What Allergy Testing Can Tell You

If you suffer from allergies, be it to foods, pet dander, pollens, or materials like wool, avoiding them (or at least having a trusty antihistamine at the ready) has probably become a top priority. And if you find yourself sniffling, sneezing, and feeling a bit itchy—or worse—whenever you come into contact with any one of these things, know that you’re not alone.

That’s because more than 50 million Americans experience various types of allergies each year. What you may not know, though, is that there are several different tests your doctor can perform to help narrow down the culprit.

“We can test for food allergies, for environmental allergies, and for some drug allergies,” says Sherry Farzan, MD, an allergist and immunologist and director of the Asthma Center at Northwell Health’s Cohen Children’s Medical Center. “If there is a concern about any of those being an issue for an individual, the allergy testing is done as a supportive guide.” However, Farzan stresses, the history of the patient—what they tell us about past exposures and reactions—is the most important piece of the puzzle, “and we use the allergy testing to help support that history.”

How allergy testing works

First, there are some important differences in the types of allergies a person may suffer from, and their potential reactions. In the case of more serious allergies, such as to foods, drugs, bee stings, and latex, exposure can lead to anaphylaxis, a potentially life-threatening form of allergic reaction. Seasonal allergies, which do not typically cause anaphylactic reactions, can contribute to significant hay fever symptoms, as well as the worsening of asthma and eczema.

To get to the bottom of what might have caused an allergic reaction, your doctor will run down anything that you might have come into contact with that could be an allergen. Your own notes or diary on what you think led to allergic symptoms are often one of the doctor’s most powerful tools.

For example, let’s say your teen ate at a restaurant and had a wrap with chicken, pesto, and avocado, then got hives. “We ask them if they’ve had any of those things since then,” says Farzan. “If they say, ‘I’ve had bread since then, and I’ve had chicken, but I am too afraid to have avocado and pine nuts,’ we would test for the foods they haven’t eaten.”

Most allergists use both skin and blood tests to identify a potential allergen.

How skin tests work. Your allergist will mark areas of the skin and place a droplet of a solution with the allergen next to each mark, before using a needle to gently prick the allergen extract into the skin. After about 15 minutes, your skin is checked for redness and a wheal (or hive)—signs of an allergic response. If you show a positive reaction to any of the substances applied, you’ll develop a red, itchy bump, which the allergist and their team will measure to gauge your body’s reaction before the marks are washed off.

How blood tests work. These tests measure IgE levels—or antibodies—in your blood, which your immune system creates in response to specific allergens, and are often used in tandem with skin tests, particularly if someone has suspected food allergies or a history of anaphylaxis. They’re also used in place of skin tests if someone has recently taken steroids, antihistamines, or some antidepressants that might interfere with the results of a skin test. People with severe asthma, unstable heart conditions, or those with skin conditions such as eczema, dermatitis, or psoriasis, may also receive blood testing.

False positives and what they mean

While generally helpful in determining what a patient may be allergic to, both skin and blood tests can show false positives—meaning the test shows an allergic reaction where there is none. “We can pick up a lot of false positives for food testing,” says Farzan. “Skin testing has a greater specificity for environmental allergies.”

That’s why allergists now say it’s essential to not give across the board skin prick tests unless you strongly suspect that a particular food or substance is causing a reaction.

“If you test 100 people off the street for a peanut allergy, both skin and blood, about 50 would have a positive test varying in size,” says Farzan. “But those 50 people probably can eat peanuts without a problem. This is a false positive.” It demonstrates sensitization—meaning your body has been introduced to a peanut allergen and has created antibodies to peanuts, but it doesn’t necessarily react when we eat a peanut. In these cases, Farzan explains, the only way to know if a person is actually truly allergic is to give them peanuts.

For example, a toddler who developed welts after eating a cracker with peanut butter would at first be prescribed a strict avoidance and then “would continue to be monitored with blood and skin tests as they grew,” says Farzan. “The parents would be instructed to carry an EpiPen at all times in case of an accidental exposure, and if or when skin and blood tests become negative, or the welt came to a size that we felt comfortable with, we would offer a food challenge.” Food challenges are when a patient ingests a full serving of their allergen—such as peanuts—over the course of several hours at their doctor’s office or in a hospital setting. There, they’re able to be closely monitored for a reaction to see if the allergy has been outgrown.

How to prepare for testing

If you suspect you are allergic to a substance, keep a diary of what you ate and what reaction you noticed (hives, itchy mouth, etc.). That way, you can share detailed information with your allergist to help them zero in on what might be causing your allergy.

Don’t take an antihistamine, including cold or allergy medications, five to seven days before the test, says Farzan, because they can cause you to have false negatives. When you make the appointment, ask if there are any other medications you need to stop taking. (They usually will stress that if you need to take an antihistamine before the test, take it and reschedule the appointment.) You’ll also be instructed to skip applying any lotions or perfumes on the day of your test.

“The skin test causes red, itchy bumps if you are allergic,” says Farzan, “so we apply a topical steroid to the site after we have done our reading, and give patients an antihistamine if any itchiness is causing them discomfort.”

It’s important to remember that identifying the source of your allergic reaction can take time and patience, but together you and your allergist can pinpoint the offender and come up with a plan to keep you safe.

To schedule an appointment with an allergist at Northwell Health Physician Partners/Westchester Heath for testing, call us today at (914) 232-1919.

By Lucy Maher, Writer, The Well by Northwell

by WHA-Admin