What is the initial approach in primary care for a 36-year-old female with postprandial (after eating) nausea, bloating, facial erythema (red face), and pruritus (itchy neck/face)?

Suspect Food Allergy with Possible Histamine-Mediated Reaction

This constellation of postprandial nausea, bloating, facial flushing (red face), and pruritus (itchy neck/face) strongly suggests an IgE-mediated food allergy or histamine-related reaction that requires immediate evaluation with detailed food history, skin prick testing or specific IgE testing, and consideration of emergency epinephrine prescription. 1

Initial Clinical Assessment

Key Historical Features to Elicit

• Timing and consistency: Document the exact time interval between eating and symptom onset (IgE-mediated reactions typically occur within minutes to 2 hours), whether symptoms occur with specific foods or all meals, and if there's a pattern of symptom-free intervals 1
• Food triggers: Identify any newly introduced foods, common allergens (shellfish, tree nuts, peanuts, fish, eggs, milk, soy, wheat), and whether symptoms occur with restaurant meals versus home-cooked food 1
• Severity progression: Ask specifically about any respiratory symptoms (throat tightness, wheezing, shortness of breath), cardiovascular symptoms (dizziness, syncope, chest pain), or gastrointestinal symptoms beyond nausea (vomiting, diarrhea, cramping) that would indicate anaphylaxis 1
• Cannabis use history: Explicitly ask about marijuana use, as cannabinoid hyperemesis syndrome can present with postprandial nausea and requires 6 months abstinence or 3 typical cycles for diagnosis 2

Physical Examination Priorities

• Vital signs with orthostatics: Check for tachycardia (typical in anaphylaxis) versus bradycardia (suggests vasovagal reaction), hypotension, and orthostatic changes that might indicate postprandial hypotension 1, 3
• Cutaneous findings: Look for urticaria, angioedema, persistent flushing pattern (sunburn-like flush suggests scombroid poisoning from histamine in spoiled fish, while urticaria with pruritus suggests IgE-mediated allergy) 1
• Respiratory and cardiovascular: Auscultate for wheezing or stridor, assess for signs of respiratory distress, and evaluate for signs of cardiovascular compromise 1

Differential Diagnosis Framework

Primary Consideration: IgE-Mediated Food Allergy

• The combination of facial erythema and pruritus occurring postprandially is highly characteristic of IgE-mediated food allergy, which represents the majority of food allergies and requires immediate recognition 1
• Nausea and bloating can be part of the gastrointestinal manifestations of systemic allergic reactions 1

Alternative Diagnoses to Exclude

• Scombroid fish poisoning: Consider if symptoms followed fish consumption, as histamine from bacterial decomposition causes identical symptoms to anaphylaxis but with more sunburn-like flush, normal serum tryptase, and potential for multiple affected individuals 1
• Flushing syndromes: Rule out carcinoid syndrome, pheochromocytoma, postmenopausal flushing, medication-induced flushing (niacin, nicotine, ACE inhibitors, alcohol), and red man syndrome from vancomycin 1
• Functional dyspepsia with postprandial distress syndrome: Consider if symptoms are limited to nausea and bloating without cutaneous manifestations, though the presence of facial flushing and pruritus makes this less likely 3, 4
• Vasovagal reaction: Distinguished by pallor, bradycardia, and absence of urticaria/pruritus, though can have nausea and diaphoresis 1

Immediate Diagnostic Testing

First-Line Laboratory Studies

• Skin prick testing (SPT) or serum specific IgE: The 2010 NIAID guidelines recommend SPT or specific IgE as first-line diagnostic tests for suspected IgE-mediated food allergy, directed by the clinical history to test relevant food allergens 1
• Timing consideration: If the patient presents within 1-2 hours of a reaction, obtain serum tryptase (peaks 60-90 minutes after anaphylaxis onset and persists to 6 hours), though this is more relevant for emergency department evaluation 1
• Basic metabolic panel, CBC, liver function tests: Exclude metabolic causes of nausea and assess for complications 2, 4

