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

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