Transcript Assessment of Allergy

Assessment of Allergy
Allergy testing
Presented by
Sylvie Daigle, RN, BSc
Université
de Montréal
Assessment of Allergy
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The term "Allergy"
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Allergic reaction
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Assessment of atopy
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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 20-30 years, why?
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Less exposure to parasitic disease?
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Lower rate of breast-feeding ?
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Exposures to air pollution?
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Exposure to allergens in town vs in the country
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The "hygiene hypothesis"?
Classification of allergic reaction
by Gell & Coombs *
Type I
Anaphylaxis (IgE)
(immediate)
Type II
Cytotoxic
Atopic diseases
Type III
Immune complex
(IgG)
Farmer’s lung
Type IV
Delayed allergy
Skin reaction to tuberculin
Autoimmune hemolytic
anemia
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
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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
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Relevant allergens (ubiquitous,
occupational)
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House dust mite
Ragweed, tree pollen
Pets
Cockroaches
Molds
Occupational protein
allergens
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 whealand-flare reaction
Technique
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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
extracts on the skin
Prick the skin through
the drop
INTERPRETATION
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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.
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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
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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; longacting 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.
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.
Predictive value of specific skin reactivity for W-R symptoms
Skin reactivity
W-R symptoms
Skin
Nasal
Ocular
Nasal and/or ocular
Respiratory
before
same time
PPV
21
18
14
17
9
22
17
16
19
4
28%
30%
21%
30%
9%
PPV of W-R RC symptoms for probable OA : 11.4%
references
- 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