Allergy Tests – Skin, Blood, Food Testing and More

Allergy Tests

Summary

Allergy tests help physicians pinpoint the cause of allergy-related symptoms. Healthcare providers who use these tests can identify the responsible allergen and then use that information to build an effective allergy treatment plan for the patient. Oftentimes, this will include a combination of lifestyle-modification suggestions and allergy medication therapy.

There are three primary groups of allergy tests:

  • Skin tests
  • Blood tests
  • Food allergy tests

In addition, tests to determine lung or nasal function may be used in cases where breathing-related symptoms are present.

Each test offers advantages and disadvantages, and may be appropriate for some patients and inappropriate for others. No single test is 100 percent accurate in diagnosing allergies. However, in conjunction with the patient’s specific symptoms (e.g., hives, wheezing, watery eyes) and medical history, these tests offer physicians the best window into the nature of an individual’s allergies.

About allergy tests

Allergy tests are often used by allergists/immunologists to determine if a person’s symptoms are caused by allergies. However, the primary goal of most allergy tests is to identify the specific allergen that triggers the patient’s allergic reactions.

An allergy occurs when the body overreacts to an allergen (a substance that is normally harmless) because it mistakes it for a threat. In such cases, the immune system produces certain antibodies to protect the body from the allergen. This, in turn, triggers an allergic reaction, which may include symptoms such as hives, runny nose, watery eyes and wheezing.

The most effective method of preventing an allergic reaction is to completely avoid contact with the triggering allergen. Therefore, identifying the specific allergen to which a person is sensitive is a crucial part of overall allergy management.

Allergy testing can be conducted on patients of any age. They are frequently used to uncover reactions to a variety of potential allergens, including:

  • Tree, weed and grass pollens
  • Molds
  • Household dust and dust mites
  • Animal dander and feathers
  • Foods (e.g., milk, eggs, peanuts, wheat)
  • Medications
  • Insect venom
  • Cockroach debris
  • Latex 

The first step in diagnosing any condition is usually a thorough physical examination that includes a complete medical history. A physician may ask detailed questions about any family history of allergies. Patients can help by preparing a list of allergies other family members may experience, as well as by detailing how, when and under what conditions their allergies occur. The physician may also ask the patient to keep a diary of this information.

Patients will also be questioned about their lifestyle at home, work or school (e.g., eating habits, hobbies). This information will further help the physician pinpoint the allergens causing the patient’s allergic reactions.

The three primary groups of allergy tests used in the diagnosis and treatment of allergy-related problems include:

  • Allergy skin test. A small amount of an allergen is introduced to the patient’s skin to determine if the person is allergic to that substance.

  • Allergy blood tests. A patent’s blood sample is tested for substances that indicate an allergic reaction has occurred to a particular allergen.

  • Food allergy tests. Patients may be asked to avoid (or, in rare cases, to ingest) specific foods according to a timetable to determine if allergic reactions can be prevented.

In addition, physicians may order tests to determine lung or nasal function or to rule out other causes of allergy-like symptoms.

In some rare circumstances, a physician may recommend a patient participate in an allergen inhalation challenge. This type of testing involves the inhalation of allergens to see if an allergic reaction occurs. Because this test can potentially cause a dangerous allergic response in a patient, it is used only when more conventional testing methods are impractical. This type of test is occasionally used to confirm the effectiveness of immunotherapy treatments.  

Complementary and alternative medicine (CAM) allergy tests also have been introduced as alternative approaches to diagnosing allergies. Proponents of these tests contend that substances such as chemicals in products, micro-organisms such as yeasts, and human hormones play a major role in allergic reactions.

These claims have not been proven, and many of these tests have been shown to be ineffective. If a physician recommends these tests, patients may want to consider getting a second opinion:

  • Cytotoxicity blood test, in which white blood cells are examined to see if an allergen causes them to change shape.

  • Applied kinesiology, in which the patient’s arm strength is tested after test substances are placed in the patient’s mouth or hand.

  • Subcutaneous or sublingual provocation testing, in which substances are injected under the skin or placed under the tongue until symptoms appear.

  • Neutralization testing, in which decreasing amounts of a substance are given until symptoms no longer appear.

  • ELISA/ACT (not to be confused with ELISA, which is often used to test for autoimmune diseases and sometimes for allergies).

  • Electroacupuncture biofeedback, which involves the measurement of electrical properties at a person’s acupuncture points.

  • Urine autoinjection, which involves the intramuscular injection of sterilized urine back into the person who excreted the urine.

  • Skin titration (Rinkel method).

  • Candidiasis allergy theory.

  • Basophil histamine release.

Patients are encouraged to discuss any alternative tests or therapies they are considering with a board certified allergist/immunologist.

