Diagnosing Allergies: Physical Exam and Allergy Tests

Reviewed by: HU Medical Review Board | Last reviewed: March 2022

Between 40 to 50 million people in the United States have allergies or asthma. While these conditions are common, it is still important to get an accurate diagnosis. This helps you find the right treatments and ways to manage your specific allergies.1

You will probably first see your primary care doctor for your allergy symptoms. However, to find out exactly what you are allergic to, you will need to see an allergist. Allergists have special training in diagnosing and treating allergies.1-4

Allergies are usually diagnosed using a combination of an in-person exam, a medical history, symptom history, and various tests. These tests may include skin tests, blood tests, lung function tests, or food challenge tests. The type of tests ordered will depend on what your doctor suspects you may be allergic to.1-5

Some tests may be ordered to rule out other health conditions.

Medical history and physical exam

A physical exam usually begins with some basic checks, including your weight, oxygen levels, and blood pressure. They will look at your eyes, ears, skin, and listen to your lungs. Your doctor will then ask about a wide variety of things, including:2-4

  • What you think you are allergic to
  • Your allergy symptoms
  • When you react and how severe those reactions are
  • How these symptoms impact your daily life
  • Your exercise and eating habits
  • Family history of allergies
  • What kinds of drugs and supplements you take

Allergies are sometimes inherited. The specific allergen is typically not inherited, just the likelihood of developing an allergy. Knowing your family history of allergies can be helpful to your doctor.2

Skin tests

Skin tests can provide your allergist with more information about a suspected allergy. There are 2 different kinds of skin tests: a skin prick test (SPT) and an intradermal test.2

Skin prick tests

For a skin prick test a small amount of a possible allergen is dropped on the skin. A needle is then used to gently prick the site. There is no bleeding. The site is checked after 15 minutes for signs of an allergic reaction. Reactions include itching, and a raised, red bump.

SPTs can help confirm many common allergies, including allergies to pollen, food, penicillin, and bees. It may also be called a scratch test. For some allergies, it can be an inexpensive and accurate test to confirm an allergy.3

With an SPT, the nurse or doctor will put a tiny amount of the potential allergen on your skin. They then use a needle to prick or scratch your skin and let the potential allergen seep in. If you are sensitive to the substance, you will have a reaction like redness, swelling, or itching. You may be tested for several allergens at once.3,4

A wheal may develop, which is a swollen, round area, similar to a hive. This usually happens within 15 minutes. If you develop a reaction, there is a good likelihood that you may be allergic to that substance.3

After 15 minutes, your reaction is measured, and this shows whether you are allergic or not. Any reaction has to be 3mm larger than the control, so even if you have a reaction, it might still be considered negative.3

Scratch tests are not recommended for all types of allergies. This test is usually safe for all ages. It may not be right if you have had a severe allergic reaction to an earlier skin test, if you have certain skin conditions, or if you take certain drugs.2,3

Patch tests

With a patch test, a tiny amount of allergen is placed on the skin, then covered. The patch is left on for 2 days. Allergic reactions may develop between 2 and 7 days.

Patch tests may be done to see if an allergen is causing skin inflammation. Your doctor may call this inflammation "contact dermatitis."2,4

With a patch test, allergens are put on the skin and covered with a bandage. Up to 20 to 30 substances may be tested at a time. Depending on your symptoms, your doctor may test you for allergies to:1-3

  • Latex
  • Certain drugs
  • Preservatives
  • Dyes
  • Metals
  • Fragrances

Your doctor will check your skin for a reaction after 2 to 4 days. Patch tests are usually done by a dermatologist. This means your allergist may refer you to one for this test.1-3

Intradermal skin tests

An intradermal skin test works slightly differently than a skin prick test. With an intradermal skin test, a doctor or nurse injects a small amount of potential allergen into the outer layer of the skin. It works in much the same way as a test for tuberculosis.2

After about 15 minutes, the area is examined. This test is often used for drug or venom allergies, or if SPT results were negative but allergies are still suspected.2

Blood tests (specific IgE)

In some cases, blood tests are performed. If you have a skin condition or are taking drugs that may interfere with skin tests, a blood test may be done. This is also called a specific IgE blood test.1,2

With this test, a sample of blood is taken and sent to a lab. The lab adds the potential allergen to your blood sample. It then measures the amount of antibodies your blood produces to attack the allergen. This test has a high rate of false positives, so it is not a good screening test. However, it may be useful in certain situations.2,3

Challenge tests

A challenge test may be ordered to diagnose certain drug and food allergies. With this test, you take a tiny amount of an allergen while your doctor watches for a reaction. You may be asked to take several doses in increasing amounts to see if you tolerate the drug or food.2

These tests are closely supervised by your doctor due to the risk of anaphylaxis, a severe, life-threatening reaction.2

Other tests

If you report breathing problems, the doctor may order a lung function test to see if you have asthma. This is a test that measures how much air you are able to breathe in and out. Some lung function tests also measure the amount of inflammation in the lungs.5

Sometimes, X-rays of the lungs or sinuses are done. All of these tests, along with the physical exam, help provide your doctor with information that can lead to a diagnosis.5

Getting an accurate diagnosis allows your doctor to properly identify the allergen causing your immune reaction. This helps your doctor create a plan to treat and prevent your allergy symptoms. An accurate diagnosis also gives you the information you need to avoid that allergen as much as possible.

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