Diagnosing Allergy | Skin and Blood Tests to Identify Allergen

That you have developed allergy can easily be spotted by your dermatologist (skin specialist). He is experienced and knows the signs and the symptoms.

Then there is the allergy specialist, who is called an allergist. He is specially trained to diagnose and treat allergies.

He will conduct different types of skin and blood tests at his office to diagnose your allergy. He will not do the tests at home because he has to be equipped in case you develop a severe reaction during the testing procedure.

He may ask you to do the skin test or the blood test or sometimes both the tests to rule out any doubt.

These tests will help to check if you have an allergy and also to detect and identify the specific substance (allergen) that has caused your allergy reaction and symptoms.

Once these allergy triggers are identified after the testing, you should avoid them.

What is important is to know is what has caused your allergy. Identifying the allergen, most of the times is difficult, sometimes impossible.

So what does your doctor do to try and pinpoint the source of your allergy? He asks you to test yourself for your hypersensitivity.

What will allergy tests help to detect?

The allergy skin and IgE blood tests will help to identify what you are allergic to. It could be:

  • Allergy to foods such as gluten in wheat, or seafood
  • Drugs such as penicillin
  • Triggers of an asthma attack such as weed pollen, mold or animal dander , dust, chemical fumes, smoke, cold and humid weather
  • Triggers of a skin rash such as nickel, latex, poisonous plants, weeds, or cosmetics

 When are allergy tests ordered?

Your doctor will ask you to undergo the allergy tests if you have had an attack of an allergic reaction with symptoms such as skin rash, itching, hives, dermatitis, chronic cold, cough, asthma, chronic abdominal pain, and diarrhea.

Allergy testing in children and adults

The allergy testing procedure and the interpretation of the result are the same for adults and children of all ages.

Medical history and physical examination

Your doctor first takes a very detailed history of the last 48 hours of your whereabouts, what you have eaten (food or drugs), any new  clothing you have worn – in short he tries to find out any new and unusual contact you have had during the last two days.

Your doctor will also ask you to write down a report in detail of all the things you have told him. Writing always brings out things you have missed in the oral history taking.

He will also enquire about your family history to determine if any of your immediate family members, namely father, mother, and siblings, suffer from or have suffered from hypersensitivity reactions. Allergies are known to run in families.

He then does a physical examination. This examination helps him to determine the type of allergy you have.

  1. Do you have a skin rash? What do they look like?
  2. Do you have any respiratory symptoms like asthma?
  3. Do you have abdominal symptoms like pain or tenderness?

The history and the physical will help him to identify the type of allergy – whether it is skin or food or allergy due to breathing in of some allergen.

Your doctor then proceeds to carry out certain tests. Certain drugs can alter the results of the tests. Your doctor will give you a list of drugs to avoid a few days (at least 5 days) prior to the testing procedure.  One such example is Avil, an antihistamine, which can alter the test result.

Different types of tests to diagnose allergy reaction

Choosing and interpreting the right test is important because you may be sensitive to many allergens but they all do not cause clinical symptoms.

Your allergist chooses the most relevant test based on the following factors:

  • Age
  • symptoms
  • family history and the allergens involved
  • history of exposure to environmental and occupational factors
  • your lifestyle
  • your eating habits

How is allergy skin testing done and how does it work?

The allergy skin test is preferred over the blood test because it is easier to carry out, and less expensive. This test  also gives more specific information to detect the antigen responsible for causing the allergy. Results are also available much faster.

Skin test for allergy is useful for diagnosing sources of

  • Food Allergy,
  • Allergy due to mold, pollen, animal dander,
  • Drugs such as penicillin,
  • Sources of contact dermatitis.

There are two skin testing methods to diagnose allergy and check for the allergen:

1. Scratch/prick/puncture test – procedure.

  • In this method, your allergist marks and labels several small spots on the skin surface.
  • Prior to that, he cleans the skin surface with an alcohol swab.
  • Usually, the back or the forearm is used.
  • Then a drop of the extract of commonly known skin allergens is placed on these marked sites. These allergens could be, for example, pollen, dust mite, animal dander, mold, and certain foods.
  • A device called the multitest is used to scratch, prick or superficially puncture the skin surface. It does not go deeper than the skin, hence does not bleed. The multitest resembles a plastic needle and is sharply pointed at one end. A fine needle can also be used for the purpose.
  • Multiple pricks are made on the marked sites, each for one allergen.
  • The allergy skin test takes about 15 minutes.

2.  Intradermal skin test

  • Why is it done? This test is more sensitive and is done when the skin test has shown a negative result to an allergen, which is strongly suspected by your allergist to be the cause of your allergy.
  • In this test, the allergen is injected intradermally (ie. into the skin).
  • Your allergist views the results after 20 minutes
  • Chances of false positive reactions are present – meaning that there may be positive reaction even when you are not allergic to that allergen.

