Allergy skin test results chart are a doctor’s most important diagnostic tool. There are different types of skin tests. The purpose of these tests is to bring suspected allergens into contact with the skin, in sufficient quantity, and to interpret the reaction they cause. The allergens in question will cause an allergic skin reaction.
What are skin tests?
The skin tests are tests performed on the skin (hence the name: skin) of the patient for allergy diagnosis. Not all allergic diseases justify doing this type of exam. It must be done by trained people, in specialized centers and the results must always be interpreted by specialists.
Types of skin tests:
We can find 3 types of skin tests, as summarized in the following section:
1. Sting tests
A drop of the allergen is applied to the skin of the arm and a light prick is made, with a special lancet, over each drop. Skin tests for allergies are painless. In this type of test, needles (lancets) are used that lightly penetrate the surface of the skin. There will be no bleeding and no more than a slight, momentary discomfort will be felt.
After cleaning the test site with alcohol, the nursing staff draws small marks on the skin and applies a drop of allergen extract next to each mark. Then, use a lancet to puncture the surface of the skin so that the extracts penetrate it. A new lancet is used for each allergen.
They are positive if a pimple appears at the sting site after 15 minutes.
Useful for respiratory allergies, food allergies, drug allergies, and Hymenoptera venom allergies.
With a skin prick test, also called a “prick test” or “scrape test,” immediate allergic reactions to 50 different substances can be detected at the same time. This test is typically done to identify allergies to pollen, mold, pet dander, dust mites, and food. In adults, the test is usually done on the forearm. In children, it can be done on the upper back.
To see if your skin is reacting normally, two additional substances are applied to the surface of the skin:
Histamine In most people, this substance causes a skin reaction. If you don’t react to histamine, the allergy skin test might not show an allergy even if you have one.
Glycerin or saline solution. In most people, these substances do not cause any reaction. If you react to glycerin or saline, you may have sensitive skin. Test results should be interpreted with care to avoid a false allergy diagnosis.
- About 15 minutes after puncturing the skin, the nursing staff looks at the skin for signs of allergic reactions. If you are allergic to one of the substances tested, a raised, red itchy bump (welt) will occur, which may look similar to a mosquito bite. The nurse will then measure the size of the lump and record the result. He will then clean your skin with rubbing alcohol to remove the marks.
2. Intradermal tests
Intradermal injection of a small amount of the diluted drug, or of the diluted hymenopteran venom, with progressive increases in concentration until a positive result or until reaching a maximum non-irritating concentration.
Readings are taken at 15-20 minutes for an immediate reaction, or in the following days if it is a late reaction.
You may need a test in which a needle is used to inject a small amount of allergen extract into the skin of your arm (intradermal test). The injection site is examined after about 15 minutes for signs of an allergic reaction. Your doctor may recommend this test to check for an allergy to insect venom or penicillin.
3. Patch tests
It consists of applying adhesives on the back that remain glued for 48 hours.
The readings of these tests are taken at 48, 72, or 96h.
These types of tests are very useful in allergic contact dermatitis and some late drug reactions.
Patch testing is usually done to see if a particular substance is causing allergic skin inflammation (contact dermatitis). Patch tests can detect late allergic reactions, which can take several days to develop.
In patch testing, no needles are used. Instead, allergens are applied to patches, which are then placed on the skin. During a patch test, the skin can be exposed to 20-30 extracts of substances that can cause contact dermatitis. Some of them can be latex, drugs, fragrances, preservatives, hair dyes, metals, and resins.
You must wear the patches on your arm or back for 48 hours. During this time, you should avoid bathing and doing activities that cause a lot of sweating. The patches are removed when you return to the doctor’s office. If the skin is irritated at the patch site, this may indicate an allergy.
How are skin tests used?
The skin tests that are most often performed are the prick tests ( prick tests or prick tests ), usually for diagnosing immediate allergic reactions, that is, possibly mediated by IgE.
They consist of applying a drop of the suspected allergen to the skin of the forearm, followed by a slight prick with a small lancet.
