Allergy skin tests help to identify the substances to which a person is allergic.
An allergen is a substance that can cause an allergic reaction. During skin testing, the skin is exposed to the suspected allergen and observed for signs of an allergic reaction.
Read more about Skin Anatomy and Physiology
Besides skin testing, other tests that may be used to detect allergies include blood tests (that detect the presence of antibodies to a particular allergen) or diet elimination ( excluding a suspected food item from the diet to see if it relieves the symptoms).
Allergy skin tests can diagnose allergies to common allergens like mold, pollen, animal fur, insect stings, and foods.
Indications of Allergy Skin Tests
Allergy tests provide information about which substances or allergens should be avoided by the patient. It also helps the treating doctor to plan treatment including medications or immunotherapy.
They help to identify allergies to airborne substances, such as pollen, pet dander, and dust mites.
Allergic skin tests are used in the following conditions:
- Hay fever (allergic rhinitis)
- Atopic dermatitis (eczema)
- Food allergies
- Penicillin allergy
- Other drugs/medication allergy
- Stinging insect (bee venom) allergy
Read more about Atopic Dermatitis: Causes, Self-care Tips, and Treatment
Read more about Angioedema – Causes and Treatment
How to Prepare for Allergy Skin Testing?
All anti-allergic and cold medicines (containing antihistamines) should be avoided for at least 5 – 7 days before undergoing allergy skin tests. This is because these medicines may result in false-negative results ( meaning the test result is negative while the patient is actually allergic to that substance).
Other medicines like antidepressants, heartburn medications, steroids, etc may also interfere with the test results and hence should be avoided. It is best to inform your treating doctor about all the medicines you are taking so that he/she can guide you accordingly.
Types and Procedure of Allergic Skin Tests
Before performing the test, the doctor will take a detailed clinical history along with your symptoms, and factors that trigger your allergy. Based on this information, the doctor will decide which allergens should be used for testing. As few as two or three substances or as many as 40 substances may be used for testing.
Allergic skin tests are of three types.
Skin prick test (scratch test or puncture test)
- The most common sites where the test is performed include the forearm, upper arm, or back.
- The skin is cleaned with alcohol.
- Multiple areas are marked and labeled on the skin with a pen.
- A small amount of different allergens (substances that are suspected of causing an allergic reaction) is placed on each of these areas marked on the skin.
- The skin is then pricked or scratched so that the allergen goes beneath the skin’s surface.
- The skin is then closely observed for swelling, redness, or other signs of a reaction.
- The results are read within 15 to 20 minutes.
- Several allergens can be tested at the same time.
Intradermal skin test
- The area of your skin where the test will be performed is cleaned with alcohol.
- A small amount of allergen is injected into the skin.
- The site of injection is then observed for swelling, redness, or other signs of a reaction.
This test is performed to find out whether the patient is allergic to bee venom or penicillin. It may also be done if the skin prick test was negative but the doctor strongly believes that the patient is allergic to a particular allergen.
- The skin is cleaned with alcohol.
- The suspected allergen is applied on a patch which is then taped to the skin of the arm or back.
- This patch has to be worn for 48 hours or even longer.
- After 48 to 72 hours, the patch is removed and the skin site is examined for any reaction.
This method is used to diagnose delayed reaction to a particular allergen.
Side Effects of the Test
Skin tests may cause mild discomfort when the skin is pricked.
In case the person is allergic to a particular allergen, there may occur symptoms such as swelling, itching, or a skin rash at that particular site. There may also be symptoms such as a stuffy nose, sneezing, red watery eyes, etc.
In very rare cases, the person can develop a severe allergic reaction involving the whole-body (called anaphylaxis). This is a life-threatening condition and needs to be treated urgently. This mostly occurs with intradermal testing. For this reason, allergic skin tests need to be performed by a trained person and at a place that is equipped to deal with any emergency.
Skin tests are usually safe for adults, children, and even infants. In certain circumstances, however, these tests should not be performed.
- Known severe allergic reaction. In case you are severely allergic to a substance, even the small amount used in skin tests could result in a life-threatening reaction (anaphylaxis).
- Certain skin conditions. If large areas of the skin ( especially on the arms and back) are affected by severe eczema or psoriasis, there may not be sufficient uninvolved skin to perform the test.
- Intradermal tests are not used to test for food allergies because of high false-positive results and the danger of causing a severe allergic reaction.
If skin tests can’t be performed for any of the above reasons, blood tests which detect the presence of antibody in the blood sample may be performed. Blood tests however are not used for penicillin allergy.
Interpretation of Results
Normal or negative results
A negative or normal test result means there were no skin changes in response to the applied or injected allergen. It usually implies that you are not allergic to that substance.
In very rare cases, the allergy test may be negative and you may still be allergic to the substance. (false-negative result).
Abnormal or positive results
A positive result occurs when the skin or the body reacts to the allergen resulting in a red, raised swollen area called a wheal.
A positive test result usually means that you are allergic to that substance and have been exposed to it earlier. A very strong positive result means you are highly sensitive to that substance.
It is possible that a person has a positive response to a substance with skin testing, but that substance is not causing any problems or symptoms in daily life. If a large dose of allergen is used, it is possible that even those who are not allergic may have a positive reaction.
Accuracy of Allergy Skin Tests
Allergy skin tests are not always accurate.
- At times, they indicate an allergy when actually you are not (false-positive).
- In some cases, the test may not induce a skin reaction to a substance to which you are allergic to (false-negative).
- It is possible that you may react positively to a substance during the test, but exposure to the same substance in everyday life does not cause problems.
Hence the results of these tests must be interpreted cautiously taking into account the history and symptoms of the person.
What to do in the case of Positive Test Result?
In case you test positive in allergy skin tests, the following steps need to be undertaken
- You should try to avoid or at least limit contact with your allergens. This would entail changes in your home or work environment or dietary changes. For example, in case you are allergic to dust mites, make sure to clean and vacuum your house thoroughly, and avoid unnecessary clutter.
- Take appropriate medicines as advised by your doctor. Depending upon your condition, you may be may prescribed medicines such as antihistamines, decongestants, nasal sprays, topical or oral steroids, etc.
- Immunotherapy or allergy shots: It involves giving gradually increasing doses of the substance to which you are allergic ( allergen). It may be given in the form of allergy shots or placed under the tongue. The increasing amount of allergen given over a period of time causes the immune system to become less sensitive to it. Thus in case of any contact with the allergen in the future, the allergic symptoms produced will be of much lesser intensity.