Diagnosing Allergy: A Short Review

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

Relation between age and allergies occurring for first time

Atopic dermatitis, food allergies

Early childhood

Allergic rhinitis

Early adulthood
Urticaria, angioedema

Allergy to insect venom; nasal polyps

Figure 1. Relation between age and allergies onset.

Taking a history: main points

Previous allergies
Seasonality of symptoms
Triggering factors
Life threatening events
Intolerance to aspirin
Family history
Environmental risk: home, work, and outdoor
Pets and lab animals
Any current treatment: antihistamines, topical and oral corticosteroids, EpiPen

Cross reaction can occur between allergens, for example, between birch tree pollen and some foods (apple, carrot, celery, potato, orange, tomato, hazelnut, and peanut). Another example is thecrossreactivity between latex and some fruits (banana, avocado and kiwi).

If both parents have a history of allergies, the risk for development of allergy in the offspring is 75%; if one parent is allergic, the risk is 50%.

Allergy tests

Figure 2. Allergy tests.

Skin prick test

The most widely used allergy test, performed during the initial consultation.

Allergen solutions: PONI F

Positive control (histamine)
Occupational allergens (ammonium persulphate, platinum salts, antibiotics, and latex)
Negative control (saline)
Inhaled allergens (house dust mite (HDM), grass pollen, cat dander, dog hair), TGR MI
Food allergens

See BMJ figure: Skin prick test kit.

How is the the skin prick test done?
1. A drop of allergen is placed on the skin of the forearm
2. A sterile 25 G (orange) needle is used to prick the skin through the allergen solution (a separate needle for each allergen)
3. Reaction is evaluated in 15 minutes.

A positive reaction is a skin weal 2 mm greater than that observed with the negative control (diluent).

See BMJ figure: skin prick test results.

Disadvantages of the skin prick test: FISH
- Food allergens (less standardized) are less reliable inhaled allergens
- Itching causes a discomfort
- Skin conditions (eczema or dermatographism) may interfere with interpretation
- Histamine antagonists (antihistamines) may suppress skin reaction

Patch test

Used for diagnosis of contact dermatitis

Food challenge

Algorithm for diagnosing food allergy: SAD F

1. Symptoms
2. Allergy testing
3. Diagnostic diet
4. Food challenge

Nasal smear

Nasal eosinophilia means that more than 10% of the stained cells in nasal smear are eosinophils.

RAST (radioallergosorbent test) or ImmunoCAP (not a RAST)

RAST measures concentration of allergen specific IgE. See BMJ figure: The principle of the RAST.

RAST and ImmunoCAP advantages: MSSS

- Medications (antihistamines) do not influence the test
- Skin conditions do not interfere with the test
- Safe
- Specificity 90% for inhaled allergens

8 top allergens account for 90 percent of food allergies. Specific IgE levels (sIgE) that predict the likelihood of passing an oral food challenge are shown in the figure. (click to enlarge the image).


Diagnosing allergy. Clinical review: ABC of allergies. BMJ 1998;316:686.
Diagnosis, treatment and prevention of allergic disease: the basics. Jo A Douglass and Robyn E O’Hehir. MJA 2006; 185 (4): 228-233.
Interactive Allergy Map by Greer Labs. Click your state to find region-specific, common airborne allergens there.

Audio and Video

Allergies. Case Notes. BBC 4.
Michael Marcus, M.D., a Pediatric Pulmonologist from the Maimonides Medical Center, discusses different aspect of allergies in this 15-minute video from ICYou.com.

Published: 07/08/2007
Updated: 02/26/2010

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