Allergy testing

 

An allergy testing machine being operated in the diagnostic immunology lab at Lackland Air Force Base

Before a diagnosis of allergic disease can be confirmed, the other possible causes of the presenting symptoms should be carefully considered. Vasomotor rhinitis, for example, is one of the many maladies that shares symptoms with allergic rhinitis, underscoring the need for professional differential diagnosis. Once a diagnosis of asthma, rhinitis, anaphylaxis, or other allergic disease has been made, there are several methods for discovering the causative agent of that allergy.

Effective management of allergic diseases relies on the ability to make an accurate diagnosis. Allergy testing can help confirm/rule out allergies and consequently reduce adverse reactions and limit unnecessary avoidance and medications. Correct diagnosis, counseling and avoidance advice based on valid allergy test results will help reduce the incidence of symptoms, medications and improve quality of life. For assessing the presence of allergen-specific IgE antibodies, you can use two different methods—a skin prick test or an allergy blood test. Both methods are recommended by the NIH guidelines and have similar diagnostic value in terms of sensitivity and specificity.



A health care provider can use the test results to identify the specific allergic triggers that may be contributing to the symptoms. Using this information, along with a physical examination and case history, the doctor can diagnose the cause of the symptoms and tailor treatments that will help the patient feel better. A negative result can help the doctor rule out allergies in order to consider other possibilities.

NIH Guidelines state that: “sIgE tests are useful for identifying foods potentially provoking IgE-mediated food-induced allergic reactions, and specified ‘‘cutoff’’ levels, defined as 95% predictive values, may be more predictive than skin prick tests of clinical reactivity in certain populations.” It further states, “sIgE tests are very useful for detecting the presence of sIgE antibodies, which indicates the presence of allergic sensitization. Fluorescence-labeled antibody assays have comparable sensitivity to that of skin prick tests, and the absolute levels of sIgE antibodies may directly correlate with the likelihood of clinical reactivity when compared with oral food challenges for the identification of foods provoking IgE mediated FA.”

According to NICE Guidelines, skin prick tests and blood tests are equally cost-effective and health economic evidence show that both the IgE antibody test and the skin prick test were cost effective compared with no test. Also, earlier and more accurate diagnoses save cost due to reduced GP consultations, referrals to secondary care, misdiagnosis and emergency admissions.

Allergy undergoes dynamic changes over time. Regular allergy testing of relevant allergens provides information on if and how patient management can be changed, in order to improve health and quality of life. Annual testing is often the practice for determining whether allergy to milk, egg, soy, and wheat have been outgrown and the testing interval is extended to 2 to 3 years for allergy to peanut, tree nuts, fish, and crustacean shellfish. Results of follow-up testing can guide decision-making regarding whether and when it is safe to introduce or re-introduce allergenic food into the diet.



Skin testing

Skin testing is also known as “puncture testing” and “prick testing” due to the series of tiny puncture or pricks made into the patient’s skin. Small amounts of suspected allergens and/or their extracts (pollen, grass, mite proteins, peanut extract, etc.) are introduced to sites on the skin marked with pen or dye (the ink/dye should be carefully selected, lest it cause an allergic response itself). A small plastic or metal device is used to puncture or prick the skin. Sometimes, the allergens are injected “intradermally” into the patient’s skin, with a needle and syringe. Common areas for testing include the inside forearm and the back. If the patient is allergic to the substance, then a visible inflammatory reaction will usually occur within 30 minutes. This response will range from slight reddening of the skin to a full-blown hive (called “wheal and flare”) in more sensitive patients similar to a mosquito bite. Interpretation of the results of the skin prick test is normally done by allergists on a scale of severity, with +/- meaning borderline reactivity, and 4+ being a large reaction. Increasingly, allergists are measuring and recording the diameter of the wheal and flare reaction. Interpretation by well-trained allergists is often guided by relevant

literature. Some patients may believe they have determined their own allergic sensitivity from observation, but a skin test has been shown to be much better than patient observation to detect allergy.

If a serious life threatening anaphylactic reaction has brought a patient in for evaluation, some allergists will prefer an initial blood test prior to performing the skin prick test. Skin tests may not be an option if the patient has widespread skin disease or has taken antihistamines sometime the last several days.