Skin tests performed by specialists are the fastest way (the result is ready in 15 minutes!), simple and cheap to identify what causes us allergies. Prick tests are bloodless and virtually painless. They can be repeated whenever there is a suspicion of a new allergy, or overtime to check the evolution.
A variation of the prick test is sometimes needed when investigating some food allergies. It is called prick-prick or minced-sting and uses foods in nature, which are minced with a lancet and then applied to the forearm.
In case of suspected allergy to hymenopteran poisons (bee and/or wasp) or to medications (for example, penicillin) or if the sting tests are negative, the next step is to perform intradermal tests, usually with immediate reading (15- 20 minutes). These consist of introducing a small amount of the diluted allergen under the skin with a small needle. It may be necessary to repeat intradermal injections with progressive increases in concentration until a positive result or until reaching a maximum non-irritating concentration. There are several concentrations that are standardized, and that must be respected in order not to cause skin irritations that do not mean allergy.
It is important to note that there is a risk of generalized reaction with this type of procedure, which should only be performed by experienced professionals in a hospital environment. These tests are a little more painful, but they can also be done at any age, depending on individual tolerance. They have a greater sensitivity.
Also for intradermal administration, there is the autologous serum test, but it is only useful in some specific types of urticaria and is not routinely used in clinical practice.
The patch tests, skin tests also designated contact (patch tests), consist of applying the test in the shape of an adhesive on the back. These tests are important in the study of allergic contact dermatitis caused by various products that come into contact with the skin, namely, metals, cosmetics, as well as topical medicines (in ointment or cream). Should be done between 6 weeks to 6 months after the symptoms stop.
The results of these tests are only known after 48 and 96 hours after application (or 7 days if a reaction to corticosteroids is suspected) and the interpretation of the results requires experience, especially as an irritant reaction to the skin that does not mean an allergy can occur! The risk of these tests is low, even in patients with a history of severe drug reactions. False-negative results may be due to poor skin penetration (larger molecules) or low concentration.
What types of allergens are tested?
We can test almost any type of allergen, depending on medical suspicion. It is important to be aware that the tests to be chosen must be suitable for each clinical suspicion, that is, it only makes sense to perform them if the patient presents symptoms compatible with an allergy:
Only skin prick tests are suitable: allergy to mites, animals, fungi, [grass pollens, grass or tree pollens, etc. can be identified. In Portugal, all these allergens are frequent causes of respiratory symptoms and, therefore, it is usual to test a typical battery that includes all of them - between 10 to 14 allergens usually.
Prick skin tests are commonly used if the commercial extract (which comes in the vials) is available. In this case, test batteries are not recommended as in the case of respiratory allergy. These tests detect most cases of food allergy, but the main problem is that not all positive results are true allergies! In other words, it is possible to have positive results without having an allergy! Tests should only be performed for foods suspected to be related to symptoms because these tests have many false-positive results., that is, there are many people with positive results, but without any allergies! When it comes to food, this can have very serious implications! And the fact that you are allergic to one of the foods in the test does not imply that you are to the others.
The results must be interpreted by an Allergist, because there may also be situations in which they are negative and this does not necessarily exclude the existence of an allergy or the risk of serious reactions. For example, the allergy may not be IgE-mediated; in addition, extracts of some foods are not always sold in bottles and these extracts do not always give reliable results, that is, they can be negative even in patients with allergies! In these cases we can use the prick-prick method with the food if there is no extract available if we have a negative prick method result, or if we want to evaluate different presentations of the food (eg raw versus cooked).
Very rarely and in very specific situations, patch tests with food are sometimes used.
In the case of suspected drug allergy, the diagnostic procedure starts with the prick tests, and if negative, the immediate reading intradermal tests are followed. In suspected cases of delayed reaction, intradermal delayed reading tests or patch tests are used.
There are medications for which we do not have skin tests available, or are not reliable.
Hymenoptera sting (bee or wasp)
In this case, we also start with the prick tests, and if they are negative, the immediate reading intradermal tests are followed with various concentrations of the poisons.
