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Allergy tests
1Serials
Skin
1
Allergy tests
T. J. David Prick testing
The principle of skin tests is that the weal and flare reaction to an allergen introduced into the skin demonstrates the presence of mast-cell-fixed antibody, mainly IgE antibody.
A drop of allergen solution is placed on the skin which is then pricked with a hypodermic needle. Prick tests can also produce variable results, but the introduction of standardised precision needles for prick testing (e.g. the Morrow Brown needle) has made the method potentially more reproducible. The delegation of skin testing to untrained staff, and the continued use of unstandardised hypodermic needles leads to frequent errors and poor reproducibility.
Allergen extracts Aqueous extracts are unstable, and 50% glycerol (for prick tests) or 0.03% human serum albumin (for intradermal tests) is added to prevent allergen degradation.
Method Control solutions Because of the variability of cutaneous reactivity, it is necessary to include positive and negative controls whenever performing skin tests. The negative control solution consists of the diluent used to preserve the allergen extracts. The positive control solution usually contains histamine 10 mg/ml, and is mainly used to detect suppression of reactivity, which may last for several weeks or more with some newer Hl receptor antagonists (e.g. astemizole).
A small drop of each test extract and control solution, at least 4 cm apart, is placed on the volar aspect of the forearm or the back of the trunk. Using the Morrow Brown device, the needle is pressed through the drop of allergen extract into the skin at an angle of 90 degrees to the skin, to a standard depth of 1 mm. Other devices are passed through the drop, penetrating the skin at a 45 degree angle, and the skin is then gently lifted to create a small break in the epidermis.
Scratch testing
Timing
A drop of allergen solution is placed on the skin which is then scratched so as to superficially penetrate the skin. As well as providing non-specific weal reactions as a result of trauma, the scratch test introduces a variable amount of allergen through the skin and is therefore poorly standardised and produces results which are too variable for routine clinical use.
The skin prick test induces a response that reaches a peak in 8 to 9 min for histamine, 10 to 12 min for compound 48/80, a histamine-releasing agent, and 12 to 15 min for allergens. Occasionally reactions take up to 30 min to develop. Late reactions (induration and inflammation that begins, peaks and terminates within l-2 days) can also occur, but are not usually sought as their significance is unclear. Which is important: weal or flare?
Dr T. J. David, PLD, MD, FRCP, DCH, Senior Lecturer in Child Health, University of Manchester, Department of Child Health, Booth Hall Children’s Hospital, Charlestown Road, Blackley, Manchester M9 2AA, UK Correspondence and requests for offprints to TJD. Currenr Faediuwics (1991) I, 145-146 0 1991 Longman Group UK Ltd
Most recommend measurement of the size of the weal alone, but a few suggest that the size of the flare should also be measured. 145
146 CURRENT PAEDIATRICS Criteria of positivity There is no agreed definition about what constitutes a positive reaction. Most definitions of a positive reaction are based on the absolute diameter of the weal, with arbitrary cut-off points for positivity at 1 mm, 2 mm or 3 mm. A major difficulty is that the size of the weal depends on the potency of the extract.
Prick tests: problems with interpretation Prick tests are easier to perform than to interpret. The main problems are: Skin prick test reactivity may be present in subjects with no clinical evidence of allergy Skin prick test reactivity may persist after clinical evidence of allergy has subsided Skin prick tests may be negative in some patients with genuine allergies Skin prick tests mainly detect IgE antibody, and are negative in non IgE-mediated reactions (e.g. to foods) False negative results may be due to recent therapy with H 1 receptor antagonists, though if skin testing is done properly this should be detected by lack of reaction to a histamine control False negative results may occur in infants and toddlers, when the weal size is much smaller than later in life There is a poor correlation between the results of provocation tests (e.g. double-blind food challenges) and skin prick tests. This poor correlation is mainly due to the high rate of false positive skin tests There is great variation in potency between extracts of different substances and between those produced by different manufacturers, affecting both extracts of inhalants and foods There is a correlation, which varies for different allergen extracts, between the total serum IgE concentration and the degree of positivity of prick test results Intradermal testing Intradermal tests are far more sensitive than prick tests, produce many more false positive reactions, and carry the risk of fatal anaphylaxis.
