What is skin allergy test

Allergy skin tests are used to find out which substances cause a person to have an allergic reaction. During allergy skin tests, your skin is exposed to suspected allergy-causing substances (allergens) and is then observed for signs of an allergic reaction.

Along with your medical history, allergy tests may be able to confirm whether or not a particular substance you touch, breathe or eat is causing symptoms.

Information from allergy tests may help your doctor develop an allergy treatment plan that includes allergen avoidance, medications or allergy shots (immunotherapy).

Allergy 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’re 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 skin testing. Your doctor will be prepared to treat this serious response.

Allergy skin tests are generally safe for adults and children of all ages, including infants. In certain circumstances, though, skin tests aren’t recommended. Your doctor may advise against skin testing if you:

• Have ever had a severe allergic reaction. You may be so sensitive to certain substances that even the tiny amounts used in skin tests could trigger a life-threatening reaction (anaphylaxis).
• Take medications that could interfere with test results. These include antihistamines, many antidepressants and some heartburn medications. Your doctor may determine that it’s better for you to continue taking these medications than to temporarily discontinue them in preparation for a skin test.
• Have certain skin conditions. If severe eczema or psoriasis affects large areas of skin on your arms and back — the usual testing sites — there may not be enough clear, uninvolved skin to do an effective test. Other skin conditions, such as dermatographism, can cause unreliable test results.

Blood tests (in vitro immunoglobulin E antibody tests) can be useful for those who shouldn’t undergo skin tests. Blood tests aren’t done as often as skin tests because they can be less sensitive than skin tests and are more expensive.

In general, allergy skin tests are most reliable for diagnosing allergies to airborne substances, such as pollen, pet dander and dust mites. Skin testing may help diagnose food allergies. But because food allergies can be complex, you may need additional tests or procedures.

Reasons for skin allergy test

Allergy tests are done to find out which 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 or eczema), in which the skin becomes red, sore, or swollen after contact with the substance
• Penicillin allergy
• Insect venom allergy
• Latex allergy

Allergies to penicillin and 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 allergy skin tests are not used to test for food allergies because of high false-positive results and the danger of causing a severe allergic reaction.

How to prepare for your skin allergy test

Before recommending a skin allergy test, your doctor will ask you detailed questions about your medical history, your signs and symptoms, and your usual way of treating them.

Before any allergy testing, your doctor will ask about:

• Illnesses
• Where you live and work
• Lifestyle
• Foods and eating habits

Your answers can help your doctor determine if allergies run in your family and if an allergic reaction is most likely causing your symptoms. Your doctor may also perform a physical examination to search for additional clues about the cause of your signs and symptoms.

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.

Medications can interfere with skin allergy test results

Before scheduling a skin allergy test, bring your doctor a list of all of your prescription and over-the-counter medications. Some medications can suppress allergic reactions, preventing the skin testing from giving accurate results. Other medications may increase your risk of developing a severe allergic reaction during a test.

Because medications clear out of your system at different rates, your doctor may ask that you stop taking certain medications for up to 10 days. Medications that can interfere with skin tests include:

• Prescription antihistamines, such as levocetirizine (Xyzal) and desloratadine (Clarinex).
• Over-the-counter antihistamines, such as loratadine (Claritin, Alavert), diphenhydramine (Benadryl, others), chlorpheniramine (Chlor-Trimeton), cetirizine (Zyrtec, others) and fexofenadine (Allegra).
• Tricyclic antidepressants, such as nortriptyline (Pamelor) and desipramine (Norpramin).
• Certain heartburn medications, such as cimetidine (Tagamet) and ranitidine (Zantac).
• The asthma medication omalizumab (Xolair). This medication can disrupt test results for six months or longer even after you quit using it (most medications affect results for days to weeks).

How the skin allergy test is performed

Allergy skin testing is usually done at a doctor’s office. A nurse generally administers the test, and a doctor interprets the results. Typically, allergy skin test takes about 20 to 40 minutes. Some allergy skin tests detect immediate allergic reactions, which develop within minutes of exposure to an allergen. Other allergy skin tests detect delayed allergic reactions, which develop over a period of several days.

There are three common methods of allergy skin testing.

Skin prick test

A skin prick test, also called a puncture or scratch test, checks for immediate allergic reactions to as many as 40 different substances at once. This test is usually done to identify allergies to pollen, mold, pet dander, dust mites and foods. In adults, the test is usually done on the forearm. Children may be tested on the upper back.

Allergy skin tests aren’t painful. This type of testing uses needles (lancets) that barely penetrate the skin’s surface. You won’t bleed or feel more than mild, momentary discomfort.

After cleaning the test site with alcohol, the nurse draws small marks on your skin and applies a drop of allergen extract next to each mark. He or she then uses a lancet to prick the extracts into the skin’s surface. A new lancet is used for each allergen.

The skin prick test involves:

1. Placing a small amount of substances that may be causing your symptoms on the skin, most often on the forearm, upper arm, or back.
2. The skin is then pricked so the allergen goes under the skin’s surface.
3. Your 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.
4. Several allergens can be tested at the same time. Allergens are substances that cause an allergic reaction.

To see if your skin is reacting normally, two additional substances are scratched into your skin’s surface:

• Histamine. In most people, this substance causes a skin response. If you don’t react to histamine, your allergy skin test may not reveal an allergy even if you have one.
• Glycerin or saline. In most people, these substances don’t cause any reaction. If you do react to glycerin or saline, you may have sensitive skin. Test results will need to be interpreted cautiously to avoid a false allergy diagnosis.