Allergy skin tests results explained

The allergist inspects the pricked sites and reads the results after 20 minutes. A positive reaction to a particular allergen will be indicated by a red, swollen, itchy area, which is called the “wheal”. These reactions will vary in intensity and are graded from 1+ to 4+. The more severe reaction can be in the form of a hive.

Allergy IgE blood tests and when they are done?

Since allergy blood tests are more costly and not as sensitive, allergy skin testing is the preferred method. Yet, you may be required to undergo an allergy blood test instead. This is done in the following indications:

  • As explained above, it becomes necessary to stop certain medicines before undergoing an allergy skin test because they interfere with the test result. Among others, these include antihistamines, steroids, and tricyclic antidepressants. In certain patients, it is not possible to stop the medicines because of the seriousness of the problem, for which those medicines are prescribed. Your doctor then orders an allergy blood test.
  • The patient has a poorly controlled asthma, which can flare up if subjected to a skin test.
  • The patient has severe widely spread dermatitis, psoriasis or eczema.
  • The patient has a history of a severe life-threatening allergic reaction called anaphylaxis. Your doctor may fear of a recurrence if exposed to the allergen during the skin test.
  • The patient may refuse to be scratched multiple times and may insist on the blood test, which requires just one prick to withdraw blood.
  • In infants and young children, a single needle prick required for the allergy blood test is more convenient than pricking the baby several times as is done in the skin test.
  • When skin testing is not easily available
  • A blood test is also indicated to know how well your allergy treatment is succeeding and whether you have outgrown the allergy.

How are allergy IgE blood tests done? Procedure

The blood is withdrawn from your vein, usually of your forearm and put on an insoluble material and sent to the lab for testing.

The lab tests the blood for the presence of various specific IgE antibodies, which form in the blood due to the exposure to various types of allergens. One blood draw is enough to test for various types of IgE antibodies.

Each allergen produces a specific type of IgE antibody in the blood.

Detection of that antibody tells you that you are allergic to that specific allergen. Higher the level of a particular IgE antibody in your blood, the more likely you are allergic to that particular food or allergen.

Types of blood tests

Allergy blood tests are of two types:

  1. Enzyme-linked immunosorbent assay (ELISA, or EIA)
  2. Radioallergosorbent test (RAST)

These tests are done to detect and measure the IgE antibodies that our immune system forms in the blood in response to the presence of antigens. Here the antigen is the allergen. ((IgE, short for “immunoglobulin E)

The antibodies attack the allergen and this results in an antigen-antibody reaction, which triggers the allergy symptoms.

Each type of allergen produces antibodies, which are specific to it. Blood tests help to identify the type of antibody produced and subsequently identify the allergen. These tests are, therefore, also called allergen-specific IgE antibody test.

These tests use a concentrated liquid form of common allergens. The common allergens that cause allergy include;

  • Pollen
  • Dust mites
  • Animal dander
  • Mold
  • Seafood

However, in the course of your history taking, if your doctor suspects a particular allergen, he will add that to the common list.

The ELIZA blood test for allergy works on this very principle and helps to identify the allergen-specific antibody.

The RAST test also works on the same principle but since the introduction of the ELIZA test, RAST is rarely used.

Allergy IgE blood tests results explained

The following interpretation of the blood tests will tell you how accurate and reliable the allergy test results are.

A positive result means that IgE antibodies have been detected in your blood and that you are allergic to the particular allergen.

A negative result indicates that no IgE antibodies have been detected and you are not allergic to the allergens you have been tested for.

However, your allergist will take great care to interpret the results of your IgE blood tests. The reason:

  • Even if no IgE antibodies have been detected (ie the test is negative), there is still a chance that the person could be allergic to the allergen tested for.
  • Similarly, if IgE antibodies have been detected for a particular allergen, the person may not develop any clinical symptoms when exposed to the allergen.

Disadvantages of Allergy Blood Test

  • Expensive.
  • The result takes a week as compared to the skin test which takes 20 minutes.
  • Doubtful accuracy  with some antigens.

How long do allergy ELISA and RAST blood tests results take?

The blood sample collected for allergy IgE blood testing, both for ELISA and RAST, is sent to the laboratory and the results are available anywhere between 7 to 14 days.

Food Allergy Test

This food allergy test or the food challenges test, as the name suggests is done to find the food allergen that may have caused allergy.  The patient is administered a capsule or pill containing the suspected allergen  and he or she is watched for any reaction. This test is performed under supervision for fear of a severe allergic reaction, preferably in a hospital. By trial and error method, an effort is made to find the source of the allergy, with the help of this food allergy testing.

However,  you  yourself can play a vital role in identifying the substance that has caused an allergy, by being vigilant of what you eat or come in contact with,  which triggers off this reaction.

Allergy testing has its drawbacks, in the sense that, it may give false positive or a false negative test report. It may show you as sensitive to a particular substance while the allergy may not be due to that substance.

 

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