Value of skin testing The role for skin prick tests in paediatric practice, if any, is far from clear. The fact that skin tests are still
in use reflect both the unscientific nature of allergy practice and the absence of better tests. The results of skin tests cannot be taken alone, and standard textbooks of allergy acknowledge that the proper interpretation of results requires ‘a thorough knowledge of the history and physical findings’. The current doctrine is that a positive prick test is an indication of clinical sensitivity, past, present or future, and that prick tests are worth doing to provide confirmatory evidence. From a carefully taken history, one might suspect a particular allergen, and the finding of a positive prick test would increase the likelihood that the allergen was causing symptoms. Few people, however, would be prepared to ignore a strong history of allergy in the face of a negative prick test, yet it is illogical to regard the prick test as significant when it confirms the history and to disregard it when it fails to do so. Tests for specific IgE antibodies, whether in the skin or the circulation, are of little use in dealing with the most common clinical issues, which are whether a child’s symptoms are related to allergen exposure, whether a condition will improve if allergens are avoided, and trying to pinpoint which allergen (if any) caused a specific exacerbation (e.g. worsening of asthma). The major problems are the large number of false positive and false negative reactions, the fact that not all allergic reactions are IgE mediated, and the fact that allergen avoidance may fail for a number of reasons such as incomplete avoidance or multiple allergies. Skin prick tests for food allergy are especially unreliable because of the large number of false positive and false negative reactions. Commercial food extracts (sometimes heat treated) and fresh or frozen raw extracts may give different results (more positives with raw foods), reflecting the fact that some patients are intolerant to certain foods only when taken in a raw state. while in others the reverse is the case.
References Bousquet J. In vivo methods for study of allergy: skin tests, techniques, and interpretation. Chapter 19, pp. 419-436, in Middleton E, Reed CE, Ellis EF, Adkinson NF, Yunginger JW (editors). Allergy. Principles and Practice. Third edition, volume 1. St Louis, Mosby, 1988. Dreborg S. Skin tests used in type I allergy testing. Position paper. Prepared by the sub-committee on skin tests of the European Academy of Allergology and Clinical Immunology. Allergy 1989; 44 (suppl.10): l-59.
Skin
1
Allergy tests
T. J. David Prick testing
The principle of skin tests is that the weal and flare reaction to an allergen introduced into the skin demonstrates the presence of mast-cell-fixed antibody, mainly IgE antibody.
A drop of allergen solution is placed on the skin which is then pricked with a hypodermic needle. Prick tests can also produce variable results, but the introduction of standardised precision needles for prick testing (e.g. the Morrow Brown needle) has made the method potentially more reproducible. The delegation of skin testing to untrained staff, and the continued use of unstandardised hypodermic needles leads to frequent errors and poor reproducibility.
Allergen extracts Aqueous extracts are unstable, and 50% glycerol (for prick tests) or 0.03% human serum albumin (for intradermal tests) is added to prevent allergen degradation.
Method Control solutions Because of the variability of cutaneous reactivity, it is necessary to include positive and negative controls whenever performing skin tests. The negative control solution consists of the diluent used to preserve the allergen extracts. The positive control solution usually contains histamine 10 mg/ml, and is mainly used to detect suppression of reactivity, which may last for several weeks or more with some newer Hl receptor antagonists (e.g. astemizole).
A small drop of each test extract and control solution, at least 4 cm apart, is placed on the volar aspect of the forearm or the back of the trunk. Using the Morrow Brown device, the needle is pressed through the drop of allergen extract into the skin at an angle of 90 degrees to the skin, to a standard depth of 1 mm. Other devices are passed through the drop, penetrating the skin at a 45 degree angle, and the skin is then gently lifted to create a small break in the epidermis.
Scratch testing
Timing
A drop of allergen solution is placed on the skin which is then scratched so as to superficially penetrate the skin. As well as providing non-specific weal reactions as a result of trauma, the scratch test introduces a variable amount of allergen through the skin and is therefore poorly standardised and produces results which are too variable for routine clinical use.
The skin prick test induces a response that reaches a peak in 8 to 9 min for histamine, 10 to 12 min for compound 48/80, a histamine-releasing agent, and 12 to 15 min for allergens. Occasionally reactions take up to 30 min to develop. Late reactions (induration and inflammation that begins, peaks and terminates within l-2 days) can also occur, but are not usually sought as their significance is unclear. Which is important: weal or flare?