Patch tests are used in suspected allergic contact dermatitis. There is a standard battery with 30 allergens, which should be well known, because in some situations the suspicion involves some allergens that are not contained there and it may be necessary to use specific batteries such as reactions to prostheses, some cosmetics, sunscreens, hairdressing products, metals, plants, etc…
Results of the tests
Before leaving the doctor’s office, you will learn the results of a skin test or intradermal test. A patch test can take several days or more to produce results.
A positive skin test means that you may be allergic to a particular substance. Larger welts usually indicate a higher degree of sensitivity. A negative skin test means that you are probably not allergic to a particular allergen.
Keep in mind that skin tests are not always accurate. Sometimes they indicate an allergy when there is none (false positive) or a skin test may not trigger a reaction when you are exposed to something you are allergic to (false negative). You can react differently to the same test performed on different occasions. Or you can react positively to a substance during a test, but not react to it in everyday life.
Your allergy treatment plan may include medications, immunotherapy, changes in your work or home environment, or changes in diet. Ask your doctor to explain anything you don’t understand about your diagnosis or treatment. With test results that identify allergens and a treatment plan that helps you take control, you can reduce or eliminate allergy signs and symptoms.
Blood tests make it possible to measure the amount of antibodies to each of the allergens. There are for food, respiratory, Hymenoptera poisons (bee and wasp) and some medicines. By assessing the level of allergen-specific antibodies, ie the antibodies that cause allergic reactions (IgE), we are able to identify what causes allergy and therefore can be an alternative to skin testing. However, these tests are longer, more expensive, and less sensitive than skin tests.
However, they acquire value in some situations:
- In patients with skin problems (in these cases, skin tests may be more difficult);
- In patients who are taking antihistamines (these medications nullify the test result);
- To complete the diagnostic evaluation of more complex cases, namely when there is cross-reactivity or some specific proteins in foods;
- To follow the evolution of the allergy, that is, to understand if the allergy is “passing” or not.
Food Intolerance Tests
The food intolerance tests showed no valid to date for the diagnosis of allergy or intolerance to food (except lactose intolerance test).
Although they have become a very widespread fad, they are expensive tests and there are no published scientific studies duly substantiated that justify this type of test – nail, hair, blood, resonance, etc….
The positive results did not prove that there is a disease, often just reflecting a normal response of the body to the food ingested. For this reason, they are currently not recommended.
Why is it done?
Information from allergy testing can help your doctor develop an allergy treatment plan that involves avoiding allergens, taking medications, or getting allergy shots (immunotherapy).
Allergy skin tests are widely used to help diagnose allergic conditions, including:
- Allergic rhinitis (hay fever)
- Allergic asthma
- Dermatitis (eczema)
- Food allergies
- Penicillin allergy
- Allergy to bee venom
Skin tests are generally safe for adults and children of all ages, including babies. In certain circumstances, however, skin testing is not recommended. Your doctor may advise against skin testing if:
You ever had a serious allergic reaction. You can be so sensitive to some substances that even small amounts used in skin tests could cause a life-threatening reaction (anaphylaxis).
You take medications that could interfere with test results. These include antihistamines, various antidepressants, and some heartburn medications. Your doctor may determine that it is better for you to continue taking these medications rather than temporarily stopping them in preparation for a skin test.
You have certain skin conditions. If psoriasis or severe eczema affects large areas of skin on the arms and back (the sites where tests are usually done), there may not be enough free, unaffected skin to perform an effective test. Other skin conditions, such as dermographism, can make test results unreliable.
Blood tests (in vitro immunoglobulin E antibody tests) can be helpful for those who should not or cannot have skin tests. Blood tests are not used for penicillin allergy.
In general, allergy skin tests are reliable in diagnosing allergies to airborne substances, such as pollen, animal dander, and dust mites. Skin tests can help diagnose food allergies. However, since food allergies can be complex, you may need additional tests or procedures.
What if the skin tests are negative?
