Presentation on theme: "Assessment of Allergy Allergy testing Presented by Sylvie Daigle, RN, BSc Université de Montréal."— Presentation transcript:
Assessment of Allergy Allergy testing Presented by Sylvie Daigle, RN, BSc Université de Montréal
Assessment of Allergy The term "Allergy" Allergic reaction Assessment of atopy Skin or immunological testing
What is Allergy? Also known as Hypersensitivity Disease
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 increased Incidence of allergy has doubled in the last 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 reaction by Gell & Coombs * Type IAnaphylaxis (IgE)Atopic diseases (immediate) Type IICytotoxicAutoimmune 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 Test Widespreaded 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
Many occupational agents cause asthma by sensitization mostly high-molecular-weight proteins some low-molecular-weight agents In the case of high-molecular-weight allergens, skin prick tests are the preferred diagnostic 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 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 extracts on the skin the drop
INTERPRETATION 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 up 4-5 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 Skin Nasal Ocular Nasal and/or ocular Respiratory before same time 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 animals D Gautrin, H Ghezzo, CInfante-Rivard, J-L Malo. Eur Respir J, 2001.
references - Bernstein IL, Chan-Yeung M, Malo JL, Bernstein DI. Asthma in the Workplace. Francis & Taylor, Middleton’s Allergy: Prinnciples and practice vol. 1,chap Pepys, J. Clinical allergy, 1973, pp Pepys, J. Atopy: a study in definition. Allergy 1994;49: Bernstein DI and al.Characterization of skin prick testing responses J Allergy Clin Immunol 1994; 49: Web sites of interest