Skin allergy testing is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.
A microscopic amount of an allergen is introduced to a patient’s skin by various means:
A patient receiving a skin allergy test
- Skin Prick test: pricking the skin with a needle or pin containing a small amount of the allergen.
- Skin Scratch test: a deep dermic scratch is performed with help of the blunt botton of a lancet.
- Intradermic test: a tiny quantity of allergen is injected under the dermis wth a hypodermic syringe.
- Skin Scrape Test: a superficial scrape is performed with help of the bovel of a needle to remove the superficial layer of the epidermis.
- Patch test: applying a patch to the skin, where the patch contains the allergen
If an immuno-response is seen in the form of a rash, urticaria (hives), or (worse) anaphylaxis it can be concluded that the patient has ahypersensitivity (or allergy) to that allergen. Further testing can be done to identify the particular allergen.
The “skin scratch test” as it’s called, is not very commonly used due to increased likelihood of infection. On the other hand, the “skin scrape test” is painless, does not leave residual pigmentation and does not have a risk of infection, since it is limited to the superficial layer of the skin.
Some allergies are identified in a few minutes but others may take several days. In all cases where the test is positive, the skin will become raised, red and appear itchy. The results are recorded – larger wheals indicating that the subject is more sensitive to that particular allergen. A negative test does not conclusively rule out an allergy; occasionally, the concentration needs to be adjusted, or the body fails to elicit a response.
Immediate Reactions Tests
Skin testing on arm
In the prick, scratch and scrape tests, a few drops of the purified allergen are gently pricked on to the skin surface, usually the forearm. This test is usually done in order to identify allergies to pet dander, dust, pollen, foods or dust mites. Intradermal injections are done by injecting a small amount of allergen just beneath the skin surface. The test is done to assess allergies to drugs like penicillin or bee venom.
Skin testing on back
To ensure that the skin is reacting in the way it is supposed to, all skin allergy tests are also performed with proven allergens likehistamine or glycerin. The majority of people do react to histamine and do not react to glycerin. If the skin does not react appropriately to these allergens then it most likely will not react to the other allergens. These results are interpreted as falsely negative.
Delayed Reactions Tests
The patch test simply uses a large patch which has different allergens on it. The patch is applied onto the skin, usually on the back. The allergens on the patch include latex, medications, preservatives, hair dyes, fragrances, resins and various metals. When a patch is applied the subject should avoid bathing or exercise for at least 48 hours.
Skin end point titration
Also called an intradermal test, this skin end point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response. To prevent a severe allergic reaction, the test is started with a very dilute solution. After 10 minutes, the injection site is measured to look for growth of wheal, a small swelling of the skin. Two millimeters of growth in 10 minutes is considered positive. If 2 mm of growth is noted, then a second injection at a higher concentration is given to confirm the response. The end point is the concentration of antigen that causes an increase in the size of the wheal followed by confirmatory whealing. If the wheal grows larger than 13 mm, then no further injection are given since this is considered a major reaction.
There are no major preparations required for skin testing. At the first consult, the subject’s medical history is obtained and physical examination is performed. All consumers should bring a list of their medications because some may interfere with the testing. Other medications may increase the chance of a severe allergic reaction. Medications that commonly interfere with skin testing include the following:
- Histamine antagonists like Allegra, Claritin, Benadryl, Zyrtec
- Antidepressants like Amitriptyline, Doxepin
- Antacid like Tagamet or Zantac
Consumers who undergo skin testing should know that anaphylaxis can occur anytime. So if any of the following symptoms are experienced, a physician consultation is recommended immediately:
- Low grade Fever
- Lightheadedness or dizziness
- Wheezing or Shortness of breath
- Extensive skin rash
- Swelling of face, lips or mouth
- Difficulty swallowing or speaking
Even though skin testing may seem to be a benign procedure, it does have some risks, including swollen red bumps (hives) which may occur after the test. The hives usually disappear in a few hours after the test. In rare cases they can persist for a day or two. These hives may be itchy and are best treated by applying an over the counterhydrocortisone cream. In very rare cases one may develop a full blown allergic reaction. Physicians who perform skin test always have equipment and medications available in case an anaphylaxis reaction occurs. This is the main reason why consumers should not get skin testing performed at corner stores or by people who have no medical training.
