Sometimes, ACD occurs after many years of repeated exposure to a weak allergen that was used in the past without any problems rather than after a recent or new contact. Because the pathways for type I and type IV allergic reactions are different, the tests required to elicit these reactions also differ; the type I reaction is evaluated using either a skin prick test in vivo or a serologic test in vitro , while the type IV reaction is evaluated using a skin patch test. Allergens in the form of liquid droplets are placed on the skin surface least 2 cm apart on the flexor aspect of the forearm.
A sterile needle or lancet is introduced into the skin through the droplet, and the needle then pushes the allergen into the dermis. A new lancet is used for every allergen to prevent contamination.
An alternative method is the intradermal test, where a small amount of an allergen is injected directly into the dermis using a syringe and needle. Both techniques require a positive histamine and a negative saline control. The test area is examined after 15 to 20 minutes to evaluate the skin reaction.
Presence of a wheal with a diameter of 3 mm or greater is considered as indicative of a positive reaction. The absence of wheal formation with an allergen indicates the absence of an allergy to that specific allergen; the positive control should form a wheal, while the negative control should not produce any reaction. If a wheal formation does not occur with the positive control, the patient could have inadvertently taken an antihistamine prior to the test, which causes dampening of the reaction.
Results of the test would then be rendered invalid. Cross-reactivity may occur, meaning that the sensitivity to one allergen may cause a reaction with another even if there was no previous exposure. Indications for the prick test: 6 , Suspected food allergy e. Poorly controlled or frequent exacerbation of allergic rhinitis, rhinosinusitis, eczema or bronchial asthma, where identifying and avoiding allergens animal dander, pollen, cockroaches and house dust mites may improve the condition.
Suspected or previous allergy to a drug such as penicillin only when there are limited alternative drugs which can be used for treatment. Food allergy commonly presents as acute urticaria, angioedema or gastrointestinal symptoms. Precautions and contraindications for the prick test: 6 , Patients on treatment with oral antihistamine and antidepressants need to defer treatment before the test 3 days for 1st-generation and 10 days for 2nd-generation antihistamines to avoid wheal dampening. Application of topical steroids at the test site should be stopped 2 to 3 weeks prior to testing because it may dampen the results.
However, oral or inhaled steroids may be continued because use of these substances does not affect the results. The prick test is contraindicated in patients who experienced anaphylaxis 4 to 6 weeks prior to the test. The prick test in the elderly may not be accurate because skin reactivity declines with age. Serum IgE testing may be considered instead.
It is best to defer the prick test for children below 2 years of age and pregnant women. There is a small chance of anaphylaxis during the test. Hence, testing should be performed where health professionals and emergency resuscitation facilities are available. The serologic or blood test for diagnosing allergies is based on the principle of detecting circulating IgE antibody levels against specific allergens in the blood. Blood assays are performed using different techniques, such as radioallergosorbent tests RAST or enzyme-linked immunosorbent assay, depending on equipment availability.
Common allergens are food, plants, house dust mites, and animal dander. The skin prick test is not available or the suspected allergen is not available in skin prick test series. The patient has extensive dermatitis or dermographism that may cause a false positive with the skin prick test. Once the allergen has been identified or excluded using prick tests or serology testing, its clinical relevance must be determined.
Patients must be informed and educated regarding allergens to avoid. However, rigid food avoidance, especially in children, may lead poor growth; hence, good nutrition intake must be ensured by providing a balance with alternative sources of nutrients. Both prick tests and serum IgE testing have low positive predictive values, but their negative predictive value is high. For example, if the test did not cause a reaction to egg, patients need not avoid egg in their diet.
However, false positive results may occur following both tests, and thus, results should be interpreted with caution and always correlated with history and physical examination. Anaphylaxis can be prevented through immediate administration of intramuscular epinephrine injection in cases where accidental exposure to a known allergen has occurred.
The advantages of the skin prick test compared to serologic testing are the lower cost and the shorter time to obtain results, while the disadvantages include the facts that fewer allergens can be tested, cooperation from young children is difficult, several essential drugs must be withheld and the procedure has a small risk of anaphylaxis. Common allergens acquired through contact include fragrances, vehicles and preservatives in cosmetics, latex, plants, topical medications e. Suspected contact or occupational dermatitis where the lesion is isolated in the area of contact.
Recurrent dermatitis affecting the lips, face, hands, feet and perineum areas that are frequently exposed to contact allergens. Stasis eczema, discoid eczema and chronic recurrent eczema for which a definite cause cannot be established.
