Também conhecido como
RAST test
Allergy screen
Nome formal
Allergen-specific IgE antibody test
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em
25 de Abril de 2018.
At a Glance
Why Get Tested?
To screen for allergies; sometimes to monitor the effectiveness of immunotherapy (desensitization) treatment
When To Get Tested?
When you have symptoms such as hives, dermatitis, rhinitis (nasal congestion), red itchy eyes, asthma, or abdominal pain that your doctor suspects may be caused by an allergy
Sample Required?
A blood sample drawn from a vein in your arm
What is being tested?
Immunoglobulin E (IgE) is a protein associated with allergic reactions; it is normally found in very small amounts in the blood. IgE is an antibody that functions as part of the body’s immune system, its defense against “intruders.” When someone with a predisposition to allergies is exposed to a potential allergen (such as food, grass, or animal dander) for the first time, they become sensitized. Their body perceives the potential allergen as a foreign substance and produces a specific IgE antibody that binds to mast cells (specialized cells in your tissues) and basophils (a type of white blood cell) in your blood stream. The mast cells are found in tissues throughout your body but are highest in concentration in your skin, respiratory system, and gastrointestinal tract. With the next exposure, these attached IgE antibodies recognize the allergen and cause the mast and basophil cells to release histamine and other chemicals, resulting in an allergic reaction that begins at the exposure site.

The allergen-specific IgE antibody test is used to screen for an allergy to a specific allergen. It measures the amount of that suspected IgE antibody in the blood. Each selection is one separate test, and the tests are very specific: honeybee versus bumblebee, egg white versus egg yolk, giant ragweed versus western ragweed. Groupings of these tests, such as food panels or regional weed, grass, and mold panels, can be done. Alternatively, you and your doctor may pick and choose selectively from a long list of individual allergens suspected of causing your allergies.

The allergen-specific IgE test can be done using a variety of methods. The traditional method that has been used is the RAST (radioallergosorbent test), but it has been largely replaced in most laboratories with the newer IgE-specific immunoassay method. Some doctors refer to all IgE allergy tests as RAST even though this is a specific methodology and may not be the exact assay that the testing lab is using.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.
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Common Questions
  • How is it used?
    The allergen-specific IgE antibody test is done to screen for an allergy (a type I hypersensitivity) to a specific substance or substances when a patient presents with acute or chronic allergy-like symptoms.

    The allergen-specific IgE antibody test may be done (instead of other medically supervised allergy testing) when the patient has significant dermatitis or eczema (also a sign of allergies), is taking necessary histamines or anti-depressants that would make other testing more difficult, or if a dangerous allergic reaction could be expected to follow another test.

    The allergen-specific IgE antibody test may also be done to monitor immunotherapy or to see if a child has outgrown an allergy, although it can only be used in a general way; the level of IgE present does not correlate to the severity of an allergic reaction, and someone who has outgrown an allergy may have a positive IgE for many years afterward.
  • When is it ordered?
    The allergen-specific IgE antibody test is usually ordered when you have signs or symptoms that suggest that you have an allergy to one or more substances.
  • What does the test result mean?
    Normal negative results indicate that you probably do not have a “true allergy,” an IgE-mediated response to that specific allergen, but the results of allergen-specific IgE antibody tests must always be interpreted and used with caution and the advice of your doctor. Even if your IgE test is negative, there is still a small chance that you do have an allergy.

    Elevated results usually indicate an allergy, but even if your specific IgE test was positive, you may or may not ever have an actual physical allergic reaction when exposed to that substance. And the amount of specific IgE present does not necessarily predict the potential severity of a reaction. Your clinical history and other allergy tests, done under close medical supervision, may be necessary to confirm an allergy diagnosis.
  • Is there anything else I should know?
    Sometimes your doctor will look at other blood tests for an indirect indication of an ongoing allergic process, including your total IgE level or your complete blood count (CBC) and white blood cell differential (specifically at your eosinophils and basophils). Elevations in these tests may suggest an allergy, but they may also be elevated for other reasons.
  • Que outros exames podem ser feitos para alergia?

    Testes cutâneos (punctura, arranhadura ou contato) e testes de provocação oral são feitos em geral por um alergista ou dermatologista. O médico pode também suspender alimentos da dieta, reintroduzindo-os pouco a pouco. É importante que esses testes sejam feitos sob supervisão médica, porque podem provocar reações graves, incluindo anafilaxia.

  • Meus exames estão negativos, mas continuo tendo problemas. Que mais posso fazer?

    Pode haver um problema de hipersensibilidade genética, como a hipersensibilidade ao glúten na doença celíaca, ou uma deficiência enzimática, como a deficiência de lactase causando intolerância à lactose. Algumas reações alérgicas não são mediadas por IgE, e não há exames laboratoriais específicos para elas. Outras doenças podem provocar sintomas semelhantes aos de alergia. Seu médico saberá como investigar esses casos.

  • Minha alergia em geral é branda. Qual é a gravidade real?

    Reações alérgicas são individuais. Podem ser brandas ou intensas, variam entre diferentes exposições, podem envolver uma área ou todo o corpo, e, algumas vezes, podem ser fatais.

  • Minhas alergias desaparecerão algum dia?

    Em crianças, alergias podem desaparecer; em adultos, isso é raro. Alergias mais graves, como anafilaxia causada por amendoins, em geral não desaparecem. A conduta mais segura é evitar alergênios conhecidos e usar medicamentos, quando necessário, como anti-histamínicos e injeções de adrenalina. A imunoterapia dessensibilizante diminui os sintomas em alguns casos, mas não é útil em alergias alimentares. O tratamento com injeções dessensibilizantes pode durar anos ou pode ser mantido indefinidamente.

  • Por que devo evitar frutas frescas, se minha alergia é a pólen?

    Pode haver reações cruzadas raras entre alergênios aéreos e proteínas de frutas.

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