Skin testing for allergies

The most common form of allergy testing, skin testing for allergies involves introducing a small amount of an allergen to the patient’s skin to determine if the person is allergic to that substance.

Physicians can use skin allergy tests to test for several different allergens at the same time. Skin testing begins with the patient’s skin being marked to identify spots for each allergen to be tested. The testing is often conducted on the forearm, upper arm or the back. Tiny amounts of the allergen are then introduced to the appropriate spot marked on the skin by one of two methods:

  • Epicutaneous testing. The allergen is introduced just barely below the surface of the skin by either a prick, scratch or patch method.

  • Intradermal testing. The allergen is injected below the skin with a needle and syringe.

After a period of time, the skin is examined for reactions to any of the allergens. Patches are typically removed at 72 hours. Results from other methods of skin testing are usually apparent within 10 to 30 minutes. If a rash or small bump develops, the patient is most likely allergic to that substance. Generally, the larger the skin reaction, the more sensitive the person is to the allergen.

A skin test should also involve the injection of histamine, a chemical present in cells throughout the body that is released during an allergic reaction. Histamine is considered a positive control because most people have a skin reaction to it, whether they are allergic or not. If a patient does not react to histamine, an allergy skin test may not be useful. In addition, a substance without any allergens in it (e.g., saline) may be injected to see if there is a reaction. If a reaction occurs, it may indicate sensitive skin, which means the results of an allergy skin test should be reviewed with caution.

Allergy skin tests can be safely performed at any age. However, they may not be as useful in children under the age of 3 and patients over age 60. In addition, people with certain skin conditions (e.g., eczema, psoriasis) may yield less accurate results.  In these situations, a blood test that measures allergy-specific antibodies may provide a better alternative for diagnosing allergies.

Some types of medications (e.g., antihistamines, tricyclic antidepressants, corticosteroids) may also interfere with skin allergy testing. Therefore, patients taking these drugs may be asked to discontinue the medications several days before undergoing an allergy test.

In addition, blood tests may be ordered for patients who have a history of the severe allergic reactions anaphylaxis and anaphylactic shock.

Blood testing for allergies

Blood tests are often used for patients who are not good candidates for skin testing, such as infants or individuals with skin disorders such as eczema. They may also be useful to help confirm or rule out diagnoses.

The most commonly used blood test for allergy-related conditions is the radioallergosorbent test (RAST). This test looks for the presence of allergy-specific immunoglobulin E (IgE) in the bloodstream. During an allergic reaction, the body produces 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. The level of IgE correlates to the sensitivity a patient has to the allergen.

Other types of blood allergy tests include:

  • CAP-RAST FEIA. This blood test is similar to the RAST, but is more useful in determining reactivity to certain foods.

  • Radio allergo sorbent procedure (RASP). This blood test is a variant of the RAST, but is more sensitive to food allergens.

  • Fluorescent allergo sorbent test (FAST). This blood test is similar to RAST, but uses fluorescent instead of radioactive compounds to test the blood. This makes the equipment well-suited for use in an office (instead of a laboratory), where results can be delivered in about six hours.

  • Multiple antigen simultaneous test (MAST). A spinoff of the RAST that proponents believe is more comprehensive and sensitive than its predecessor. MAST testing allows physicians to test for 38 allergens at a time (although it cannot diagnose food allergies), while RASTs look at only a single allergen per test.

  • Immunoassay capture test. Proponents say the process used to make the patient’s blood and the test medium react leads to a result as sensitive and accurate as those obtained from allergy skin testing.

  • Paper radioimmunosorbent test (PRIST). This test measures the total number of IgE antibodies in the blood.

  • Complement test. Complement is made up of proteins that bind with antibodies, and is involved in immune system functioning. Patients with hives or angioedema often show a deficiency in blood complement. Complement levels also can help a physician determine whether a case of angioedema is related to an allergy or to hereditary factors. This is important, because treatments differ for the two conditions.

In addition to tests designed to identify specific allergens, other blood tests may be used in diagnosing allergy-related conditions. These may include:

  • Complete blood count (CBC). Counts red and white blood cells in an individual and can be used to check for many diseases and ailments. A high number of white blood cells, known as eosinophils, indicates that an allergy may be present.

  • Sedimentation rate (sed rate). Indirectly measures the degree of inflammation in the body. It measures the rate of fall of red blood cells known as erythrocytes in a tall, thin tube of blood. Patients with hives or other allergy symptoms may have their sed rate tested to rule out an underlying inflammatory illness.

  • Serum metabolic analysis (SMA). Measures several different substances used in the process of metabolism (e.g., electrolytes, blood sugar). Blood levels indicate how well the liver, kidneys or other organs are functioning. It may be used to rule out other potential causes of allergy symptoms and is most useful when used with other tests.

