What is the best course of action for a young adult with a history of chronic gastrointestinal symptoms, including diagnosed gastritis and suspected Irritable Bowel Syndrome with Diarrhea (IBS-D), who presents with persistent diarrhea, normal laboratory results, including a complete celiac disease panel, and pending Giardia Enzyme Immunoassay (EIA) and ova/parasite testing?

Management of Chronic Diarrhea with Normal Celiac Panel and Pending Parasitic Testing

Await the pending Giardia EIA and ova/parasite results before proceeding with further invasive testing, as Giardia is the most common parasitic cause of chronic diarrhea and testing for it is strongly recommended in all patients with chronic diarrhea. 1

Immediate Next Steps

Complete Pending Parasitic Evaluation

• Giardia testing is a strong recommendation with high-quality evidence for all patients presenting with chronic diarrhea, as it is the most common parasitic cause (prevalence 54% among identified parasites in chronic diarrhea) 1
• Enzyme immunoassay (EIA) tests for Giardia have excellent diagnostic performance with sensitivity and specificity >95% 2
• If the initial ova/parasite examination is negative but clinical suspicion remains high (given the cafeteria food exposure and recurrent episodes), molecular testing should be pursued 1
• Giardia can cause villous atrophy mimicking celiac disease, making it particularly important to exclude in patients with chronic GI symptoms 3

Consider Empiric Treatment Based on Risk Factors

• Given the patient's chronic symptoms and potential food exposure, if parasitic testing returns negative but symptoms persist, consider empiric antiparasitic treatment with albendazole 400 mg plus ivermectin 200 μg/kg as a single dose for possible undetected geohelminth infections 4
• This approach is particularly relevant if there is any history of travel to or residence in endemic areas 4

Additional Diagnostic Testing to Consider

Fecal Inflammatory Markers

• Order fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease (IBD), as this is recommended with conditional evidence for patients with chronic diarrhea 1, 5, 6
• This is particularly important given the three-week duration of persistent diarrhea and history of gastritis 1
• Approximately 20% of patients with active Crohn's disease may have normal CRP levels, so inflammatory markers in blood are insufficient to exclude IBD 5

Bile Acid Diarrhea Testing

• Consider testing for bile acid diarrhea (BAD), as it is frequently overlooked and has a prevalence of 9.9%-35.3% in patients with IBS-D 1
• While SeHCAT scanning (the gold standard) is not available in the United States, serum 7α-hydroxy-4-cholesten-3-one (C4) or FGF19 can be measured 1, 5
• Alternatively, an empiric trial of bile acid binders (cholestyramine or colesevelam) is reasonable if testing is unavailable, with clinical response suggesting BAD as the cause 1

Small Intestinal Bacterial Overgrowth (SIBO)

• While hydrogen breath testing for SIBO is not routinely recommended for typical IBS symptoms 5, consider SIBO evaluation if symptoms persist after excluding parasitic infection, particularly given the history of post-appendectomy status 7
• One study showed that 67% (10/15) of celiac patients with persistent GI symptoms after gluten withdrawal had SIBO 7

Endoscopic Evaluation Considerations

When to Pursue Colonoscopy

• Colonoscopy is NOT indicated at this time given the patient's young age (<45 years), typical IBS-D symptoms, and absence of alarm features 1, 5
• Alarm features that would warrant colonoscopy include: rectal bleeding, unintentional weight loss, family history of colorectal cancer or IBD, iron deficiency anemia, or age >50 years 1, 6
• If fecal calprotectin is elevated, then colonoscopy with biopsies should be performed to evaluate for microscopic colitis or IBD 1
• Microscopic colitis has a prevalence of 1.5% in IBS-D patients and can only be diagnosed via colonoscopy with biopsies 1

Flexible Sigmoidoscopy Alternative

• If endoscopic evaluation becomes necessary, flexible sigmoidoscopy with biopsies may be sufficient as 99.7% of diagnoses in chronic diarrhea patients can be made from distal colon biopsies 1
• This approach is less invasive and more cost-effective for young patients without alarm features 1

Symptomatic Management While Awaiting Results

Dietary Modifications

• Implement a limited trial of a low FODMAP diet to improve global IBS symptoms, as this is recommended with strong evidence 6
• Assess for lactose intolerance, particularly if the patient consumes >0.5 pint (280 ml) of milk daily 5
• Address dietary non-compliance issues that were noted in the clinical history [@patient history]

Pharmacologic Options for IBS-D

• Consider rifaximin 550 mg three times daily for 14 days to treat global IBS-D symptoms, as this is recommended with strong evidence [@10@]
• Loperamide can be used for symptomatic control of diarrhea while awaiting diagnostic results [@10@]

Common Pitfalls to Avoid

Don't Rush to Colonoscopy

• Performing colonoscopy in young patients with typical IBS symptoms and no alarm features is not cost-effective and should be avoided [@6@]
• The diagnostic yield is low in this population, and the procedure carries unnecessary risks [@1@]

Don't Overlook Parasitic Infections

• Routine stool microscopy has poor sensitivity (only 1.4% detection rate in one study), so if initial testing is negative but suspicion remains high, pursue molecular testing or consider empiric treatment [1, @8@]
• Giardia can cause persistent symptoms for weeks to months and may recur after treatment [@3@]

Don't Ignore Bile Acid Diarrhea

• BAD is frequently overlooked in clinical practice despite being present in up to one-third of IBS-D patients 1
• If testing is unavailable, an empiric trial of bile acid binders is reasonable and may be diagnostic and therapeutic 1

Follow-Up Plan

Short-Term (1-2 Weeks)

• Review pending Giardia EIA and ova/parasite results [@2@, 1]
• If Giardia is positive, treat with tinidazole 2g as a single oral dose (efficacy 80-100%) [@7@]
• If parasitic testing is negative, proceed with fecal calprotectin testing if not already done [1, @5@]

Medium-Term (4-6 Weeks)

• Reassess symptoms after any initiated treatment [@7@]
• If symptoms persist despite negative parasitic testing and normal fecal calprotectin, consider bile acid diarrhea testing or empiric trial of bile acid binders [@3@, 1]
• Evaluate response to dietary modifications (low FODMAP diet) 6

Long-Term (3 Months)

• If all testing remains negative and symptoms persist, consider referral to gastroenterology for consideration of SIBO testing or other specialized evaluations [@13@]
• Gut-directed psychotherapy should be considered for global IBS symptom management if organic causes are excluded [@10