Wed Jul 23, 2014
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Allergy Tests & Procedures

ALLERGY TESTS

Skin and blood tests are frequently used to determine the likelihood of a specific allergic reaction. To be truly helpful, the results must always be interpreted in the context of the patient's specific clinical history. The most important tool in the diagnosis of an allergy, is the exact details about the reaction that occurred (see new patient forms). A graded challenge is sometimes required to confirm the diagnosis.

SKIN-PRICK TESTING
Skin Prick Testing - UCLA Food & Drug Allergy Care CenterSkin-prick testing is used to identify the presence of a specific allergic antibody (IgE) to a potential trigger. This technique can be used in the diagnosis of environmental allergies, food allergy and some medication allergies.
In general, this is a quick and reliable method to detect the presence of IgE. Results are available in about 20 minutes.  These tests must be used in the appropriate context to be helpful. When skin prick or blood tests are given to people with non-specific symptoms and without proper indications, positive tests are associated with actual food allergy only half the time.

A specialized device that resembles a sharp toothpick is used to introduce extracts of potential allergens into the top layer of skin. The use of "positive" and "negative" controls with histamine and saline is essential to interpret the results. A "positive" test demonstrates that specific IgE is present, and therefore the person is "sensitized" or "at-risk" for allergy.  However, the person is allergic only if he or she has allergic symptoms when contact is made with the specific item. It is, in fact, common to be "sensitized" but not "allergic" to foods and other common proteins. 

Specialized skin prick tests

"Prick to prick" testing is a technique that has been shown to be effective in oral food allergy or pollen-food allergy.  This technique is most helpful when testing fresh fruits or vegetables as the commercial preparation often changes some of the features of trigger elements.  However, precaution should be taken to demonstrate that the food itself is not irritating to the skin.  

Penicillin skin testing
UCLA is one of the few facilities that offers complete and accurate penicillin skin testing to reveal severe acute allergies to penicillin.  The test consists of a series of skin-prick tests to penicillin and its metabolites. If these tests do not indicate the presence of allergic antibody (IgE), a series of intradermal injections is performed to further screen for IgE. Intradermal injections actually place some of the substance under the skin. It is very safe for individuals with a history of penicillin allergy to receive penicillin and related substances if this test is "negative." However, if it is "positive," penicillin and related antibiotics should be avoided, if possible. If the medication is absolutely needed, a carefully supervised desensitization can be performed.


BLOOD TESTS FOR ALLERGY ANTIBODIES
(SPECIFIC IGE FORMERLY KNOWN AS RAST)
Like skin-prick testing, these tests measure specific allergic antibody (IgE) to various proteins. While in the past, skin-prick tests were considered more sensitive, with the current generation of blood tests the tests are equivalent although not identical to skin-prick testing.  The blood tests for specific IgE are also helpful when skin testing cannot be performed. Additionally, the results of the blood tests have been extensively studied in children with food allergy, and these tests may be helpful together with skin tests in the diagnosis and management of food allergy. For some age groups, there are published guidelines that help explain the significance of values for certain foods.

PATCH TESTING
Patch testing is a technique that screens for delayed allergic reactions, (e.g. reactions that resemble poison ivy). It involves applying a sticker-like device to a person's back for 48 hours, and then serial exams of the site for irritation and blistering.

Commerical test for contact dermatitis
Commerically prepared kits allow for screening of delayed skin reactions to various medications, chemicals and metals. These results require expertise to correctly read and interpret the findings.

Custom patch tests
Custom patch testing uses specialized chambers and allows for prolonged application of specific items to the skin, such as personal cosmetics or foods.  This technique has shown promise in the diagnosis of eosinophilic esophagitis and other complex food allergies. However, because there are currently no standardized materials, methods or guidelines for interpretation, this type of testing is not considered part of the standard of care for most conditions.


ALLERGY PROCEDURES

ORAL FOOD CHALLENGE
Oral food challenges are carefully supervised procedures that allow for definitive diagnosis of food allergy when the diagnosis is unclear. They also permit "disease monitoring" to see if an allergy is still active. The location and specifics of the food challenge depend on the exact details of the prior reactions and results of skin-prick tests and blood tests for allergic antibodies. The procedure begins with a tiny amount of the problem food. Throughout the challenge, there is direct supervision and monitoring for early signs of allergic reactions. Reactions are treated with allergy medications just like accidental ingestions. The dose is doubled approximately every 15 minutes until the person undergoing the challenge has a reaction or an entire portion is tolerated without concerns.

DRUG CHALLENGE
Although a drug challenge has a similar name as an oral food challenge, it is a different type of procedure. Graded drug challenges are performed when the likelihood of a drug allergy is low. Desensitization protocols more closely resemble the techniques used in oral food challenges and are described below.

LOCAL ANESTHETIC TESTING
Adverse reactions to local anesthetics such as lidocaine are common. Acute reactions can be very concerning and are frequently related to preservatives or other additives included in the local anesthetic such as epinephrine. For people who have had acute reactions, local anesthetic testing allows for identification of safe alternatives to the problem drug. The challenge involves a series of challenges using the skin-prick test technique followed by a series of intradermal challenge injections with increasing concentrations of the drug. Intradermal injections actually place some of the medication under the skin. Screening for delayed skin irritation and swelling from local anesthetics is best done using the commercially available patch test.

DRUG DESENSITIZATIONS
A desensitization protocol is a very closely supervised, relatively safe way to give a medication to someone who has had an adverse drug reaction that is concern  for anaphylaxis. It involves starting with very tiny doses of the medication, and doubling the dose every 15 minutes until a reaction occurs or the full dose is achieved. Before the medication is increased, vital signs are taken and we monitor for breathing problems. The medication is stopped if there is any sign of allergy, and the reaction is treated. Once the acute problem resolves, the desensitization process is resumed. It is not completely clear why this works, but it seems that the constant stimulation of the allergy cells by this method renders them "unreactive." This process is not a cure, but does allow temporary use of the medication as long as it is given at least once a day. If therapy is interrupted, the process has to be repeated. A typical procedure takes about six hours to complete, and most reactions are mild.

ASPIRIN DESENSITIZATION FOR AERD
Aspirin desensitization for Aspirin Exacerbated Respiratory Disease (AERD) has a unique protocol as this disease is not mediated by allergic antibodies (IgE). Interestingly, although aspirin causes acute respiratory reactions when taken sporadically, after aspirin desensitization, high daily doses significantly improve both asthma and nasal polyps. Aspirin desensitization is an option for people with AERD who continue to have asthma and sinus problems despite taking maximal medications.

This is a two-to-three-day process of giving incrementally increasing doses of aspirin every three hours. Allergic reactions are expected and unpredictable. Various types of serious reactions can occur, and reactions can occur at multiple doses. These include acute throat swelling, skin rashes and swelling, asthma, sinus congestion, abdominal pain and blood pressure problems. These reactions respond slowly but reliably to appropriate medications. Published guidelines describe the medications, environment and staff needed to perform this procedure safely.
 
DESENSITIZATION FOR OTHER TYPES OF REACTIONS TO ASPIRIN
There are several other types of reactions to aspirin or other NSAIDS including anaphylaxis and various skin rashes but they are generally not due to allergic antibodies to aspirin.  Aspirin desensitization may be possible for some of these conditions but these require different specific protocols than AER.. Desensitization protocols do not work for predictable adverse reactions to aspirin such as "tinnitus" (aka ringing in the ears) or easy bleeding or bruising.