  • Complement test. Measures levels of blood complement (proteins involved in immune system function). This test may be used to rule out hereditary or other non-allergic causes of swelling, itching or hives – especially if a patient is very young (under age 5) or over age 50.

Because blood samples must be sent to a laboratory for analysis, it usually takes several days to receive the results of a blood test.

Food allergy tests

In addition to skin and blood testing, there are other methods that may be used to assist in the diagnosis of food allergies. These include:

  • Elimination diets. Suspected food allergens such as milk, peanuts or eggs are eliminated from a patient’s diet and then slowly reintroduced one at a time. The patient is monitored for signs of an allergic reaction (e.g., skin rash, shortness of breath).

    To further boost the odds of an accurate diagnosis, a double-blind study may be conducted. In this approach, the food being tested is disguised so that neither the patient nor the tester knows when the suspected allergen has been introduced.
  • Food challenge test. A suspected allergen is introduced to the patient in a controlled environment. After the patient consumes the food, he or she is monitored for a reaction. This test is rarely performed, and should only be conducted under the close supervision of a licensed medical professional.

Food allergy testing is most effective for patients with mild allergies. However, because some food allergies can trigger the severe and potentially life-threatening reaction known as anaphylactic shock, those with more substantial allergies may be more safely checked with blood testing.

Other related tests

A variety of other tests may be used to diagnose allergies or to rule out other conditions that may produce similar symptoms. These may include:

  • Nasal smear. Examination of nasal fluids. Patients blow their nose onto a plastic sheet. Fluids are transferred to a plastic slide and tested for the presence of eosinophils, a white blood cell that can indicate the presence of an allergy. This test is rarely done.

  • Nasal endoscopy. Viewing nose and/or sinus cavities using a special telescope (nasal endoscope). Nasal mucous membranes that are swollen, pale and bluish may indicate the presence of an allergy.

  • Chest, nasal and/or sinus x-rays. An image of the chest, nose or sinuses produced on film paper or fluorescent screens using low doses of radiation. X-rays may be taken to rule out other diseases such as emphysema, cystic fibrosis, tumors, foreign body interference and nose structure malformations.

  • Electrocardiogram (EKG). Records the electrical activity of the heart. These may be used in older patients experiencing shortness of breath to rule out a heart–related problem.

  • Pneumatic otoscopy. Viewing the patient’s ear drum both at rest and during movement in response to a change in pressure. It may be used to rule out ear infections as a source of certain symptoms.

  • Tympanometry. Determines the patient’s response to sound under different air pressures. It may be used to rule out fluid in the middle ear, damage to the ear drum (tympanic membrane), impacted ear wax and other conditions.

A physician who suspects another respiratory disorder (e.g., asthma) may order other tests, such as spirometry or other pulmonary function tests.

Potential risks with allergy testing

Anaphylactic shock is by far the greatest potential risk associated with some types of allergy tests (e.g., skin tests, food tests).

Anaphylaxis is a rare, severe allergic reaction affecting two or more body systems. The condition often begins immediately after an individual has been exposed to a particular allergen. Without prompt emergency treatment, anaphylaxis can quickly progress to life-threatening anaphylactic shock, which is characterized by difficulty breathing and a dangerous drop in blood pressure. In such cases, an emergency dose of epinephrine (adrenaline) is necessary to reverse these symptoms and potentially to save the patient’s life.

Patients should contact a physician immediately if any of the following symptoms develop after allergy testing:

  • Fever
  • Dizziness or lightheadedness
  • Wheezing
  • Shortness of breath
  • Severe swelling (angioedema) and/or itching

Treatments that may follow allergy tests

Once testing has revealed that a patient’s symptoms are due to an allergy, a physician (usually an allergy specialist) 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. This often involves avoiding the problem allergen, when possible. For instance, people with pollen allergies may benefit from staying indoors as much as possible 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 a prescription allergy medication. Prescription antihistamines, decongestants and steroid nasal sprays are among the drugs used to effectively treat allergies. Bronchodilators and leukotriene modifiers may also be recommended for people whose allergies trigger asthma attacks or asthma-like symptoms.

Most physicians recommend that individuals who are susceptible to severe reactions carry an injection of epinephrine with them at all times and understand how to self–administer the drug. A medical alert bracelet or necklace is also a good idea for these individuals.

Finally, if all other methods fail to 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 regarding their conditions. Patients may wish to ask their doctor the following questions regarding allergy tests:

  1. Why are you recommending that I undergo allergy testing?
  2. What type of allergy test will you use to determine if I have allergies?
  3. Can you explain the testing process to me in detail?
  4. How accurate is the test I am undergoing? Can I trust the results?
  5. What risks do I face by undergoing this allergy test?
  6. Will I experience any pain during testing?
  7. When will the results of my test be available?
  8. What do my test results indicate?
  9. Now that my results are in, what is the next step?
  10. Do I require further testing?
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