Penicillin Allergy Skin Test

Now we have an FDA-approved skin test to find out if you are still allergic to penicillin. 90% of persons who suspect an allergy to penicillin are not allergic to it. They have been avoiding penicillin and related drugs unnecessarily for many years and in many cases for decades! Reasons, why you may think that you are allergic to penicillin, are many. You probably had an actual allergic reaction to penicillin many years ago and with the passage of time, you have outgrown the allergy to penicillin. Sometimes you may mistake a side effect to penicillin for an allergic response. A typical example is that you take penicillin and develop nausea, vomiting and diarrhea and genital yeast infection. In some cases, it could be that your mother has told you about an unknown reaction to penicillin as a child, and you have been avoiding penicillins ever since. Occasionally you may have been avoiding penicillins just because other family members are allergic to them.

Whatever may be the reason, it is important to find out if you are indeed allergic to it. Why is it important? Penicillin and other related drugs are still drugs of choice for treating many common infections. They are bactericidal in their action meaning they kill susceptible bacteria rather than just inhibit their growth. They penetrate tissues better, are tolerated better and relatively inexpensive. If you say you are allergic to penicillin without verifying it, your healthcare providers may prescribe you alternate antibiotics that may not be as efficient, more expensive and may lead to bacterial resistance to them.

The ideal time to test for penicillin allergy is when you are not sick and not in need of immediate antibiotic treatment. You have to be off antihistamines for 7-10 days before you could be tested for penicillin allergy. Appointments for penicillin allergy skin tests should be made in advance (not a walk-in) because the test takes 2-3 hours to complete. We prefer to start the test in the morning when our office opens.
The test itself is simple. Initially, you undergo skin prick tests on your forearm. We place drops of reagents consisting of PrePen®, Penicillin G, histamine and negative controls on the skin in front of your forearms. We prick your skin through those drops using plastic needles. The prick carries the reagents through the skin to areas of skin where mast cells reside. The later cells are the ones responsible for allergic reactions to penicillin. If your mast cells carry IgE antibodies to penicillin on their surface, then you react to the reagents introduced by forming a local swelling, redness, and itching. This is measured in 15 minutes after the performance of pricks and compared to negative controls. If the wheal/ swelling size is 2-3 millimeters greater than the negative control, then you are still allergic to penicillin. Then the test is stopped, and you are told that you should still avoid penicillin and penicillin derivatives.

On the other hand, if you test negative for PrePen® and Penicillin G by prick tests (i.e., not allergic), then to confirm this we proceed to do intradermal (ID) skin tests. In this step, the same reagents (PrePen®, Penicillin, histamine and negative controls) are introduced in duplicate (twice) into your skin in front of your other forearm by syringe and needle. After 15 minutes, the results are read and interpreted in the same fashion as above. If you test positive for PrePen® or Penicillin G by forming a bigger wheal, redness or itching in comparison to negative control, then you are told that you are allergic to penicillin and its derivatives, and you should avoid them in future.

On the contrary, if you had tested negative for penicillin allergy by intradermal test also, then before we could tell you that you are not allergic to penicillin anymore we perform another safety test by asking you to swallow an amoxicillin tablet and keeping you in the office for the next 2 hours. If you pass this test also without manifesting any allergic symptoms (such as rash, itching, flushing, swelling, cough, wheezing, tightness of throat or chest, shortness of breath, low blood pressure etc.), then you are told that you are not allergic to penicillin anymore. You are also given a letter informing your health care providers of the same.

It is important to understand that testing for both PrePen® and Penicillin G will identify 97% of patients with penicillin allergy (allergy to beta-lactam ring of the penicillin molecule) and amoxicillin challenge will identify those patients who are allergic to side chain of the ring and thereby improve the sensitivity of the test. It is critical to understand that no combination of tests is 100% accurate in excluding penicillin allergy. This is the reason for doing the test by a qualified allergist in a controlled environment where emergency measures could be initiated where required.

Most of the insurances cover the penicillin allergy skin test. Insurances we accept – click on this link to learn about insurances we accept. Check with your insurance company for details. Call our office if you would like to schedule an appointment.