Intestinal microbiota. Research evaluating the role of altered intestinal microbiota in IBS has yielded mixed results. A meta-analysis of 777 IBS patients showed an increase in Firmicutes spp, a decrease in Bacteroidetes spp, and an increase in the ratio of Firmicutes spp to Bacteroidetes spp in subjects’ fecal specimens.18 Another study, of 1340 patients, found no difference in Bacteroides spp and Enterococcus spp between healthy controls and IBS patients, but did find (1) lower fecal counts of Lactobacillus spp and Bifidobacterium spp and (2) higher fecal counts of Escherichia coli and Enterobacter spp in IBS patients.19
Postinfectious IBS.The Rome Foundation introduced the diagnosis of postinfectious IBS (PI-IBS) in 2019. PI-IBS develops in 10% of patients who have had infectious enteritis. Female gender, younger age, psychological distress during or before the enteritis episode, and severity of the acute episode are risk factors for this IBS variant.20 A study of 21,421 enteritis patients found that 42% with protozoal or parasitic infection and 14% with bacterial infection developed IBS.
To identify possible "red flags" or other disorders, take a history that includes the course of symptoms, triggers, and alleviating factors.
Patients with nonviral enteritis often have a more severe course of enteritis, typically requiring antibiotics. It is believed that the resulting irregularities in the intestinal microbiota make these patients more likely to develop PI-IBS.21 PI-IBS patients are likely to improve or fully recover over time. Symptoms of PI-IBS are managed in a manner similar to how non-PI-IBS patients are managed.20
Challenges in making the IBS diagnosis
Historically, the diagnosis of IBS has been made clinically after excluding red flags (ie, signs or symptoms that might reflect other underlying medical problems) in the clinical presentation. For this reason, obtain a thorough clinical history that includes the course of symptoms, triggers, and alleviating factors. Any of the following are considered red flags1,22,23:
age > 50 years at onset of symptoms
new-onset constipation in the elderly
rectal bleeding
unexplained weight loss or anemia
family history of organic gastrointestinal disease
palpable abdominal or rectal mass
nocturnal symptoms.
New studies demonstrate that several inflammatory markers can help exclude inflammatory bowel disease from the differential diagnosis in patients in whom IBS is suspected and being investigated.24 In 2019, the American Gastroenterological Association published a clinical practice guideline updating the laboratory evaluation of functional diarrhea and IBS-D in adults,25 and made several recommendations:
Obtain the level of fecal calprotectin (normal level, ≤ 50 mcg/g) or fecal lactoferrin (≤ 4.0-7.25 mcg/g); if these tests are not available or results are not accessible, the C-reactive protein level is a reasonable option.
Do not routinely use the erythrocyte sedimentation rate or C-reactive protein level to screen for inflammatory bowel disease.
Test for Giardia lamblia with an antigen or polymerase chain reaction test.
Do not test for ova and parasites (other than Giardia) in patients who do not have a history of travel or who have not emigrated from a high-risk area recently.
Obtain testing for celiac disease with immunoglobulin A (IgA) tissue transglutaminase and with a second test, of immunoglobulin G (IgG) tissue transglutaminase and IgG or IgA deaminated gliadin peptides, to detect celiac disease in IgA-deficient patients.
Order testing for bile-acid diarrhea with selenium homotaurocholic acid nuclear medicine scanning (if available in your region; the test is available in Europe); measurement of bile acid from a 48-hour stool collection; or an assay of fibroblast growth factor 19, which measures defective feedback of bile-acid synthesis. If these tests are unavailable, consider an empiric trial of a bile-acid binder.
