Do you know what are you allergic to? (Part 2)
In part 1, we have discussed the common allergens, allergic reactions, and the reason behind an overactive immune system after coming into contact with the allergens. So now, let’s focus on how you can identify know what you are allergic to, and the treatments available.
How can I know what am I allergic to?
Avoidance of provoking triggers or allergens to prevent future allergy reactions is the best way to overcome allergies. Therefore, it is important to accurately identify the offending allergens causing the symptoms. There are several allergy tests available now:
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Skin prick test
The prick test works on the principle of activating IgE antibodies (as mentioned above), on the skin cells.
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Small drops of various allergens are placed on the skin surface of your forearm, leaving enough space between them
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Then a sterile needle or lancet is introduced into the skin through the droplet, to push the allergen(s) into the skin. Bear in mind that every new lancet is used for every allergen to prevent contamination.
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After 15-20 minutes, if your skin becomes red and swollen, with the presence of wheal, that means you're having an allergic reaction to that particular substance tested on. A larger wheal indicates a higher sensitivity to the allergen but does not necessarily cause more severe allergic symptoms.
However, not every allergy can be tested for a prick test and there are some precautions for the prick test.
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Indications for prick test |
Precautions for prick test |
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Suspected food allergy (e.g. to egg, peanut, wheat, fish, soy, or cow’s milk) that cannot be determined from food elimination. |
Has taken an oral antihistamine, a type of allergy medication before the test (3 days for 1st generation and 10 days for 2nd generation antihistamines) |
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Poorly controlled or frequent exacerbation of allergic rhinitis, eczema, or asthma; whereby identifying and avoiding allergens (animal dander, pollen, cockroaches, and house dust mites) may improve the condition. |
Application of topical steroids at the test site should be stopped 2-3 weeks before testing because it may dampen the results. However, oral or inhaled steroids may be continued as they do not affect the results. |
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Suspected or previous true allergy to a medication such as penicillin (only done when there are limited alternative drugs that can be used for treatment). |
Contraindicated in patients who experienced anaphylaxis 4-6 weeks before the test. |
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Prick tests in the elderly may not be accurate because skin reactivity declines with age. A blood test (refer below) may be considered instead. |
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It is best to defer the prick test for children below 2 years of age and pregnant women. |
The skin prick test is not particularly painful, but some do find it slightly uncomfortable. Although this test is safe, keep in mind that there is a small chance of anaphylaxis happening during the test, as the allergen is introduced into the body. Hence, testing should be performed where health professionals and emergency resuscitation facilities are available.
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Blood testing
The blood test is based on the principle of detecting circulating IgE antibodies levels against specific allergens in the blood. A single allergen or a battery of allergens can be tested for, and selection of the suspected allergen or allergens is shortlisted from the individual’s history. In general, sensitivity is high for common allergenic food like egg, peanut, milk, and soy whereas sensitivity to house dust mites and animal dander is moderately high.
| Indications for the blood test |
| Where skin prick test is not available or the suspected allergen is not available in the skin prick test series. |
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Individuals are unable to discontinue oral antihistamines. |
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There is a history suggestive of an allergy, but skin prick testing is negative. |
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There is a history of anaphylaxis to an allergen. |
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Individuals have extensive skin allergies that may cause a false positive with the skin prick test. |
The advantages of the skin prick test compared to the blood test are the lower cost and shorter time to obtain results. However, only fewer allergens can be tested, and there is a need for several essential medications to be withheld because the procedure has a small risk of inducing anaphylaxis.
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Patch test
The patch test is the gold standard test used to detect a type of allergic reaction where repeated exposure to allergens triggers a clinical reaction known as allergic contact dermatitis. Common allergens acquired through contact include fragrances, vehicles, and preservatives in cosmetics, latex, plants, topical medications (antibiotics and corticosteroids), metal (e.g. nickel), and hair dyes.
