Allergy Tests Can Yield False Positives, Here's How To Get Reliable Results

Misunderstanding the significance of food allergy test results can lead to unnecessary anxiety and dietary changes. The three tests used most often to diagnose IgE mediated food allergies* are the skin prick test (SPT), specific IgE blood testing, and a physician-supervised oral food challenge.

However, two of these tests can yield false-positive results and should never, by themselves, be the basis of a food allergy diagnosis. A doctor or allergist needs to weigh the results of the test against their patient’s complete medical history.

The Skin Prick Test

To perform a skin prick test:

  1. A drop of a suspected food allergen is put on the patient’s skin; the skin is pricked through the drop to get the allergen into the skin’s top layer.
  2. A red itchy bump called a flare or wheal should appear at the prick site within 15 minutes if the person carries a specific IgE antibody to the allergen.

Skin prick tests have a high negative predictive value, and a low positive predictive value.

  • A negative predictive value means that negative test outcomes are likely correct.
  • A low positive predictive value means positive test outcomes are less likely to be correct; false-positive results can occur.

Because false positive’s can happen, an SPT is not a good initial screening tool for food allergies. However, doctors use this test to confirm that a patient’s history is consistent with an IgE mediated allergy.

IgE Blood Testing

IgE blood testing, sometimes called RAST or ImmunoCAP testing, measures a patient’s blood level of specific IgE antibodies associated with foods.

Like the SPT, this test has a high negative predictive value, and a low positive predictive value, making it a poor screening tool. Children with non-food allergies such as eczema, allergic rhinitis, and asthma frequently have mildly elevated IgE blood test outcomes, as can some individuals with no allergies at all.

The IgE blood test’s predictive value differs with each food, but higher levels generally indicate increased likelihood of an IgE mediated allergy. Elevated levels are not related to allergy severity, and the “class level” designations used by some testing facilities are meaningless.

Oral Food Challenge

A physician-supervised oral food challenge is the definitive test for determining IgE mediated food allergy. Though the test usually takes four to eight hours to complete, it is reliable.

The patient gradually ingests the suspected food allergen while under a doctor’s or allergist’s watchful eye; if the patient is allergic symptoms will manifest. When symptoms (e.g., hives, swelling, trouble breathing) are absent during the challenge the presence of IgE antibodies toward the tested food is unlikely.

    *IgE mediated food allergy: food-specific IgE antibodies develop and attach to the body’s allergy cells; eating the specific food triggers an onset of allergic symptoms such as hives, swelling, or anaphylaxis.

Source: Kids With Food Allergies
Photo: Pexels

Community

Latest Post by sunshinestate Mon, 11/11/2019 - 1:39pm
Comments: 1
Latest Post by absfabs Mon, 11/11/2019 - 1:28pm
Comments: 2
Latest Post by absfabs Mon, 11/11/2019 - 1:23pm
Comments: 3

More Articles

Anaphylactic shock (A-nuh-fih-LAK-tik shok): A severe and sometimes life-threatening immune system reaction to an antigen that a person has been...

One of the most difficult things for a parent to do is determine whether his or her toddler has a cold or a...

You no doubt have your own way of teaching people about your child’s food allergy, a way that suits your temperament, and style of communication....

Reliable peanut allergy statistics are not that easy to come by. There is a lot of available research on food allergies in general but not too...

Most people know that to enjoy whatever food safety accommodations an airline offers they need to inform the airline of their allergy prior to...

More Articles

More Articles

A 504 plan* documents food allergy accommodations agreed to by parents and their child’s school. Plans are typically created during a 504 meeting...

If there is a child at your children's school allergic to peanuts, the school probably discourages or may not allow peanut products to be brought...

If you are on a budget, but you need to wear some sort of notification that you have a peanut...

Unless we consciously carve out time for self-care, constant food allergy management can slowly erode our sense of well-being. Signs of allergy-...

Peanuts cause more severe food allergic reactions than other foods, followed by shellfish, fish, tree nuts and eggs. Although there is only a...

If you avoid peanuts, it’s likely you know the joy of cashews. Slightly sweet and smooth in texture, cashews provide not only relief to those with...

The prevalence of food allergy has dramatically increased over the past two to three decades, and not just among children. Preliminary results...

When someone in the family is diagnosed with a food allergy, a choice must be made whether to ban the problem food or foods from the home. The...

Looking for a fun way to share what you know about your own food allergies? Or are you hoping to educate the people around you in a fun way about...

According to the results of a new study, children lacking Vitamin D may be more susceptible to food allergies. Researchers working at the Albert...

If you or your child has a peanut or nut allergy, identifying the presence of nuts in food becomes a priority, but what if the written or spoken...

Soap allergies can cause a lot of discomfort and itching. If you suddenly develop a rash or bumps on your skin, you may suspect that you have an...

Even professionals can have difficulty keeping up with the constant flow of updated information available in their field. A survey study presented...

People with pollen allergies can develop allergic reactions to fresh fruits, vegetables and/or nuts. This is called the pollen-food allergy...

There are more "peanut-free" products than ever on the supermarket shelves. This means more choices than ever for peanut-allergic shoppers and...