The American Gastroenterological Association position statement on IBS states that the diagnosis is based on identifying positive symptoms (e.g., Rome criteria) consistent with the condition while excluding other conditions with similar clinical presentations in a cost-effective manner. Spiller and colleagues formulated an algorithm for the evaluation of patients with abdominal pain and deranged bowel habits. This allows the clinician to evaluate for potential alarm symptoms via diagnostic testing and, in the absence of significant findings, to classify the patient into one of the subtypes of IBS (Fig. 16.1).

A careful history and physical should be taken. One should obtain information about the character and frequency of bowel movements, presence and characteristics of abdominal discomfort, duration of symptoms, and inquiry into alarm symptoms. In patients complaining of “diarrhea,” one should ask about fecal incontinence as often patients complain of diarrhea when they are actually describing symptoms of incontinence. In patients with complaints of “constipation,” one should ascertain whether it is an infrequent urge to defecate or whether it is more of difficulty with evacuation of stool which suggests pelvic floor abnormalities such as rectal prolapse or dyssynergic defecation. Important considerations in the differential diagnosis according to bowel habit are outlined in Table 16.2.

It is also important to obtain a social and psychosocial history, as there is a well-known association between IBS and a history of abuse and psychiatric illness. Furthermore, it is important to understand how the symptoms are affecting the patient’s quality of life. As mentioned in the case study, IBS symptoms were affecting both the patient’s functioning at work and her ability to participate in social activities. The physical exam should include a rectal exam to evaluate the perianal area and assess for stool character, rectal tone, presence of blood, and evidence of dyssynergia.

In patients who demonstrate “red flag” alarm symptoms (see Table 16.3), further investigations such as upper endoscopy and colonoscopy, a complete blood count, thyroid-stimulating hormone (TSH), inflammatory markers such as C-reactive protein, celiac serologies (serum IgA and tissue transglutaminase (tTG) antibody), and stool studies to rule out infection, such as Clostridium difficile and Giardia, should be considered on a case-by-case basis. Table 16.4 outlines the American College of Gastroenterology diagnostic recommendations for IBS. Interestingly, a recent study of 200 patients seen in an academic medical facility who met IBS criteria revealed that 70 % of patients endorsed a red flag symptom. It was found that in patients endorsing alarm symptoms, many were not tested further, and, in those who were, the yield of testing was low. This study suggests that in patients who meet criteria for IBS, further testing, even in the presence of alarm symptoms, may be of low yield.