What is the first step in investigating suspected allergy?

The first step is a detailed clinical history, assessing symptoms, allergen exposure, timing of reactions and differential diagnoses. Test selection should be guided by the clinical suspicion.

When should skin prick testing be performed?

Skin prick testing is generally the first-line test when available and not contraindicated. It allows identification of sensitisation to inhalant, food or other relevant allergens.

When should skin prick testing be avoided?

It should be avoided when the patient needs to continue antihistamines, has extensive dermatosis, significant dermographism or there is a high risk of systemic reaction to the tested allergen.

What are Phadiatop and Fx5 panels used for?

Phadiatop is used to screen for sensitisation to inhalant allergens and Fx5 to screen for food allergy. A positive result guides the subsequent selection of targeted specific IgE tests.

What is the role of specific IgE testing?

Specific IgE tests help confirm sensitisation to individual allergens, guide avoidance measures, support therapeutic decisions and select candidates for immunotherapy.

Does a positive test necessarily confirm clinical allergy?

No. A positive test demonstrates immunological sensitisation, but the diagnosis of allergy requires correlation with the clinical history and exposure to the relevant allergen.

When should referral to Allergy and Clinical Immunology be considered?

Referral should be considered in cases of anaphylaxis, complex food allergy, suspected drug allergy, consideration of immunotherapy, hereditary angioedema, diagnostic uncertainty or need for specialist investigation.

Allergy screening: diagnostic approach

Q: Should total IgE be requested routinely?

No. Total IgE has limited value in routine screening for respiratory or food allergy and should be reserved for specific situations such as allergic bronchopulmonary aspergillosis, omalizumab dosing or hyper-IgE syndromes.

Allergic manifestations are a frequent cause of respiratory, cutaneous 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, either individually or in panels, are used to confirm sensitisation to inhalant or food allergens.
This algorithm proposes a structured approach to allergy screening, helping to identify patients who may benefit from further investigation, avoidance measures and, when indicated, immunotherapy. Appropriate test selection helps optimise resources and avoid unnecessary requests.

Podcast transcript

Initial assessment

When the clinical history suggests allergic disease, structured screening should be initiated. Consider respiratory/ocular symptoms (asthma, rhinitis, conjunctivitis), food-related manifestations (abdominal pain, vomiting, diarrhoea, food refusal, faltering growth, eczema), reactions to medicines, latex and Hymenoptera venom. Confirm the temporal relationship with exposure and consider differential diagnoses.

Skin prick testing

If available and not contraindicated, skin prick testing is the first step. A positive result, when consistent with the clinical history, confirms sensitisation and guides avoidance measures and possible immunotherapy. If negative but clinical suspicion persists, proceed with specific IgE testing. When skin prick testing is unavailable or contraindicated, laboratory tests should be preferred.

When to avoid skin prick testing

Avoid skin prick testing when it is essential to continue antihistamines/tricyclic antidepressants, when extensive dermatosis/dermographism is present, or when there is a high risk of an anaphylactic reaction to the allergen. In these scenarios, use serum specific IgE testing.

Specific IgE panels (laboratory screening)

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

Interpretation of panels

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

Individual specific IgE — indications

Request individual specific IgE to: confirm the diagnosis when the clinical history is convincing; guide avoidance measures; assess patients unable to undergo skin prick testing; investigate negative skin prick testing with high clinical suspicion; confirm sensitisation before immunotherapy; monitor tolerance acquisition in food allergy or response to desensitisation. Select 1–2 allergens per group, adjusted to the clinical history and epidemiological context.

Total IgE — when to request

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

Conclusion

Recommended pathway: clinical history ➝ skin prick testing (if possible) ➝ IgE screening panels (Phadiatop/Fx5) ➝ individual specific IgE according to indication; reserve total IgE for specific cases. This sequence standardises diagnosis, avoids indiscriminate testing and supports decisions regarding avoidance and immunotherapy.

Clinical cases

Clinical cases — Allergy screening

Case 1 — Seasonal rhinoconjunctivitis in a child

A 5-year-old child presents with watery rhinorrhoea, sneezing and ocular itching in spring. Examination shows pale nasal mucosa. The child is not taking any medication. What is the most appropriate next step to confirm sensitisation?

Request total IgE
Request Phadiatop (inhalant panel)
Perform skin prick testing for house dust mites and pollens
Start intranasal corticosteroid therapy without investigation

See answer

c) Perform skin prick testing for house dust mites and pollens.
Rationale: When available and not contraindicated, skin prick testing is the first step in suspected respiratory allergy. Total IgE does not screen for allergy; Phadiatop is an alternative when skin prick testing is unavailable or contraindicated.

Case 2 — Suspected food allergy with limitation to skin prick testing

An adult presents with oropharyngeal itching and urticaria within minutes of peanut ingestion. They need to continue antihistamines due to a concomitant condition. What is the best first-line test?

Total IgE
Serum specific IgE to peanut
Phadiatop panel
Direct oral food challenge without previous testing

See answer

b) Serum specific IgE to peanut.
Rationale: If skin prick testing is contraindicated or unavailable, for example when antihistamines cannot be stopped, specific IgE to the suspected allergen should be requested. Inhalant panels are not useful for food allergy; total IgE is not useful in routine screening.

