Why Do Allergy Skin Prick and Blood Testing Results Sometimes Differ?

Image by Wolfgang Ihloff in Wikipedia

For inhalant allergies, there is a high agreement (~80-90%) in results found between skin prick and blood testing. However, occasionally the results may differ quite significantly... usually in these situations, the skin prick test comes back positive for something whereas the blood test comes back negative or normal. It's also possible that both skin prick and blood testing may come back normal even if a patient is truly allergic.

Why?

Before going into an explanation, you must first understand how these tests are performed. Don't worry, this will be a simplified explanation.

To begin, for any substance, there are protein markers called epitopes to which an allergic response can be mounted.

For example, let's take a "cat."

Cat is composed of many different epitopes, both major and minor, to which a human can mount a potential allergic response. For the vast majority of people, only a few of these epitopes can trigger an allergic reaction. Let's call this Epitope A. However, for a minority of patients, a reaction can also occur to Epitopes b and F.

Based on this knowledge, a variety of companies have created a purified "essence of cat" containing Epitopes A, b, and F that can be utilized to test patients for cat allergy rather taking an actual live cat and having it scratch a patient with its claws.

SO... when a skin prick test is performed, a doctor takes a needle and dips the tip of it into this "essence of cat" and pricks the skin of a patient depositing a small amount under the skin.

If a skin reaction occurs (redness and swelling), than the allergy test for cat would be considered positive. The bigger the reaction, the more severely the patient is considered allergic to cat.

With allergy blood testing, however, a computer analyzes the blood taken from a patient to see if it contains a large number of antibodies called IgE that is specific for cat Epitope A.

Assuming a patient is ONLY allergic to cat Epitope A (which the vast majority of patients are), both the skin prick and blood testing will give the same exact positive result.

HOWEVER, if a patient is ONLY allergic to Epitopes b and F but not A, than the skin prick test will still come back positive, BUT the blood test will come back normal! Remember, the blood test can only detect Epitope A, but not b and F.

So that's why a skin prick and blood test may disagree on whether a patient may be allergic to something or not.

And before you ask... skin prick testing typically checks more epitopes for a given substance like cat than blood testing.

Given your newfound knowledge now, can you figure out why BOTH skin prick AND blood testing may come back normal even if a patient may truly be allergic?

It rare situations, a patient may theoretically be allergic to other epitopes not present in the test. For example, what if a patient is allergic to cat Epitope R, x, and W? If that's the case, a skin prick test for cat Epitopes A, b, and F and blood test for cat Epitope A will both come back normal.

If this last situation applies to you, there may not be any good way to get tested for more unusual epitopes other than to go to a tertiary care allergy center. Hopefully with time, biotech companies will develop even better tests that incorporate even more if not all epitopes that may trigger allergic reactions in humans.