Allergy Skin Tests

Allergy Skin Tests

Also called: Intradermal Test, Skin Test, Epicutaneous Test

Summary

Allergy skin tests are used to identify the allergens that trigger a patient’s allergic reactions. There are three primary skin tests:

  • Scratch or prick test. Tiny amounts of allergens are placed on the skin, and a series of tiny scratches or needle pricks force the allergens into the skin.
  • Intradermal test. Tiny amounts of an allergen are injected just below the skin.
  • Patch test. An absorbent pad is soaked in an allergen and taped onto the skin of the patient for 24 to 72 hours.

During each of these tests, one or more allergens are introduced to the patient’s skin. For prick, scratch or intradermal tests, the healthcare provider administering the test waits 10 to 30 minutes to see if a local wheal, or hive, develops. For a patch test, the patient will return after 24 to 72 hours to be examined for a reaction. If a positive skin reaction to that allergen is detected, an allergy is likely.

Allergy skin tests are safe, fast, accurate and relatively inexpensive. In most cases, they cause little pain or discomfort. After a patient’s allergy has been diagnosed, a physician will recommend a treatment plan to reduce or eliminate symptoms.

About allergy skin tests

Allergy skin tests are a safe, fast, painless, accurate and relatively inexpensive method to identify the allergens that trigger a patient’s allergy symptoms. Identifying the specific allergens behind a person’s symptoms is important because it helps physicians create a treatment plan aimed at managing or eliminating allergy symptoms.

A patient can be tested for up to 30 allergies at the same time. However, a physician usually will test only the suspected symptom triggers. Typically, allergy skin tests are used to uncover reactions to the following potential allergens:

  • Tree, shrub, weed and grass pollens
  • Molds
  • Household dust and dust mites
  • Animal dander and feathers
  • Foods (skin testing for food allergies is not always as accurate and some foods may respond better to skin testing than others)
  • Medications
  • Insect venom

Allergy skin tests can be safely performed at any age. However, they may not be as sensitive in children under the age of 3 and adults over the age of 60. In addition, people with the following health conditions may yield less accurate results:

  • Severe eczema
  • Psoriasis
  • Chronic renal (kidney) failure
  • Cancer
  • Spinal cord injuries
  • Peripheral neuropathy (failure of the nerves that carry information to and from the brain and spinal cord)

In addition, patients on some types of medication (especially antihistamines) may also yield less accurate results.

For these individuals, a blood allergy test known as a radioallergosorbent test (RAST) may provide a better alternative for diagnosing allergies. During an allergic reaction, the body produces immunoglobulin E (IgE) antibodies to protect against what it mistakenly perceives to be a dangerous substance. The RAST test measures the amount of specific IgE produced to an individual allergen in a sample of blood.

Types and differences of allergy skin tests

There are three primary methods of allergy skin tests. All are effective at diagnosing a patient’s allergies, but each has its own distinct advantages and disadvantages. These tests are:

  • Prick test/scratch test. Tiny amounts of different allergens are placed on the skin, and a series of tiny scratches or needle pricks force the allergens into the skin. This epicutaneous test is usually the first type of skin test attempted, because it is highly unlikely to trigger anaphylaxis, a potentially dangerous reaction. Both prick tests and scratch tests are very good at identifying specific allergies. However, they are not as sensitive as intradermal tests and sometimes show no reaction to an allergen even when an allergy is present.

  • Intradermal test. Tiny amounts of an allergen are injected just below the skin. Intradermal tests are more sensitive, but less specific, than prick or scratch tests. They sometimes are done when a skin prick test does not reveal an allergy, but the allergy nonetheless is still suspected.

  • Patch test. An absorbent pad is soaked in an allergen and taped onto the skin of the patient for 24 to 72 hours. A patch test generally is used to identify allergic contact dermatitis, a rash that results from direct skin contact with an allergen. The chemicals used in these tests often come from a kit containing the 20 most common causes of contact dermatitis. These include substances used in jewelry, hair dyes, shampoos, medicinal creams, clothing, glues and cosmetics.