There may be allergy with negative tests. The most common situations are:
- Very young children when the allergic component is not yet well defined (this is seen in respiratory allergy, but rare in the case of food allergy);
- When taking antihistamines in the previous days;
- When tests are poorly performed (that’s why you should always seek help from an Allergist - it’s the best way to avoid mistakes!);
- When the mechanism is not mediated by IgE.
In cases where the tests are negative, it may be necessary to carry out tests or, even more importantly, carry out a provocation test to confirm or exclude the suspicion of allergy.
Medications can interfere with results
Before scheduling a skin test, bring your doctor a list of all the prescription and over-the-counter medications you take. Some medications can inhibit allergic reactions and this prevents skin tests from giving accurate results. Other medications can increase the risk of developing a serious allergic reaction during a test.
Because medications are cleared from the body at different rates, your doctor may ask you to stop taking certain medications up to 10 days before. Here is a list of medications that can interfere with skin tests:
Prescription antihistamines, such as hydroxyzine (Vistaril).
Over-the-counter antihistamines, such as loratadine (Claritin, Alavert), diphenhydramine (Benadryl), chlorpheniramine, cetirizine (Zyrtec Allergy), and fexofenadine (Allegra).
Tricyclic antidepressants, such as nortriptyline (Pamelor) and desipramine (Norpramin).
Some heartburn medications, such as cimetidine (Tagamet) and ranitidine.
The asthma medicine omalizumab (Xolair). This medicine can affect test results for six months or more, even after stopping it. For comparison, most medications affect results for a few days or weeks.
What you can expect after the test?
Skin tests are usually done in a doctor’s office. Usually, the nursing staff performs the test and a doctor interprets the results. This test usually takes about 20 to 40 minutes. Some tests detect immediate allergic reactions, which occur within minutes of exposure to an allergen. Other tests detect late allergic reactions, which occur over a period of several days.
Risks Associated with skin test
The most common side effect of skin tests is slightly swollen, red, and itchy bumps (hives). These hives may be more apparent during the test. However, in some cases, an area of swelling, redness, and itchiness may form within hours after the test that may persist for a couple of days.
Rarely, allergy skin tests can produce an immediate and severe allergic reaction, so it is important to perform these types of tests in an office where appropriate emergency equipment and medications are available.
1. Who can take the tests?
Can be performed at any age, from baby months to the elderly! It’s not age that matters, it’s clinical suspicion!
In childhood, we often need prick tests due to suspected food or drug allergies. In older children, we often need prick tests for respiratory allergy, and in adults, we find all situations.
2. Suspension of medication and preparation before the test?
We have to stop taking anti-allergy medications in the days before the exam.
In addition to the suspension of these medications, it is not necessary to carry out any type of prior preparation, nor is it necessary to follow a fast.
3. How long does a test take?
The duration depends on the tests, the prick, prick-prick, and intradermal are quick to apply and the results are read after 15-20 minutes.
Patch tests are quick to apply, but they should stay stuck to the back for 48-72 hours, and only after removal are the results readable.
4. Are skin tests reliable?
Usually yes. There may be situations where after discussion with your doctor it will be decided that a skin test is not indicated or that an alternative is preferable.
5. How to prepare before the test is being performed?
Before recommending a skin test, your doctor will ask you detailed questions about your medical history, signs and symptoms, and the usual way to treat them. Your answers can help your doctor determine if allergies are inherited and if the most likely cause of your symptoms is an allergic reaction. Your doctor may also perform a physical exam looking for other clues to the cause of your signs and symptoms.
These are tests that are usually performed on the forearm by the nurse or doctor. A drop of allergen extract or a nurse-prepared solution of fresh food is placed on the skin and then, using a plastic device with a pointed tip, the skin is punctured with the surface to allow penetration of the liquid. Sometimes the plastic devices are already embedded in the allergen to test their ease of use. It is important to remain still during the procedure and therefore we can ask for your cooperation in immobilizing the child. Your skin reaction will be measured after 15 minutes, during which time you will wait in the waiting room. If the test is positive, there will be swelling and redness at the test site. It is the measurement of this response that will allow the doctor to interpret the test as positive or negative. A positive and negative control test is always performed to ensure that the results are interpretable.
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