Allergy Testing Training for Physicians: Basics Part 1

Dr-dean-mitchell-2015I (Dr. Dean) am currently involved in expanding allergy training to physicians all over the United States. It is exciting to teach a mix of family physicians, pediatricians, internists and other specialists on how to utilize allergy testing in their own practices.

When assessing allergic patients, in addition to a good history, there are two options available: allergy skin tests or allergy blood tests. When performed and interpreted properly, these are two very sensitive and specific tests. As a board certified allergist, I use both of these types of testing in my practice to make an accurate allergic diagnosis.

Allergy Skin Testing

Allergy skin testing has never been more simple or safe. When I originally trained in my allergy fellowship 25 years ago, we were trained to do intradermal skin tests.

Intradermal skin tests involved injecting allergen extract under the skin to evaluate the allergic response. If you were doing many screening skin tests this was quite difficult and labor intensive. It also caused the patient’s skin to get extremely inflamed and was very uncomfortable. Just imagine getting 30 shots in your arm!!

Skin Testing Today

Today, allergy skin testing has moved in the direction of prick skin testing. The intradermal skin test has lost favor because it was felt to cause too many false positive tests, and hence less specificity.

In contrast, with prick skin testing a small amount of pressure is applied to the skin with the allergen on the tip of the testing apparatus (which is a plastic device similar to a toothpick).

There is now an effective device called The Multitest, that has eight prongs and allows eight different tests to be applied in one compression on the skin. It is mostly a pressure sensation for the patient and there should be no blood.

It is easy to set up all the relevant allergens on several panels, and the entire testing should take no more than 5 minutes to apply.

The Key To Allergy Testing

  • When doing proper allergy testing, it is key that the patient is advised to avoid antihistamines for at least 3 days prior to the testing. This is important to ensure the tests don’t return any false negatives, as the antihistamine could block a potential allergic response and the allergen could be missed.
  • The other important point for testing technicians is for the skin test to be measured accurately. The companies that make the allergen extract provide a measuring device to help make this simple.
  • Thirdly, physicians should carefully observe the positive and negative control on the skin once they have read the skin testing. The skin testing should be read 15 minutes after the last test is applied. When reading the results, look closely first at the negative control. This is just saline and should give no response. The positive control contains histamine and everyone who is not on chronic antihistamines should have a response!

What Determines A Positive Test?

The big question is: what determines a positive test? This can be tricky, but my rule of thumb from many years of clinical experience is to look at the negative control. It should not have a reaction, meaning no wheal (or induration) at this test.

Then, look carefully at the positive control. This should have a reaction, and typically if it has a wheal of 3mm or greater it is positive.

What happens if your negative control shows a reaction of 2mm? This is not a very strong reaction, but it is a reaction! In this case, I carefully check that the positive control has a 3mm differential, which means that in this case it would need to measure 5mm to be valid.

This is important because it appears that this patient’s skin is hypersensitive. If you look at other allergens on the testing and they read 3mm I would not consider them positive. There needs to be that 3mm differential, and a positive allergen test should be closer to the result from the positive control.

Once you start doing more and more skin testing you will become more comfortable in interpreting testing. However, it is important to get the basics right from the start otherwise you will perpetuate misdiagnosis.

Next time we will discuss the different allergens and their clinical significance.

Dr. Dean Mitchell
Mitchell Medical Group, NYC & Long Island

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