About 15 minutes after the skin pricks, the nurse observes your skin for signs of allergic reactions. If you are allergic to one of the substances tested, you’ll develop a raised, red, itchy bump (wheal) that may look like a mosquito bite. A nurse will then measure the bump’s size.

After the nurse records the results, he or she will clean your skin with alcohol to remove the marks.

Intradermal skin test (skin injection test)

You may need a test that uses a needle to inject a small amount of allergen extract just into the skin on 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.

The intradermal skin test involves:

1. Injecting a small amount of allergen into the skin.
2. Your health care provider then watches for a reaction at the site.
3. This test is more likely to be used to find out if you’re allergic to bee venom or penicillin. Or it may be used if the skin prick test was negative and your doctor still thinks that you’re allergic to the allergen.

Patch test

Patch testing is generally done to see whether a particular substance is causing allergic skin irritation (contact dermatitis). Patch tests can detect delayed allergic reactions, which can take several days to develop.

Patch tests don’t use needles. Instead, allergens are applied to patches, which are then placed on your skin. During a patch test, your skin may be exposed to 20 to 30 extracts of substances that can cause contact dermatitis. These can include latex, medications, fragrances, preservatives, hair dyes, metals and resins.

You wear the patches on your arm or back for 48 hours. During this time, you should avoid bathing and activities that cause heavy sweating. The patches are removed when you return to your doctor’s office. Irritated skin at the patch site may indicate an allergy.

Patch testing is a method to diagnose the cause of skin reactions that occur after the substance touches the skin:

1. Possible allergens are taped to the skin for 48 hours.
2. Your health care provider will look at the area in 72 to 96 hours.

Allergy blood test vs skin test

Allergy blood tests are used to find out if you have an allergy. One type of allergy blood test called a total immunoglobulin E (IgE) test measures the overall number of immunoglobulin E (IgE) antibodies in your blood. Another type of allergy blood test called a specific IgE test measures the level of IgE antibodies in response to individual allergens.

In allergy blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

What do the allergy blood test results mean?

If your total immunoglobulin E (IgE) levels are higher than normal, it likely means you have some kind of allergy. But it does not reveal what you are allergic to. A specific immunoglobulin E (IgE) test will help identify your particular allergy. If your results indicate an allergy, your health care provider may refer you to an allergy specialist or recommend a treatment plan.

Your treatment plan will depend on the type and severity of your allergy. People at risk for anaphylactic shock, a severe allergic reaction that can cause death, need to take extra care to avoid the allergy-causing substance. They may need to carry an emergency epinephrine treatment with them at all times.

Be sure to talk to your health care provider if you have questions about your test results and/or your allergy treatment plan.

Allergy skin test results interpretation

Before you leave your doctor’s office, you’ll know the results of a skin prick test or an intradermal test. A patch test may take several days or more to produce results. For skin prick tests to be informative, they must be interpreted in conjunction with the patient’s history and physical examination. Your doctor must also be aware of the many reasons for a false-positive and false-negative reaction to properly interpret skin allergy test results.

Normal results

A negative allergy skin 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.

Abnormal results

A positive allergy skin test result means you reacted to a substance, that you may be allergic to a particular substance. Your doctor will see a red, raised area called a wheal. Bigger wheals usually indicate a greater degree of sensitivity. A negative skin test means that you probably aren’t allergic to a particular allergen.

Often, a positive allergy skin test result means the symptoms you’re having are due to exposure to that substance. A stronger response means you are likely more sensitive to the substance.

Table 1. Allergy skin test results chart

Footnote: There are a couple of grading scales used but the size of the wheal is most accurate. The size of the wheal does not indicate the severity of the symptoms but shows your doctor the degree of sensitivity to the allergen.

People can have a positive response to a substance with allergy skin testing, but not have any problems with that substance in everyday life.

Keep in mind, skin allergy tests aren’t always accurate. They sometimes indicate an allergy when there isn’t one (false positive), or skin testing may not trigger a reaction when you’re exposed to something that you are allergic to (false negative). You may react differently to the same test performed on different occasions. Or you may react positively to a substance during a test but not react to it in everyday life.

Your doctor 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.

Your allergy treatment plan may include medications, immunotherapy, changes to your work or home environment, or dietary changes. Ask your doctor to explain anything about your diagnosis or treatment that you don’t understand. With test results that identify your allergens and a treatment plan to help you take control, you’ll be able to reduce or eliminate allergy signs and symptoms.

Common problems with skin prick testing

One of the common errors in skin prick testing is placing the tests too close together (< 2 cm apart) so that spreading of allergen solutions between test sites occurs.

There are many reasons that cause a false-positive or false-negative skin prick test result.

Causes of a false-positive skin allergy test result

• A positive reaction from one test site may affect the result of a neighboring test site (place test sites at least 2 cm apart)
• Irritant reaction

Causes of a false-negative skin allergy test result

• Medications such as antihistamines that block the effect of histamine (advise patients to stop taking medication at least 72 hours prior to skin testing)
• Decreased reactivity of the skin in infants and elderly patients
• Allergen extract too diluted (especially with foods)

Some people are surprised by negative test reactions when they have been fairly sure a particular plant has caused their symptoms. This may be because they are allergic to another material. For example, privet is commonly blamed for hay fever, but in fact is a rare cause. Instead, symptoms may be caused by ryegrass, which flowers at the same time as privet. In other instances, rhinitis is due to irritants such as pine pollen, not allergens.

Allergy skin test side effects

The most common side effects of skin allergy 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.