colitis) is characterized by chronically recurring symptoms of abdominal pain, discomfort (urgency and bloating) and alterations in bowel habits. Classic signs usually associated with the inflammatory process within the gastrointestinal tract, apart from diarrhoea, are rectal bleeding, fever and weight loss – the presence of these symptoms in particular should raise suspicions regarding a diagnosis of IBS. Histopathological examination of the gastrointestinal mucosa will reveal inflammation and ulcerations in the small and/or large intestine in patients with inflammatory bowel disease.(Stenson and Korzenik 2003)

Constipation is a common condition that may exacerbate faecal incontinence. Left untreated, chronic constipation may lead to faecal impaction with overflow incontinence. Such patients must be differentiated from IBS with diarrhoea predominance, as they require treatment directed at constipation as opposed to diarrhoea.(Scarlett 2004)