Patch testing includes a different series of allergens from the prick test, and there are about 35 to 80 allergens that are commercially available. The most commonly used series is the European Standard Battery, which consists of 34 allergens. Selection of the allergen or allergens to be tested is based on history and suspicion. Allergens are placed in small aluminium or polyethylene chambers measuring 8 mm in diameter and 0.
The patch is removed after 48 hours, and the first reading is documented. A second final reading is performed after 96 hours. A third reading on day 7 may be necessary for certain allergens, such as metals, that require longer contact on the skin before an assessment can be made.
Precautions and contraindications for the patch test: 10 , Pre-test counselling is important because patients must refrain from bathing and exercise during the test period. Sweating and wetting of the patch causes it to dislodge, leading to test failure. Refraining from bath or ablution may not be acceptable in certain cultures; hence, informed consent must be obtained. Patients should be counselled regarding side effects such as erythema, pruritus, or hyper- or hypopigmentation at the test site.
Occasionally, blisters may appear, and eczema may flare. Any skin discoloration that may occur at the test site will resolve with time. The test should be planned about 6 months from the time of suspected contact to avoid a flare of the existing dermatitis, especially if it is poorly controlled. Topical steroid application must be avoided at the test site both 1 week before and during the test. The patch test should be deferred in pregnant women because immunological changes of pregnancy may affect patch results.
A positive reaction may spread from a test site, causing a false positive result at adjacent sites. Hence, results need to be interpreted with caution.
Once the offending allergen is identified, the patient must be informed and counselled to avoid the offending allergens to prevent recurrence or persistence of symptoms. This online information and written documentation will help to generate a list of products that contain the specific allergen that the patient needs to avoid. The approach to select a suitable allergic test based on the presenting symptoms and signs is summarized in Figure 3. A few other tests are available for further confirmation of allergies.
However, these tests are not widely available and are performed in specialized allergy centres because more advanced expertise is required for patient selection, preparation, test related materials, procedures, interpretation and application of the results. The prick to prick test is usually performed for food allergies when commercially prepared extracts of the allergen in question are not available or a different allergenic component of the food is suspected in a negative prick test.
Other test procedures, reading and interpretation are similar to those of the prick test. Basophils, just as mast cells, will degranulate when IgE attached to high-affinity IgE receptors on the cell surface is cross-linked in the presence of an allergen. The basophil activation test is relatively new but has the advantage of high sensitivity and specificity.
This is the gold standard test in diagnosis of food allergies. It involves meticulous preparation of the food and the placebo, stringent dosing schedules and multiple assessments of the clinical reactions.
The prick test, the serum IgE test, and the patch test are all useful to identify common allergens responsible for clinical manifestations of allergies. The prick and serum IgE tests detect type I reactions usually related to food and aeroallergens while the patch test detects type IV reactions, which are responsible for contact dermatitis. Indication for testing must be clear because testing asymptomatic individuals is not beneficial.
Clinicians must be wary of false positive or negative results, interpret the results with caution, and always match the results with a good history and physical examination. Management of allergies may be exhausting because some allergens, such as house dust mites and pollen, are constantly present in the environment, thereby rendering incessant reactions.
Some allergens may be difficult to detect because they are weak and produce a reaction only after repeated exposures over months or years.
Once the relevant allergens are identified and symptomatic treatment is instituted, patients must be informed and educated on allergen avoidance; this would reduce the likelihood and incidence of future reactions and ensure a satisfactory management outcome. National Center for Biotechnology Information , U. Journal List Malays Fam Physician v. Malays Fam Physician.
Published online May Leelavathi Muthupalaniappen and Adawiyah Jamil. Author information Copyright and License information Disclaimer. Abstract This article provides information on allergy testing and serves as a simple guide for physicians who are considering using allergy testing as a step in patient management.
Keywords: Hypersensitivity, immunoglobulin E, skin tests, patch test, prick test. Introduction Allergies are described as abnormal harmful immune responses to harmless substances, or allergens. Precautions and contraindications for the prick test: 6 , 14 Patients on treatment with oral antihistamine and antidepressants need to defer treatment before the test 3 days for 1st-generation and 10 days for 2nd-generation antihistamines to avoid wheal dampening.
Indications for serum IgE testing: The skin prick test is not available or the suspected allergen is not available in skin prick test series. The patient is unable to discontinue oral antihistamines. There is a history suggestive of an allergy, but skin prick testing is negative. There is a history of anaphylaxis to any allergen. Post-prick test and serum IgE test Once the allergen has been identified or excluded using prick tests or serology testing, its clinical relevance must be determined.
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