Case 3 — Panel interpretation and next step

A patient presents with perennial rhinitis. The inhalant panel (Phadiatop) is positive (≥0.35 PAU/L). What is the most useful next step?

Total IgE to quantify atopy
Request targeted individual specific IgE tests (e.g. Dermatophagoides, relevant pollens)
Repeat the same panel in 1 week
Establish a definitive diagnosis based only on the panel

See answer

b) Request targeted individual specific IgE tests.
Rationale: A positive panel indicates atopy and guides the selection of specific IgE tests to confirm clinically relevant sensitisation. Total IgE adds no value in routine screening.

FAQ

Frequently Asked Questions about Allergic Disease (screening and testing)

1) When should I choose skin prick testing instead of blood tests?

Whenever available and not contraindicated, skin prick testing is the first step in screening for respiratory/inhalant allergy. It confirms sensitisation when consistent with the clinical history. Blood tests, such as specific IgE or IgE panels, are alternatives when skin prick testing is unavailable or contraindicated.

2) When should I request IgE panels such as Phadiatop or Fx5?

They are used as laboratory screening when skin prick testing cannot be performed or as an initial targeted complement. Phadiatop covers inhalant allergens; Fx5 covers common food allergens. A positive panel guides the selection of individual specific IgE tests.

3) How should I interpret a positive mixed-allergen panel?

Results equal to or above approximately 0.35 PAU/L indicate atopy/sensitisation to the tested mixture. They do not confirm clinical disease on their own. The next step is to request individual specific IgE tests for the allergen(s) most likely according to the clinical history.

4) Is total IgE useful for screening?

No, not for routine screening of respiratory or food allergy. It should be reserved for specific situations, such as ABPA, defining the initial omalizumab dose or hyper-IgE syndromes.

5) When is skin prick testing contraindicated?

When the patient cannot stop antihistamines or tricyclic antidepressants, has extensive dermatosis or dermographism, or there is a high risk of reaction. In these cases, targeted serum specific IgE should be prioritised.

6) How should individual specific IgE tests be selected?

Select 1–2 allergens per group, guided by the history and exposure pattern, such as house dust mites and grass pollens in rhinitis, or peanut, milk and egg in food allergy. Avoid indiscriminate use of extensive panels.

7) Skin prick testing is negative but clinical suspicion remains strong: what should I do?

Request targeted serum specific IgE. Reconsider timing, including seasonality and age, as well as differential diagnoses. In food allergy, an oral food challenge under specialist supervision may be considered when indicated and safe.

8) Are there differences between children and adults in food allergy?

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

9) How should the patient be prepared for skin prick testing?

Antihistamines should be stopped beforehand according to their half-life, creams should be avoided on the testing area, and the patient should be clinically stable. Drugs that interfere with the test, such as tricyclic antidepressants, should be documented.

10) When should allergen-specific immunotherapy be considered?

In patients with persistent symptoms despite avoidance measures and optimised pharmacological treatment, with documented sensitisation and a clear clinical relationship with the allergen. It should be considered and prescribed by an experienced team.

References

Authorship and updates

Author(s): Filipe Cerca, MD

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17/06/2026

Algorithm for the approach to allergic disease

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Allergy screening: diagnostic approach

Initial assessment

Skin prick testing

When to avoid skin prick testing

Specific IgE panels (laboratory screening)

Interpretation of panels

Individual specific IgE — indications

Total IgE — when to request

Conclusion

Clinical cases — Allergy screening

Case 1 — Seasonal rhinoconjunctivitis in a child

Case 2 — Suspected food allergy with limitation to skin prick testing

Case 3 — Panel interpretation and next step

Frequently Asked Questions about Allergic Disease (screening and testing)

1) When should I choose skin prick testing instead of blood tests?

2) When should I request IgE panels such as Phadiatop or Fx5?

3) How should I interpret a positive mixed-allergen panel?

4) Is total IgE useful for screening?

5) When is skin prick testing contraindicated?

6) How should individual specific IgE tests be selected?

7) Skin prick testing is negative but clinical suspicion remains strong: what should I do?

8) Are there differences between children and adults in food allergy?

9) How should the patient be prepared for skin prick testing?

10) When should allergen-specific immunotherapy be considered?

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Register

Allergy screening: diagnostic approach

Initial assessment

Skin prick testing

When to avoid skin prick testing

Specific IgE panels (laboratory screening)

Interpretation of panels

Individual specific IgE — indications

Total IgE — when to request

Conclusion

Clinical cases — Allergy screening

Case 1 — Seasonal rhinoconjunctivitis in a child

Case 2 — Suspected food allergy with limitation to skin prick testing

Case 3 — Panel interpretation and next step

Frequently Asked Questions about Allergic Disease (screening and testing)

1) When should I choose skin prick testing instead of blood tests?

2) When should I request IgE panels such as Phadiatop or Fx5?

3) How should I interpret a positive mixed-allergen panel?

4) Is total IgE useful for screening?

5) When is skin prick testing contraindicated?

6) How should individual specific IgE tests be selected?

7) Skin prick testing is negative but clinical suspicion remains strong: what should I do?

8) Are there differences between children and adults in food allergy?

9) How should the patient be prepared for skin prick testing?

10) When should allergen-specific immunotherapy be considered?

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