Blood testing

An allergy blood test is quick and simple and can be ordered by a licensed health care provider e.g. an allergy specialist, GP or PED. Unlike skin-prick testing, a blood test can be performed irrespective of age, skin condition, medication, symptom, disease activity and pregnancy. Adults and children of any age can take an allergy blood test. For babies and very young children, a single needle stick for allergy blood testing is often more gentle than several skin tests.

An allergy blood test is available through most laboratories, and a sample of the patient’s blood is sent to a laboratory for analysis and the results are sent back a few days later. Multiple allergens can be detected with a single blood sample.

Allergy blood tests are very safe, since you are not exposed to any allergens during the testing procedure.

How does the test work?

The test measures the concentration of specific IgE antibodies in the blood. Quantitative IgE test results increases the possibility of ranking how different substances may affect your symptoms. A general rule of thumb is that the higher the IgE antibody value, the greater the likelihood of symptoms. Allergens found at low levels that today does not result in symptoms can nevertheless help predict future symptom development. The quantitative allergy blood result can help determine what a patient is allergic to, help predict and follow the disease development, estimate the risk of a severe reaction and explain cross-reactivity.

A low total IgE level is not adequate to rule out sensitization to commonly inhaled allergens. Statistical methods, such as ROC curves, predictive value calculations, and likelihood ratios have been used to examine the relationship of various testing methods to each other. These methods have shown that patients with a high total IgE have a high probability of allergic sensitization, but further investigation with allergy tests for specific IgE antibodies for a carefully chosen of allergens is often warranted.

History

Radiometric assays include the radioallergosorbent test (RAST) test method, which uses IgE-binding (anti-IgE) antibodies labeled with radioactive isotopes for quantifying the levels of IgE antibody in the blood. Other newer methods use colorimetric or fluorescence-labeled technology in the place of radioactive isotopes.

The market-leading RAST methodology was invented and marketed in 1974 by Pharmacia Diagnostics AB, Uppsala, Sweden, and the acronym RAST is actually a brand name. In 1989, Pharmacia Diagnostics AB replaced it with a superior test named the ImmunoCAP Specific IgE blood test, which uses the newer fluorescence-labeled technology. American College of Allergy Asthma and Immunology (ACAAI) and the American Academy of Allergy Asthma and Immunology (AAAAI) issued the Joint Task Force Report “Pearls and pitfalls of allergy diagnostic testing” in 2008, and is firm in its statement that the term RAST is now obsolete:

“The term RAST became a colloquialism for all varieties of (in vitro allergy) tests. This is unfortunate because it is well recognized that there are well-performing tests and some that do not perform so well, yet they are all called RASTs, making it difficult to distinguish which is which. For these reasons, it is now recommended that use of RAST as a generic descriptor of these tests be abandoned.” The new version, the ImmunoCAP Specific IgE blood test, is the only specific IgE assay to receive FDA approval to quantitatively report to its detection limit of 0.1kU/l.

Other

Challenge testing: Challenge testing is when small amounts of a suspected allergen are introduced to the body orally, through inhalation, or other routes. Except for testing food and medication allergies, challenges are rarely performed. When this type of testing is chosen, it must be closely supervised by an allergist.

Elimination/Challenge tests: This testing method is utilized most often with foods or medicines. A patient with a particular suspected allergen is instructed to modify his/her diet to totally avoid that allergen for determined period of time. If the patient experiences significant improvement, he/she may then be “challenged” by reintroducing the allergen to see if symptoms can be reproduced.

Patch testing: Patch testing is used to help ascertain the cause of skin contact allergy, or contact dermatitis. Adhesive patches, usually treated with a number of different commonly allergic chemicals or skin sensitizers, are applied to the back. The skin is then examined for possible local reactions at least twice, usually at 48 hours after application of the patch, and again two or three days later.

Some “screening” test methods are intended to provide qualitative test results, giving a “yes” or “no” answer in patients with suspected allergic sensitization. One such method has a sensitivity of about 70.8% and a positive predictive value of 72.6% according to a large study.

Unreliable tests: There are other types of allergy testing methods that the American Academy of Allergy, Asthma, and Immunology considers to be unacceptable.

These unreliable allergy testing methods are:

Applied kinesiology (allergy testing through muscle relaxation), Cytotoxicity testing, Urine autoinjection, Skin titration (Rinkel method), and Provocative and neutralization (subcutaneous) testing or sublingual provocation.

 

Pathophysiology