Dr T. J. David, PLD, MD, FRCP, DCH, Senior Lecturer in Child Health, University of Manchester, Department of Child Health, Booth Hall Children’s Hospital, Charlestown Road, Blackley, Manchester M9 2AA, UK Correspondence and requests for offprints to TJD. Currenr Faediuwics (1991) I, 145-146 0 1991 Longman Group UK Ltd
Most recommend measurement of the size of the weal alone, but a few suggest that the size of the flare should also be measured. 145
146 CURRENT PAEDIATRICS Criteria of positivity There is no agreed definition about what constitutes a positive reaction. Most definitions of a positive reaction are based on the absolute diameter of the weal, with arbitrary cut-off points for positivity at 1 mm, 2 mm or 3 mm. A major difficulty is that the size of the weal depends on the potency of the extract.
Prick tests: problems with interpretation Prick tests are easier to perform than to interpret. The main problems are: Skin prick test reactivity may be present in subjects with no clinical evidence of allergy Skin prick test reactivity may persist after clinical evidence of allergy has subsided Skin prick tests may be negative in some patients with genuine allergies Skin prick tests mainly detect IgE antibody, and are negative in non IgE-mediated reactions (e.g. to foods) False negative results may be due to recent therapy with H 1 receptor antagonists, though if skin testing is done properly this should be detected by lack of reaction to a histamine control False negative results may occur in infants and toddlers, when the weal size is much smaller than later in life There is a poor correlation between the results of provocation tests (e.g. double-blind food challenges) and skin prick tests. This poor correlation is mainly due to the high rate of false positive skin tests There is great variation in potency between extracts of different substances and between those produced by different manufacturers, affecting both extracts of inhalants and foods There is a correlation, which varies for different allergen extracts, between the total serum IgE concentration and the degree of positivity of prick test results Intradermal testing Intradermal tests are far more sensitive than prick tests, produce many more false positive reactions, and carry the risk of fatal anaphylaxis.
Value of skin testing The role for skin prick tests in paediatric practice, if any, is far from clear. The fact that skin tests are still
in use reflect both the unscientific nature of allergy practice and the absence of better tests. The results of skin tests cannot be taken alone, and standard textbooks of allergy acknowledge that the proper interpretation of results requires ‘a thorough knowledge of the history and physical findings’. The current doctrine is that a positive prick test is an indication of clinical sensitivity, past, present or future, and that prick tests are worth doing to provide confirmatory evidence. From a carefully taken history, one might suspect a particular allergen, and the finding of a positive prick test would increase the likelihood that the allergen was causing symptoms. Few people, however, would be prepared to ignore a strong history of allergy in the face of a negative prick test, yet it is illogical to regard the prick test as significant when it confirms the history and to disregard it when it fails to do so. Tests for specific IgE antibodies, whether in the skin or the circulation, are of little use in dealing with the most common clinical issues, which are whether a child’s symptoms are related to allergen exposure, whether a condition will improve if allergens are avoided, and trying to pinpoint which allergen (if any) caused a specific exacerbation (e.g. worsening of asthma). The major problems are the large number of false positive and false negative reactions, the fact that not all allergic reactions are IgE mediated, and the fact that allergen avoidance may fail for a number of reasons such as incomplete avoidance or multiple allergies. Skin prick tests for food allergy are especially unreliable because of the large number of false positive and false negative reactions. Commercial food extracts (sometimes heat treated) and fresh or frozen raw extracts may give different results (more positives with raw foods), reflecting the fact that some patients are intolerant to certain foods only when taken in a raw state. while in others the reverse is the case.
References Bousquet J. In vivo methods for study of allergy: skin tests, techniques, and interpretation. Chapter 19, pp. 419-436, in Middleton E, Reed CE, Ellis EF, Adkinson NF, Yunginger JW (editors). Allergy. Principles and Practice. Third edition, volume 1. St Louis, Mosby, 1988. Dreborg S. Skin tests used in type I allergy testing. Position paper. Prepared by the sub-committee on skin tests of the European Academy of Allergology and Clinical Immunology. Allergy 1989; 44 (suppl.10): l-59.