Allergies can cause symptoms ranging from the minor to the life threatening. If you have an allergy, you’ll want to know what it is that your body sees as a threat. That way, you and your doctor can work together to find ways to stop or lessen your symptoms. In some cases, you may even be able to avoid the allergen.

Blood testing and skin prick testing are the most common tests used today to help determine your likelihood of having an allergy. Allergy symptoms are the result of an overreaction of your immune system to an irritant or allergen, such as dust, mold, or cat dander. Your immune system releases immunoglobulin (IgE) antibodies in an attempt to fight off the irritant, or allergy. Allergy tests attempt to detect these IgE antibodies in different ways. This helps your doctor to identify your allergies. These tests are available for children and adults.

Skin prick testing is the most common way doctors test for allergies. Your doctor may order both tests for you, or one test may be more suitable for you than the other.

Skin Prick Test

Skin prick testing will be done at your doctor’s office. For this test, a doctor or nurse will lightly prick the skin on your back or arm with a comb-like tool. Then, they will add a small amount of a suspected allergen over the pricked area.

You’ll know, and feel, the results more quickly than with a blood test. If the doctor sees swelling or if the area begins to itch, that’s a positive reaction. This means you’re more likely to be allergic to that particular allergen. A positive reaction could happen right away, or it could take 15 to 20 minutes. If there’s no reaction, it’s unlikely you’re allergic to the substance.

Skin prick testing is more sensitive than blood testing. It’s also less expensive. However, there’s more risk. Though rare, it’s possible to have a serious reaction. For this reason, a doctor may avoid skin testing if your risk for anaphylaxis or a severe reaction is high. This is also why your doctor will give you a skin test in their office. Doctors and staff should be trained to deal with any reaction that might occur.

“For drug allergy, often times skin testing is the preferred method of diagnosis,” says Niti Choksh, MD, a practicing allergist and immunologist in New York. Particularly for penicillin allergy, she says, it tends to be more accurate.

If you’re getting a skin prick test, you’ll be asked to stop taking antihistamine medication a few days before the test. If you don’t think that’s feasible, then discuss further options with your doctor.

RAST or Other Blood Tests

Blood testing is another common way to measure the potential for an allergy. Radioallergosorbent or RAST testing used to be the go-to blood test for helping to diagnose an allergy. However, newer allergy blood tests are now available. ImmunoCAP testing is a more common allergy blood test. Your doctor could also order an enzyme-linked immunosorbent assay, or ELISA test.

These blood tests look for IgE antibodies in your blood that are specific to a certain food or other allergen. The higher the level of IgE, the more likely you are to have an allergy to that particular food.

While skin testing results are available promptly, usually within 20 to 30 minutes of placement, you won’t know your blood test results for several days. You’ll likely have it done at a lab instead of your doctor’s office. On the plus side, there’s no risk that the test will trigger a severe reaction. Because of this, blood testing is considered the safer option. This is particularly important for people who are at higher risk for a life-threatening anaphylactic reaction, as well as for those with unstable heart disease or asthma.

Because blood tests offer absolute values, Choksh says it’s easier to see trends over time. One blood draw can also be used to test for multiple allergens.

Blood testing may also be better for people who aren’t able to or would rather not stop using certain medication for a few days before the test. This is required for an accurate skin prick test. Blood testing may also be better for anyone with an extensive rash or eczema, which can make skin testing more difficult.

What to Ask Your Doctor

If you think you may have an allergy, you should make an appointment with your primary care doctor or an allergy specialist. If your doctor doesn’t address any of the following questions, you may want to bring them up yourself:

  • What’s the most likely culprit of my symptoms?
  • Do I need allergy testing?
  • What types of allergy testing do you recommend and why?
  • How accurate are these tests?
  • Are there any risks to doing this testing?
  • Should I stop taking any medication prior to this testing?
  • When will I know the results?
  • What do these results mean?
  • What should I do next?

Your doctor should explain what the test results mean in the greater context of your overall history and circumstances. If not, ask. Allergy testing isn’t an exact science and false positives, even false negatives, are possible. It’s important to note that neither skin nor blood tests will predict the type or severity of any potential allergic reaction.

In fact, 50 to 60 percent of blood and skin testing could give false positives. This means that if your skin testing shows a positive result, you may not react to that allergen in everyday life. You don’t want to avoid a food when you don’t need to. For this reason, a doctor may schedule follow-up testing weeks or even months after your first test to compare results. They may also order additional blood and skin testing.

Your doctor won’t just consider allergy test results when determining whether you have an allergy. Instead, allergy tests can be useful when your medical history and specific symptoms are also considered.

A doctor will use all the information available to them to help determine which allergens are most likely to give you problems. Because allergies can cause life-threatening reactions, it’s important you work with your doctor to find the testing and treatment plan that works best for you.