According to a recent study, up to 95% of people identified as having a penicillin allergy are found to be incorrectly labeled following comprehensive allergy testing. Just over 4 million people in England have been told that they are allergic to penicillin. However, in 2.7 million cases, this is incorrect. Unfortunately, due to the potential severity, once you are coded as having a penicillin allergy, it’s going to be there for life. The only way to remove this and correct your record is to undergo penicillin allergy testing that shows otherwise.
By being incorrectly labeled as having a penicillin allergy, you are at a high risk of not getting the right antibiotics when needed. This could subsequently lead to poorer health outcomes down the line.
What is a Penicillin allergy?
A penicillin allergy occurs when the immune system mistakenly identifies penicillin or its derivatives as harmful molecules. This can trigger an allergic reaction of varying severity.
Symptoms
The symptoms following an allergic reaction to penicillin can range from mild rashes to more severe and life-threatening conditions such as anaphylaxis.
Understanding whether one has a true penicillin allergy is also vital. Misconceptions about allergic reactions can lead to the unnecessary avoidance of penicillin and its derivatives, which are often the most effective antibiotics available. They also tend to have fewer side effects than penicillin alternatives.
Moreover, mislabeling individuals as penicillin-allergic can contribute to the growing problem of antibiotic resistance. Alternative antibiotics may not be as effective, requiring higher doses or longer treatment durations.
I see a lot of patients who say they had an upset tummy, felt nauseous, or even vomited after taking a penicillin antibiotic. These are not signs of an allergy. They are merely side effects of the antibiotic in most cases.
Penicillin Allergy Testing Methods
We can test for penicillin allergy via different methods, similar to other allergens.
- Skin prick testing
- Oral challenge
- IgE blood tests for penicillin related substances
Before undergoing testing, you must also be assessed by a specialist to determine suitability. This will be based on the previous reactions you have had and the likelihood of there being a true penicillin allergy reaction.
Please note that in cases of severe or clear-cut reactions that indicate a high probability of penicillin allergy, testing will not go ahead due to the risks.
Out of the above, skin prick testing and an oral challenge are the most sensitive. The oral challenge is performed by administering a small amount of low-concentration penicillin and observing for any reaction.
The challenge with testing for penicillin via skin prick testing or an oral challenge is that the immune system reactions can be very severe. This means that some people could develop systemic swelling and anaphylaxis after a single drop of penicillin on their skin or tongue. This may happen even with very diluted concentrations.
Penicillin IgE Testing
The safest initial test is therefore the indirect test of IgE blood test where we can safely check for IgE antibiodies to penicillin. This is an entirely safe procedure that involves taking 5ml of blood and sending it to the lab to have it checked for these antibodies.
This blood test usually checks for allergies to Amoxicillin, Ampicillin, Cephalosporins, Benzylpenicillin (Penicilin G) and Phenoxymethylpenicilin (Penicilin V). Here is more information about each. The column on the left shows the exact substance the blood test checks for. The column on the right shows the corresponding antibiotic.
| Substance | Description |
|---|---|
| Amoxicilloyl | A determinant for amoxicillin, this test measures antibodies against amoxicilloyl to determine allergies specifically to amoxicillin. |
| Ampicilloyl | Similar to amoxicilloyl but for ampicillin, testing for ampicillin-specific antibodies can identify allergies to ampicillin. |
| Cefaclor | As a cephalosporin, testing for cefaclor is crucial due to potential cross-reactivity in individuals with penicillin allergies, providing insights into broader antibiotic allergies. |
| Pen G (Benzylpenicillin) | Testing for IgE antibodies against Pen G helps confirm a general penicillin allergy, crucial for determining overall sensitivity to penicillin antibiotics. |
| Pen V (Phenoxymethylpenicillin) | This test identifies allergies to Pen V, a specific form of penicillin, enabling more precise allergy identification. |
What Happens After Testing?
The blood test results usually take about 2-3 days to come back. After the results, a follow up consultation is required to analyse them and discuss the next options. There are two options:
Negative result
Completely negative result. Due to the limitations of the IgE blood test, however, we may still need to perform a further oral challenge or skin prick test. This is the only way to be 100% confident that you are not allergic to penicillin. A negative IgE blood test to penicillin cannot confidently say that you are not allergic. A small percentage of patients with a negative IgE blood test will still be allergic to penicillin.
Follow up confirmatory testing with a skin prick test or oral challenge can now be considered. Although possible, we have mitigated for the high risk of a reaction, knowing that the IgE test showed no major reaction.
Following either the skin prick or oral challenge test, you need to be observed for at least 2-3 hours in a clinical environment before you can leave. This is for your own safety in case of delayed reactions.
Positive result
Positive results confirm a penicillin allergy. This means that it is necessary to avoid penicillin and related or cross-reactive antibiotics in the future. In some cases, a desensitisation procedure may be considered, depending on the severity of the allergy and your own medical needs. This can be performed in more specialist clinics.
FAQs about Penicillin Allergy Testing
Here are some commonly asked questions that you may still have related to allergy testing for penicillin antibiotics.
Q: How accurate is penicillin allergy testing?
A: Penicillin allergy testing, including both skin tests and IgE blood tests, is highly accurate. Skin prick tests in adults have been shown to have a high negative predictive value, meaning if the test is negative, it is very likely that you are not allergic to penicillin. IgE blood tests provide additional specificity for certain penicillin components and must also be done as the first step to rule out a very severe allergy.
Q: Is penicillin allergy testing safe?
A: Penicillin allergy testing is conducted under controlled conditions in a clinical environment and with medical supervision to manage any adverse reactions immediately. An IgE blood test should be performed as the initial step. The second stage of an oral challenge or skin prick test should only go ahead if the IgE blood test is negative. The risk of a severe allergic reaction is therefore low but not insignificant. You have to fully understand the risks of this type of testing before going ahead with it.
Q: What should I do if I test positive for a penicillin allergy?
A: If you test positive, we will let your GP know and recommend avoiding penicillin and related antibiotics. We may also suggest alternative antibiotics for future treatments and discuss other potential options if penicillin is critically needed.
Q: Can penicillin allergies go away over time?
A: Yes, some people may outgrow their allergy, especially if they have not had an allergic reaction in many years. Your allergist may recommend retesting to determine if the allergy still exists.
Q: How should I prepare for penicillin allergy testing?
A: Preparation may involve stopping certain medications that could interfere with test results, such as antihistamines. Before arranging the appointment, we can give you specific instructions based on your medical history.
Conclusion
Penicillin allergy testing plays a critical role in ensuring patients receive the safest and most effective antibiotic treatment when needed. If you think you’ve had an allergic reaction to penicillin, it may be worth getting a test. It’s also recommended to have a test done if you have a penicillin allergy coded on your GP record with no recollection of previous allergic reactions.
One of the largest studies from date, published in the Lancet in July 2025 has further encouraged those with uncertain penicillin allergy to be tested, in an effort against antimicrobial resistance.
We know the statistics show that the majority of people with a penicillin allergy record don’t actually have a true reaction. Getting tested is therefore key in managing your health effectively both now and in the future. To enquire about penicilin allergy testing, please contact us.