Additional Testing Based on Clinical Suspicion

• 24-hour urinary histamine metabolites: If the patient presents more than 2 hours after symptom onset, as urinary histamine remains elevated up to 24 hours 1
• Component-resolved diagnostics: Consider measuring IgE to specific food allergen components when SPT or specific IgE results are in the intermediate range to better profile the patient 1
• H. pylori testing: Obtain stool antigen or urea breath test, as eradication resolves symptoms in a subset of patients with dyspeptic symptoms 3, 4

When to Perform Upper Endoscopy

• Upper endoscopy is not immediately indicated for suspected food allergy with cutaneous manifestations, but should be performed if the patient is ≥60 years old with new-onset dyspeptic symptoms, has alarm features (unintentional weight loss >10%, progressive dysphagia, GI bleeding), or if symptoms persist despite allergen avoidance and empiric treatment 3, 4

Immediate Management and Safety Planning

Emergency Preparedness

• Prescribe epinephrine auto-injector: For patients >25 kg, prescribe 0.3 mg epinephrine auto-injector; for 10-25 kg, prescribe 0.15 mg auto-injector, with instructions to administer intramuscularly to the anterior-lateral thigh at the first sign of systemic symptoms 1
• Anaphylaxis action plan: Provide written instructions on recognizing early signs of anaphylaxis (throat tightness, difficulty breathing, dizziness, widespread hives) and when to use epinephrine followed by immediate 911 call 1
• Medical identification: Recommend medical alert jewelry or wallet card documenting the food allergy 1

Adjunctive Pharmacotherapy

• H1 antihistamine: Prescribe diphenhydramine 25-50 mg every 6 hours as needed for mild cutaneous symptoms (flushing, pruritus, isolated urticaria), or consider a non-sedating second-generation antihistamine for daily use 1
• H2 antihistamine: Consider ranitidine (or famotidine given ranitidine's market withdrawal) twice daily if symptoms are frequent 1
• Proton pump inhibitor trial: If nausea and bloating are prominent and H. pylori testing is pending, initiate high-dose PPI (omeprazole 20-40 mg daily) for 4-8 weeks, as PPIs are first-line for ulcer-like dyspepsia with 80-90% healing rates 3

Allergen Avoidance and Dietary Management

• Strict elimination diet: Once specific food allergens are identified through testing, implement complete avoidance of the offending food(s), with careful label reading and awareness of cross-contamination risks 1
• Dietary counseling: Refer to a registered dietitian experienced in food allergies to ensure nutritional adequacy while avoiding allergens, as excessive restriction risks malnutrition 4
• Food diary: Have the patient maintain a detailed food and symptom diary to identify patterns and potential triggers if initial testing is negative 1

Specialist Referral Criteria

• Allergist/immunologist referral: Refer for confirmatory testing (SPT, specific IgE, component-resolved diagnostics), consideration of oral food challenge if diagnosis is uncertain, and development of a comprehensive management plan 1
• Gastroenterologist referral: Consider if symptoms persist despite allergen avoidance, if upper endoscopy is indicated, or if gastroparesis or functional dyspepsia is suspected (gastric emptying scintigraphy may be needed) 1, 4

Follow-Up and Monitoring

• Short-term follow-up (1-2 weeks): Review food diary, assess response to allergen avoidance and medications, ensure proper epinephrine auto-injector technique, and review laboratory results 1
• Long-term management: Annual reassessment of food allergy status, as some food allergies (particularly to milk, egg, soy, wheat) may resolve over time, though adult-onset food allergies typically persist 1

Critical Pitfalls to Avoid

• Delaying epinephrine prescription: Even if the initial reaction was mild, prescribe epinephrine immediately, as subsequent reactions can be more severe and unpredictable 1
• Relying solely on antihistamines: Antihistamines are adjunctive only; epinephrine is the first-line treatment for anaphylaxis and should never be delayed in favor of antihistamines if systemic symptoms develop 1
• Missing scombroid poisoning: If symptoms occurred after fish consumption, consider scombroid poisoning, which requires no long-term allergen avoidance but does require education about proper fish storage 1
• Overlooking cannabinoid hyperemesis syndrome: Failure to ask about cannabis use can lead to extensive unnecessary testing, as CHS requires only cessation of cannabis for diagnosis and treatment 2