What is the best course of action for a 15-year-old male with a history of GI issues, presenting with dizziness, nausea, diarrhea, and allergy symptoms, who has been diagnosed with gastritis and is suspected to have IBS (Irritable Bowel Syndrome), and is currently being treated with ondansetron (ondansetron) for nausea?

Management Recommendations for This 15-Year-Old with Prolonged GI Symptoms

Immediate Priority: Address the Upper Respiratory Symptoms

This patient requires treatment for his purulent nasal drainage and nasal congestion, which has been present for one week and may represent acute bacterial rhinosinusitis. 1 The combination of purulent nasal drainage lasting more than 7 days warrants consideration of antibiotic therapy if symptoms are not improving or are worsening.

Critical Reassessment of Ondansetron Use

Ondansetron should be discontinued or used only intermittently for acute vomiting episodes, not as ongoing therapy for this patient's chronic diarrhea. 1 Here's why:

• Ondansetron is indicated for acute gastroenteritis-associated vomiting in children >4 years to facilitate oral rehydration, but it may paradoxically increase stool volume and worsen diarrhea 1
• The IDSA guidelines explicitly state that ondansetron can increase diarrhea as a side effect 1
• This patient's primary problem is now chronic diarrhea (3 weeks), not acute vomiting, making ondansetron counterproductive 1
• Antimotility drugs like loperamide are contraindicated in children <18 years with acute diarrhea 1, so ondansetron's diarrhea-promoting effect is particularly problematic here

Essential Diagnostic Workup Still Needed

Complete the stool analysis that was ordered and add the following tests:

• Stool culture, ova and parasites, and Giardia antigen to rule out infectious causes given the 3-week duration 1
• Fecal calprotectin to screen for inflammatory bowel disease, particularly important given his age and prolonged symptoms 2
• Celiac serology (tissue transglutaminase IgA with total IgA) as celiac disease can present with chronic diarrhea and abdominal pain 2
• Complete blood count, C-reactive protein or ESR to assess for inflammation or anemia 2

Dietary Intervention Must Be Enforced

The patient must completely eliminate school cafeteria food AND noodles immediately. 1 The fact that he continues eating noodles despite counseling is likely perpetuating his symptoms:

• Wheat is one of the most common food triggers in functional GI disorders 1
• His symptoms are clearly food-triggered (exacerbated by noodles and spicy foods) 1
• Implement a strict elimination diet avoiding wheat, dairy, spicy foods, greasy foods, and all school cafeteria meals 1
• Consider referral to a dietitian for structured dietary guidance, as this significantly improves adherence 3

Address the Dizziness and Blood Pressure

The dizziness is most likely orthostatic hypotension from dehydration secondary to chronic diarrhea and vomiting. 1 His blood pressure readings (133/77 and 128/74) are actually at the upper end of normal for a 15-year-old, making primary hypertension unlikely:

• Ensure aggressive oral rehydration with electrolyte-containing solutions 1
• Monitor for signs of dehydration: decreased urine output, dry mucous membranes, prolonged capillary refill 1
• Recheck orthostatic vital signs (lying, sitting, standing blood pressures and heart rates) 1

If IBS-D is Confirmed After Excluding Organic Disease

Only after completing the above workup and ruling out infectious, inflammatory, and celiac causes should you proceed with IBS-D management:

First-Line Treatment for IBS-D in Adolescents:

• Dietary modification with elimination of wheat, dairy, and trigger foods 3, 2
• Regular physical exercise as foundational therapy 3
• Soluble fiber (psyllium) 3-4 g/day, gradually increased for global symptoms, though this may worsen symptoms if he has IBS-D 3
• 12-week trial of probiotics (no specific strain recommended) 3, 2

Second-Line Treatment (if symptoms persist after 3 months):

• Low-dose tricyclic antidepressant (amitriptyline 10 mg at bedtime) is the most effective second-line treatment for adolescents with IBS-D, with careful explanation that this is for gut-brain neuromodulation, not depression 3, 2
• Ondansetron 4 mg once daily, titrated to maximum 8 mg three times daily can be considered for refractory IBS-D in adolescents, but only after other measures have failed and with close monitoring for constipation 3, 2, 4, 5

Critical Pitfalls to Avoid

• Do not continue ondansetron as chronic therapy for diarrhea - it worsens diarrhea and is only indicated for acute vomiting 1
• Do not give loperamide - it is contraindicated in patients <18 years with acute diarrhea 1
• Do not pursue colonoscopy unless alarm features develop (blood in stool, unintentional weight loss ≥5%, fever, anemia) or stool studies reveal inflammatory markers 3
• Do not allow continued consumption of known trigger foods (noodles, school cafeteria food) - this undermines all other interventions 1

Monitoring and Follow-Up

• Recheck in 1 week after treating sinusitis, discontinuing ondansetron, enforcing dietary elimination, and ensuring adequate hydration 1
• Review stool studies and bloodwork when available 2
• Reassess symptoms after 3 months of dietary modification and first-line interventions before escalating to pharmacological therapy 3
• Consider gastroenterology referral if symptoms persist despite these measures or if alarm features develop 1