As part of the skin testing process, physicians perform control tests that help to determine how accurate skin testing will be on the patient. These control substances are administered in the same manner as the allergen (e.g., prick, scratch, injection, patch) and should produce a predictable result. Two control substances commonly used are:

  • Histamine (used in a positive control test). Patients should automatically have a reaction to the histamine – if they do not, allergy tests are unlikely to reveal interpretable results. A positive control test is not used for patch testing.

  • Saline (used in a negative control test). Saline should not provoke any reaction. If it does, the patient likely has very sensitive skin, and any results from the allergy testing may be less conclusive.

Before the allergy skin test

Prior to the scheduling of allergy testing, the patient will undergo a full physical examination. As part of this process, the physician will take a complete medical history and ask the patient for specific details about the patient’s past allergic reactions. It is crucial that patients explain in as much detail as possible the particulars of their symptoms – when they occur and under what circumstances – as well as identify possible allergens that may have been encountered at the time of the reactions.  

Patients scheduled to undergo allergy skin testing need minimal preparation before the test. However, it is crucial that patients inform a physician of any medications they are taking. Medications that can interfere with test results include:

  • Antihistamines. Antihistamines can prevent allergic reactions, thus potentially skewing test results. Antihistamines block the work of histamine, a chemical released during allergic reactions. Histamine contributes to symptoms such as hives, sneezing, runny nose and itchy eyes. Antihistamines attach to cell receptors – known as H1, H2 and H3 – and prevent histamine from doing the same. The histamine (which primarily does its work at the H1 receptor) is blocked, short-circuiting the allergic process. All over-the-counter antihistamines should be avoided for at least 48 hours before the test, while prescription antihistamines should be discontinued for five to seven days before the test (or as directed by the physician).

  • Tricyclic antidepressants. These can cause the skin to become more sensitive to light than normal and may therefore impact the accuracy of allergy skin test results. They also may have strong anti-H1 and H2 activity. Tricyclic antidepressants are used in the treatment of depression, obsessive compulsive disorder, bedwetting and other conditions.

  • H2 antagonists. These also block the release of histamine and can therefore skew test results.

  • Corticosteroid creams. These can suppress reactions in the areas they are applied.

  • Tranquilizers (including sleep medications).

  • Anti-emetics (used to treat nausea and vomiting).

Patients should not stop taking any medication without first consulting a physician. In some cases, it is not prudent to stop taking the medication. A physician is then likely to perform a control test that will indicate if the particular drug interferes with a skin test. Two control substances – histamine (used in a positive control test) and saline (used in a negative control test) – are tested on the patient. The control substances help a physician to determine how valid the skin test is likely to be. 

During and after the allergy skin test

The entire test procedure itself is likely to take less than one hour from the time patients arrive at the testing facility until the time they leave.

First, a physician or nurse examines the site to be tested and cleans it with alcohol. Most physicians use the back of the arm or the forearm for testing. The back is actually more reactive and offers a larger testing area, but it is difficult for patients to see the reaction in that location.

Each area of the skin at the test site is marked by pen with the identity of the allergen to be tested. A tiny amount of the allergen is prepared for introduction to the patient’s skin. In addition, two control substances also are prepared – histamine is used as a positive control test and saline is used as a negative control test.

Control substances help a physician to determine how valid the skin test is likely to be.  Patients should automatically have a reaction to the histamine – if they do not, allergy tests are unlikely to reveal interpretable results. Meanwhile, the saline should not provoke any reaction. If it does, the patient likely has very sensitive skin and any results from the allergy testing are likely to be less conclusive. 

Once the allergens and control substances have been prepared, they are introduced to the skin. This will be accomplished in different ways, depending on the type of test:

  • Scratch or prick test. Drops of solutions containing allergens are placed on the skin at least 1 inch (2.5 centimeters) apart. Then, a device called a lancet is used to create a series of tiny scratches or needle pricks that push these substances into the skin. This procedure may cause minor discomfort, but is not generally considered to cause pain.

  • Intradermal test. Allergens are injected with a needle just below the skin at least 2 inches (5 centimeters) apart. The needle itself is very thin and does not cause the same pain associated with deep, muscular injections that people usually associate with injections of medicines.

  • Patch test. Absorbent pads are soaked in the allergens and taped onto the skin of the patient for 24 to 72 hours.