Skin testing can be done on individuals of all ages. However, there are times when a skin test should not be done. Individuals who take medications for depression, gastric acidity or antihistamines should not undergo this test. In such cases, stopping the medications for a skin test may not be worthwhile as one may develop symptoms from the untreated medical disorders. Individuals who have severe, generalized skin disease or an acute skin infection should not undergo skin testing. One needs uninvolved skin for testing.
There are some individuals who are highly sensitive to even the smallest amount of allergen and in such scenarios, allergic testing is not recommended. Whenever the chances of an anaphylactic shock are high, the test is best avoided.
Besides skin tests, there are blood tests which measure a specific antibody in the blood. The IgE antibody plays a vital role in allergies but its levels in blood do not always correlate with the allergic reaction.
There are many alternative health care practitioners who perform a variety of provocation neutralization tests, but the vast majority of these tests have no validity and have never been proven to work scientifically.
How the Test is Performed
There are three common methods of allergy skin testing.
The skin prick test involves:
- Placing a small amount of substances that may be causing your symptoms on the skin, most often on the forearm, upper arm, or back.
- The skin is then pricked so the allergen goes under the skin’s surface.
- The health care provider closely watches the skin for swelling and redness or other signs of a reaction. Results are usually seen within 15 to 20 minutes.
- Several allergens can be tested at the same time.
The intradermal skin test involves:
- Injecting a small amount of allergen into the skin.
- The health care provider then watches for a reaction at the site.
- This test is more likely to be used to find out if you are allergic to bee venom or penicillin. Or it may be used if the skin prick test was negative and the provider still thinks that you are allergic to the allergen.
Patch testing is a method to diagnose the cause of skin reactions that occur after the substance touches the skin:
- Possible allergens are taped to the skin for 48 hours.
- The health care provider will look at the area in 72 to 96 hours.
How to Prepare for the Test
Before any allergy testing, the health care provider will ask about:
- Where you live and work
- Foods and eating habits
Allergy medicines can change the results of skin tests. Your doctor will tell you which medicines to avoid and when to stop taking them before the test.
How the Test will Feel
Skin tests may cause very mild discomfort when the skin is pricked.
You may have symptoms such as itching, a stuffy nose, red watery eyes, or a skin rash if you allergic to the substance in the test.
In rare cases, people can have a whole-body allergic reaction (called anaphylaxis), which can be life-threatening. This usually only occurs with intradermal testing. Your health care provider will be prepared to treat this serious response.
Why the Test is Performed
Allergy tests are done to determine what substances are causing your allergy symptoms.
Your doctor may order allergy skin tests if you have:
- Hay fever (allergic rhinitis) and asthma symptoms that are not well controlled with medicine
- Hives and angioedema
- Food allergies
- Skin rashes (dermatitis), in which the skin becomes red, sore, or swollen after contact with the substance
- Penicillin allergy
- Venom allergy
Allergies to penicillin and closely related medicines are the only drug allergies that can be tested using skin tests. Skin tests for allergies to other drugs can be dangerous.
The skin prick test may also be used to diagnose food allergies. 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.
A negative test result means there were no skin changes in response to the allergen. This negative reaction most often means that you are not allergic to the substance.
In rare cases, a person may have a negative allergy test and still be allergic to the substance.
What Abnormal Results Mean
A positive result means you reacted to a substance. Your health care provider will see a red, raised area called a wheal.
Often, a positive result means the symptoms you are having are due to exposure to that substance. In general, a stronger response means you are more sensitive to the substance.
People can have a positive response to a substance with allergy skin testing, but not have any problems with that substance in everyday life.
Skin tests are usually accurate. However, if the dose of allergen is large, even people who are not allergic will have a positive reaction.
Your health care provider will consider your symptoms and the results of your skin test to suggest lifestyle changes you can make to avoid substances that may be causing your symptoms.
The most common side effect of skin testing is slightly swollen, red, itchy bumps (wheals). These wheals may be most noticeable during the test. In some people, though, an area of swelling, redness and itching may develop a few hours after the test and persist for as long as a couple of days.
Rarely, allergy skin tests can produce a severe, immediate allergic reaction, so it’s important to have skin tests performed at an office where appropriate emergency equipment and medications are available.