Before recommending a skin test, you will be asked detailed questions about your medical history, your symptoms and your usual way of treating them. Your answers can help determine if allergies run in your family and if an allergic reaction is most likely causing your symptoms. Your healthcare professional also may do a physical examination to search for more clues about the cause of your symptoms.

Medicines can interfere with results

Before scheduling a skin test, bring your healthcare professional a list of all of your prescription and nonprescription medicines. Some medicines can suppress allergic reactions, preventing the skin testing from giving accurate results. Other medicines may increase your risk of developing a severe allergic reaction during a test.

Because medicines clear out of your system at different rates, you may be asked to stop taking certain medicines for up to 10 days. Medicines that can affect the results of skin tests include:

• Prescription antihistamines, such as hydroxyzine (Vistaril).
• Nonprescription antihistamines, such as loratadine (Claritin, Alavert), diphenhydramine, chlorpheniramine, cetirizine (Zyrtec Allergy) and fexofenadine.
• Tricyclic antidepressants, such as nortriptyline (Pamelor) and desipramine (Norpramin).
• Certain heartburn medicines, such as cimetidine and ranitidine.
• The asthma medicine omalizumab (Xolair). This medicine can disrupt test results for six months or longer even after you quit using it. For comparison, most medicines affect results for days to weeks.

During allergy skin tests, the skin is exposed to suspected allergy-causing substances, called allergens, and then checked for signs of an allergic reaction.

Along with a medical history, allergy tests may be able to confirm whether a particular substance a person touches, breathes or eats is causing symptoms.

Before you leave the medical 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 get results.

A positive skin test means that you may be allergic to a particular substance. Bigger wheals usually mean a greater degree of sensitivity. A negative skin test means that you probably aren't allergic to a particular allergen.

Keep in mind, skin tests aren't always accurate. They sometimes show an allergy when there isn't one. This is called a false-positive. In some cases, skin testing may not trigger a reaction when you're exposed to something that you are allergic to, called a 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 allergy treatment plan may include medicines, immunotherapy, changes to your work or home environment, or dietary changes. Ask your allergy specialist 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 get rid of allergy symptoms.

The most common side effect of skin testing is slightly swollen, red, itchy bumps, called 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 remain for a couple of days.

Rarely, allergy skin tests can produce a severe, immediate allergic reaction. For this reason, it's important to have skin tests performed at an office where appropriate emergency equipment and medicines are available.

Information from allergy tests may help a healthcare professional develop an allergy treatment plan that includes allergen avoidance, medicines or allergy shots, called immunotherapy.

Allergy skin tests are widely used to help diagnose allergic conditions, including:

• Hay fever, also called allergic rhinitis.
• Allergic asthma.
• Dermatitis, called eczema.
• Food allergies.
• Penicillin allergy.
• Bee venom allergy.

Skin tests are generally safe for adults and children of all ages, including infants. In certain circumstances, though, skin tests aren't recommended. A healthcare professional 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, known as anaphylaxis.
• Take medicines that could interfere with test results. These include antihistamines, many antidepressants and some heartburn medicines. Your care professional may determine that it's better for you to continue taking these medicines 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 known as in vitro immunoglobulin E antibody tests can be useful for those who shouldn't or can't undergo skin tests. Blood tests aren't used for penicillin allergy.

In general, allergy skin tests are 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.