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Indications for the patch test |
Precautions for the patch test |
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Suspected contact due to work-related exposures (e.g. barber) where the lesion is isolated in the area of contact. |
Individuals must refrain from bathing and exercising during the test period. This is because sweating and wetting of the patch cause it to dislodge, leading to test failure. |
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Worsening of existing skin allergies despite adequate treatment. |
Topical steroid application must be avoided at the test site both 1 week before and during the test. |
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Recurrent skin allergies affect the lips, face, hands, and feet. |
Contraindicated in pregnant women because immunological changes of pregnancy may affect patch results. |
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Chronic recurrent eczema for which a definite cause cannot be established. |
Individuals who are on treatment with a high daily oral steroid (>10mg), steroid injections, immunosuppressants, or phototherapy or those who have recent excessive sun exposure (within 1 week) should defer patch test because it may affect test results. |
As shown in the picture below, the aluminium chambers were filled with allergens (A). Then the allergens were patched to the upper back with hypoallergenic tape (B). After 3 days, there was a positive reaction to nickel (C). 2 days later there was a persistence of the reaction to nickel, while a negative reaction to other allergens remained (D).
Disadvantage: A positive reaction may spread from a test site, causing a false positive result at adjacent sites.
What are the treatments available?
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Avoid contact with allergens
It is quite easy to avoid certain food or chemical substances. However, it is almost impossible to avoid pollen and house dust mites. There are pollen calendars and forecasts available on the internet to give you an idea of the pollen count in a certain region. By cleaning the house regularly, and using mite-proof bedding, you can somewhat lower the exposure to dust mites. Read more about avoiding asthma triggers.
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Medications
Antihistamine or steroids are often used. These medications are available in various forms, including tablets, nasal sprays, and injections. Steroid lotions or creams can be used to treat allergic skin reactions.
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Allergen-specific immunotherapy
This therapy involves exposing people to small amounts of the allergen. Similar to vaccination, it is done at regular intervals by either injecting the allergen under your skin or placing it under your tongue in the form of a tablet or drops. It takes about 3-5 years to complete allergen-specific immunotherapy. This approach is mainly used for the treatment of allergies to pollen, dust mites, and insect venom.
It's often not possible to completely prevent an allergy from developing. Certain measures may lower the risk of it happening too frequently. Non-smokers and children who grow up in smoke-free environments are less likely to develop allergies. If you have a high risk of developing an allergy, it might be a good idea to not get a pet cat to avoid developing a cat fur allergy. But if you don't have a greater risk, then there's no reason not to have a pet.
All in all
The prick test, blood test, and patch test are all useful to identify common allergens that are responsible for allergic reactions. The prick test and blood test detect allergic reactions related to food and aeroallergens (airborne substances) while the patch that detects allergic reactions related to skin allergies. Get your advice from a healthcare professional about the treatment that is suitable for your condition and avoid allergens as much as you can for a better quality of life.
Allergy tests are available in the AMC clinic.
Medically reviewed by Ashwini Nair, MB BCh BAO.
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References:
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Best Practice Advocacy Centre New Zealand Appropriate use of allergy testing in primary care. Best Tests. 2011
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Li JT. Allergy testing. Am Fam Physician. 2002 Aug;66(4):621–624.
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Anvari S, Miller J, Yeh CY, et al. IgE-mediated food allergy. Clin Rev Allergy Immunol. 2019 Oct;57(2):244–260. DOI: 10.1007/s12016-018-8710-3.
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Hong SD, Ryu G, Seo MY, et al. Optimal cutoff values of allergen-specific immunoglobulin E to house dust mites and animal dander based on skin-prick test results: Analysis in 16,209 patients with allergic rhinitis. Am J Rhinol Allergy. 2018 Jan;32(1):23–26. DOI: 10.2500/ajra.2018.32.4483.
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Prick, patch, or blood test? A simple guide to allergy testing - PMC. Accessed April 14, 2022. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8346756/
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Allergies: Overview. Institute for Quality and Efficiency in Health Care (IQWiG); 2020. Accessed April 14, 2022. https://www.ncbi.nlm.nih.gov/books/NBK447112/
