assessment of allergy allergy testing presented by sylvie daigle, rn, bsc université de montréal
TRANSCRIPT
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