Allergy Screening: Diagnostic Approach

Allergic manifestations are a frequent cause of respiratory, skin, and digestive symptoms. Diagnosis should begin with a detailed clinical history, followed by the appropriate selection of skin or laboratory tests. Among the available methods, the prick test and the measurement of specific IgE or screening panels stand out, as they help confirm sensitization to inhalants or foods.
This algorithm proposes a structured approach to allergy screening, supporting the identification of patients who may benefit from further investigation, avoidance measures, and, when appropriate, immunotherapy. Proper test selection helps optimize resources and prevent unnecessary requests.

Initial assessment

Given a clinical history suggestive of allergic disease, structured screening is initiated. Consider respiratory/ocular symptoms (asthma, rhinitis, conjunctivitis), food-related manifestations (abdominal pain, vomiting, diarrhoea/diarrhea, feeding refusal, failure to thrive, eczema), and reactions to drugs, latex, and Hymenoptera venoms. Confirm the temporal relationship with exposure and consider differential diagnoses.

Skin prick testing (prick test)

If available and not contraindicated, the prick test is the first step. A positive result, when consistent with the history, confirms sensitisation and guides allergen avoidance and, where indicated, immunotherapy. If negative but clinical suspicion persists, proceed with specific IgE testing. When the prick test is unavailable or contraindicated, prefer laboratory tests.

When to avoid the prick test

Avoid when antihistamines/tricyclic antidepressants must be continued, in extensive dermatoses/dermographism, or where there is a high risk of anaphylactic reaction to the allergen. In these scenarios, use serum specific IgE.

Specific IgE panels (laboratory screening)

Phadiatop (inhalants): mixture of mites, animal dander, moulds and pollens; useful for screening respiratory atopy.
Fx5 (foods): mixture of common food allergens (e.g., egg, milk, fish, wheat, peanut, soya/soy); useful for screening food allergy. Reporting is qualitative/semi-quantitative with a usual threshold of 0.35 PAU/l.

Panel interpretation

Values above the threshold suggest atopy/sensitisation; increasing classes (1–6) reflect higher clinical likelihood. A positive panel guides selection of individual specific IgE by allergen group. A negative panel with strong clinical suspicion requires reassessment (other diagnoses, seasonal repeat testing, or an alternative method).

Individual specific IgE — indications

Order to: confirm diagnosis when history is compelling; guide avoidance measures; when the prick test cannot be performed; prick test negative with high suspicion; confirmation prior to immunotherapy; monitoring tolerance (food allergy) or response to desensitisation. Select 1–2 allergens per group, tailored to history and epidemiological context.

Total IgE — when to order

Not indicated for routine screening of respiratory or food allergy. Reserve for specific situations: assessment/follow-up of allergic bronchopulmonary aspergillosis, determination of initial omalizumab dose, and hyper-IgE syndromes.

Conclusion

Recommended pathway: clinical history ➝ prick test (if possible) ➝ screening IgE panels (Phadiatop/Fx5) ➝ individual specific IgE as indicated; reserve total IgE for particular cases. This sequence standardises diagnosis, avoids indiscriminate testing, and supports decisions on avoidance and immunotherapy.

Urticaria: diagnosis and guidance

Clinical Cases — Allergy Screening

Case 1 — Seasonal rhinoconjunctivitis in a child

5-year-old child with watery rhinorrhea, sneezing, and itchy eyes in spring. Examination: pale nasal mucosa. No current medication. What is the most appropriate next step to confirm sensitisation?

  1. Order total IgE
  2. Order Phadiatop (inhalant panel)
  3. Perform a skin prick test for mites and pollens
  4. Start intranasal corticosteroids without investigation
See answer
✅ c) Perform a skin prick test for mites and pollens.
Rationale: When available and not contraindicated, the prick test is the first step in suspected respiratory allergy. Total IgE does not screen for allergy; Phadiatop is an alternative when prick testing is unavailable/contraindicated.

Case 2 — Suspected food allergy with limitation to prick testing

Adult with oropharyngeal pruritus and urticaria minutes after peanut ingestion. Needs to continue antihistamines for a comorbid condition. What is the best first-line test?

  1. Total IgE
  2. Serum specific IgE (peanut)
  3. Phadiatop panel
  4. Direct oral food challenge without prior testing
See answer
✅ b) Serum specific IgE (peanut).
Rationale: If prick testing is contraindicated/unavailable (e.g., antihistamines cannot be stopped), order specific IgE to the suspected allergen. Inhalant panels are not for food allergy screening; total IgE is not useful for routine screening.

Case 3 — Panel interpretation and next step

Patient with perennial rhinitis. Inhalant panel (Phadiatop) positive (≥0.35 PAU/L). What is the most useful next step?

  1. Total IgE to quantify atopy
  2. Request targeted individual specific IgE (e.g., Dermatophagoides, relevant pollens)
  3. Repeat the same panel in 1 week
  4. Make a definitive diagnosis based on the panel alone
See answer
✅ b) Request targeted individual specific IgE.
Rationale: A positive panel indicates atopy and guides the selection of specific IgE to confirm clinically relevant sensitisation. Total IgE adds no value for routine screening.

Frequently Asked Questions about Allergic Disease (screening and testing)

1) When should I choose a prick test instead of lab tests?

Whenever available and not contraindicated, the prick test is the first step for screening respiratory/inhalant allergy. It confirms sensitisation when consistent with the clinical history. Laboratory tests (specific IgE/panels) are alternatives if prick testing is unavailable or contraindicated.

2) In which situations should I order IgE panels (Phadiatop/Fx5)?

Use as laboratory screening when a prick test cannot be performed or as an initial focused complement. Phadiatop covers inhalants; Fx5 covers common foods. A positive panel guides the selection of individual specific IgE.

3) How do I interpret a positive mixed panel?

Results at or above ~0.35 PAU/L indicate atopy/sensitisation to the tested mixture. They do not, by themselves, confirm disease. The next step is to order individual specific IgE to the most likely allergen(s) based on history.

4) Is total IgE useful for screening?

No for routine screening (respiratory or food). Reserve it for particular situations (e.g., ABPA, determining the initial omalizumab dose, hyper-IgE syndromes).

5) When is the prick test contraindicated?

If the patient cannot withhold antihistamines/tricyclic antidepressants, has extensive dermatosis/dermographism, or there is a high risk of reaction. In such cases, prioritise serum specific IgE.

6) How should I choose individual specific IgE tests?

Select 1–2 allergens per group, guided by history/exposure (e.g., mites and grasses in rhinitis; peanut/milk/egg in food allergy). Avoid indiscriminate “large panels”.

7) Negative prick test but strong clinical suspicion: what to do?

Order targeted serum specific IgE. Reconsider timing (seasonality, age) and differential diagnoses. In food allergy, consider supervised oral challenge when indicated and safe.

8) Differences between children and adults in food allergy?

In children, key allergens include milk, egg, fish, wheat, soy, peanut; in adults, fresh fruits, shellfish and fish are prominent. Test selection should reflect these profiles and exposure history.

9) How to prepare the patient for a prick test?

Stop antihistamines in advance (according to half-life), avoid creams/dermographism at the site, and ensure clinical stability. Document interfering drugs (e.g., tricyclic antidepressants).

10) When to consider specific immunotherapy?

In patients with persistent symptoms despite avoidance and optimised pharmacotherapy, with documented sensitisation and a clear clinical relationship to the allergen. It should be assessed and prescribed by an experienced team.

Autor: Filipe Cerca, MD