After the allergens and control substances have been introduced, patients are monitored for potential reactions for a period of 10 to 30 minutes. With scratch, prick or interdermal testing, results can be known at this time.

For patch testing, the patient will have to leave the facility and return after a couple of days before results of the testing can be examined. Patients should avoid washing the test area or immersing it in water. They also should avoid activities that cause excessive perspiration.

In general, skin rashes at the testing site indicate an allergy. If a wheal appears at the site where a substance was introduced, the patient is very likely producing antibodies to the allergen and therefore is allergic to the substance. The size of the wheal may be compared with any wheals produced by the control substances to confirm the allergy diagnosis.

For instance, a physician may diagnose an allergy if a wheal, provoked by a prick, scratch or patch test, is both:

  • Bigger than any wheal that formed for the negative control substance (saline)

  • Substantially bigger than any wheal formed for the positive control substance (histamine)

Intradermal tests are very sensitive and therefore have a higher standard of comparison to controls. Though interpretation varies, an allergy diagnosis in intradermal testing is generally made if a wheal produced by an allergen is both:

  • As big or bigger than a wheal provoked by the negative control substance (saline)

  • Substantially bigger than a wheal produced by the positive control substance (histamine)

After testing, the test site will be cleaned with alcohol. A mild cortisone cream may be applied to relieve any potential itching at the site. Patients should keep the area uncovered for a few hours afterward. Local swelling may be treated with cold packs.

Potential risks with allergy skin tests

The biggest risk with allergy skin tests is the possibility of anaphylactic shock, a potentially life-threatening condition.

Anaphylaxis is a rare, severe allergic reaction that involves two or more body systems. It often begins immediately after an individual has been exposed to a particular allergen. Without prompt emergency treatment, anaphylaxis may progress to anaphylactic shock, which is characterized by difficulty breathing and a dangerous drop in blood pressure. An emergency dose of epinephrine (adrenaline) is necessary to reverse these symptoms.

Anaphylaxis is rare in all allergy skin testing, though the incidence is slightly higher in intradermal testing. However, the potential for anaphylaxis – albeit slight – dictates that allergy skin tests should never be attempted except under the strict monitoring of a physician.

In some cases, allergy skin tests have provoked milder reactions as long as a day after testing. Patients should call their physician if any of the following symptoms develop:

  • Fever
  • Dizziness or lightheadedness
  • Wheezing
  • Shortness of breath
  • Severe swelling or itching

It is not unusual for patients to feel itching, tenderness or swelling at the testing site. These symptoms usually subside within minutes to hours. A nonprescription steroid cream such as cortisone may help relieve these symptoms. Applying cool cloths can also provide relief.

Treatments that may follow allergy skin tests

Once allergy skin testing has revealed the exact nature and severity of a patient’s allergy, a physician will devise a treatment plan. In cases of very mild allergies, the patient may be urged to make certain lifestyle modifications that may be effective in preventing symptoms on a day-to-day basis. For instance, people with pollen allergies may be advised to avoid outdoor activities when pollen counts are high.

Over-the-counter antihistamines or decongestants also might be suggested as a means of treating minor flare-ups. Those with moderate to severe allergies may be given prescription antihistamines, decongestants, steroid nasal sprays and leukotriene modifiers.  

Finally, if all other methods fail to adequately treat the problem, allergy shots (immunotherapy) may be recommended. In this treatment, a small dose of the offending allergen is injected under a patient’s skin on a periodic basis.  In many cases, the patient’s tolerance level to these allergens rises dramatically, causing a significant reduction in symptoms.

Questions for your doctor

Preparing questions in advance can help patients to have more meaningful discussions with their physicians. Patients may wish to ask their doctor the following questions about allergy skin tests:

  1. What type of allergy skin test will you use on me?
  2. Are there special steps I should take on the day of my allergy skin test?
  3. Will I experience pain during or after the test?
  4. Do I face any risks by undergoing an allergy skin test?
  5. How accurate is the test I am undergoing?
  6. When will the results of my test be available?
  7. What do my test results indicate?
  8. Will I need additional tests to confirm the diagnosis?
  9. Now that I have been diagnosed, what are my treatment options?
  10. Do you recommend that my children undergo allergy skin testing as well?
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