Assessment of AllergyAssessment of Allergy

Allergy testing

Presented by Sylvie Daigle, RN, BSc

Université de Montréal

Assessment of AllergyAssessment of Allergy

The term "Allergy"

Allergic reaction

Assessment of atopy

Skin or immunological testing

What is Allergy?What is Allergy?

Also known asHypersensitivityDisease

Definition

⇨ The term allergy ( von Pirquet -1906),

can be summarized as the acquired,

specific, altered capacity to react.

⇨ From Greek words: allos "change, altered"

+ ergon "reaction, reactivity".

⇨ Acquired means prior adequate

antigenic or allergenic exposure.

Allergy has increasedAllergy has increased

Incidence of allergy has doubled in the last 20-30 years, why?

Less exposure to parasitic disease?

Lower rate of breast-feeding ?

Exposures to air pollution?

Exposure to allergens in town vs in the country

The "hygiene hypothesis"?

Classification of allergic reactionby Gell & Coombs *

Type I Anaphylaxis (IgE) Atopic diseases(immediate)Type II Cytotoxic Autoimmune hemolytic

anemia

Type III Immune complex Farmer’s lung(IgG)

Type IV Delayed allergy Skin reaction to tuberculin

First published in 1968 : «Clinical aspects of immunology

Assessment of Atopy

• Clinical essential for asthma management⇨

(in particular if pets at home, in relation to the

pollen seasons, etc.)

• Epidemiologic studies

• Occupational investigation

Type I Hypersensitivity Detection

Skin Prick Testing, recommended to assess atopic status

RAST (ELISA), serum specific antibodies Intradermal Skin Testing: more sensitive

than prick testing but less specific, with risk of anaphylactic reaction; also, difficulty of interpretation (local trauma due to injection)

Skin Prick TestWidespreaded in the 1970s after its

modification by J. Pepys Advantages Mechanisms Technique Interpretation Factors affecting skin test

Relevant allergens (ubiquitous, occupational)

House dust mite Ragweed, tree pollen Pets Cockroaches Molds Occupational protein

allergens

Occupational protein allergens

Many occupational agents cause asthma by sensitizationmostly high-molecular-weight proteinssome low-molecular-weight agentsIn the case of high-molecular-weight allergens , skin prick tests are the preferreddiagnostic correlates of Ig-E sensitization

Advantages

• Skin prick testing is cheap, rapid and accurate

• High degree of specificity

• Safe and painless

• Wide range of allergens

• Objective evidence of sensitization

Technique and reactionTechnique and reaction

• Introduction of allergen extract into the dermis

• Ig-E-mediated response

• Allergen-induced wheal-and-flare reaction

Technique

Use the inner forearm Mark the area to be tested (2 cm apart) Place a drop of each allergen extract on each mark Prick the skin through the drop Use a new lancet/needle for each allergen Negative (saline solution) and positive control

(histamine phosphate, 10 mg/ml) must be included: to exclude false positive reactions (dermographism) and false negative reactions(intake of antihistamines)

Put drops of allergen Prick the skin through Put drops of allergen Prick the skin through extracts on the skinextracts on the skin the drop the drop

INTERPRETATIONINTERPRETATION

Read at their peak (15-20 minutes) Measure with a millimeter rule Largest + smallest of wheal and erythema

2 The wheal is principally used (diameter) What if the negative control is positive? What if the positive control is negative? The size of the wheal does not relate to the severity

of symptoms

Common errors in prick testing

Tests too close together (< 2 cm) Induction of bleeding, leading possibly to

false-positive results Insufficient penetration of skin by lancet

leading to false-negative Spreading of allergen solutions during the tests.

Causes of false-positive skin prick tests• Irritant reaction• Dermographism• Contamination of an allergen extract• Enhancement from a nearby strong reaction

Causes of false-negative skin prick tests• Extract of diminished potency • Medications modulating allergic reaction• Diseases attenuating the skin response, e.g.

eczema • Improper technique (no or weak puncture)

Factors affecting skin test results

Quality of the allergen extract (standardized) Area of the body, wrist least reactive Age, less reactive after 50 Circadian rhythms do not affect the skin reaction Drugs: short acting antihistamines inhibit the

wheal-and-flare reaction for up to 24 h; long-acting antihistamines may affect reaction for up4-5 days.

CONCLUSIONS

• When properly performed, skin tests represent one of the major tools for diagnosis of Ig-E-mediated diseases.• Assessment of the atopic status of subjects is often included in epidemiological studies of asthma and occupational asthma because atopy is a risk factor.

Predictive value of specific skin reactivity for W-R symptoms

W-R symptoms

SkinNasalOcularNasal and/or ocularRespiratory

before

211814179

same time

221716194

PPV

28%30%21%30%9%

Skin reactivity

PPV of W-R RC symptoms for probable OA : 11.4%

Natural history of sensitization, symptoms and diseases in apprentices exposed to laboratory animalsD Gautrin, H Ghezzo, CInfante-Rivard, J-L Malo. Eur Respir J, 2001.

referencesreferences

- Bernstein IL, Chan-Yeung M, Malo JL, Bernstein DI. Asthma in the Workplace. Francis & Taylor, 2006

- Middleton’s Allergy: Prinnciples and practice vol. 1,chap 38.- Pepys, J. Clinical allergy, 1973, pp 491-509.- Pepys, J. Atopy: a study in definition. Allergy 1994;49: 397-399 - Bernstein DI and al.Characterization of skin prick testing responses J Allergy Clin Immunol 1994; 49:498-507

Web sites of interest- www.asthma-workplace.com- www.asthme.csst.qc.ca/document/Info_Gen/AgenProf/- www.remcomp.com/asmanet/asmapro/index.htm