Penicillin Allergy Testing: Are You Really Allergic?

If you've been told you're allergic to penicillin, you're not alone. Around 10% of people carry this label. The problem is that the evidence suggests more than nine in ten of them may no longer be truly allergic — and for many, it's not clear they ever were.

This matters more than it might seem. People labelled as penicillin-allergic are routinely prescribed alternative antibiotics that can be less effective, carry more side effects, or contribute to antibiotic resistance. Understanding whether that label actually applies to you has real consequences for your health.

Why so many people carry the label inaccurately

The most common reason is a rash in childhood. A child is given amoxicillin for an ear infection, develops a rash a few days later, and the family is told — quite reasonably, given the uncertainty in the moment — to note a penicillin allergy. What's rarely explained is that many of those rashes are caused by the underlying viral infection, not by the antibiotic. The two simply happened at the same time.

Even when a genuine reaction did occur, penicillin allergy is known to diminish over time. The majority of patients who report a penicillin allergy may not truly be allergic, and most people lose their sensitivity over the years — even those with a history of severe reactions. A label acquired in childhood may have no clinical relevance by adulthood, but it tends to follow a patient through their entire medical record unless someone actively investigates it.

Can't I just have a blood test?

This is one of the most common questions we hear, and it's understandable — a blood test seems like a quick, convenient answer. Unfortunately, it isn't a reliable one.

Blood tests for penicillin allergy measure specific IgE antibodies in the blood. The difficulty is that these antibodies decline over time, often to undetectable levels, even in people who were genuinely allergic. As a result, penicillin-specific IgE blood tests are almost always negative — not because the person isn't allergic, but because the test simply doesn't have the sensitivity to detect it reliably.

A negative blood test cannot confidently rule out penicillin allergy, and a positive result doesn't necessarily confirm clinical allergy either. This is a limitation acknowledged in NHS clinical guidance: blood tests for penicillin allergy are not considered a reliable diagnostic tool and do not help to reach a confident conclusion either way.

The test is not entirely useless — it may contribute to the clinical picture — but it cannot provide the yes or no answer most patients are hoping for. If your GP has suggested a blood test to settle the question, it is worth knowing that a clear result is unlikely.

What proper testing actually involves

For adults, the investigation of penicillin allergy follows a structured pathway, usually beginning with a blood test. A blood sample is taken to measure penicillin-specific IgE antibodies. As discussed above, this test is almost always negative — not because the allergy is absent, but because the sensitivity of the blood test is simply too low to detect it reliably. When the result comes back negative, as it usually does, it confirms that a more thorough investigation is needed rather than providing the all-clear.

The next step is skin testing, carried out at the Fitzrovia Hospital. A skin prick test first checks for an immediate IgE-mediated response. If that is negative, an intradermal test follows — a more sensitive version in which a small amount of solution is injected just beneath the skin surface. If both remain negative, a graded oral challenge is performed: the patient takes increasing doses of amoxicillin under close observation, with monitoring for any reaction over several hours.

Depending on the clinical picture, the skin testing and oral challenge may be completed in a single session or split across two separate appointments. The process requires proper clinical supervision and emergency equipment to be available throughout — which is why it is conducted in a hospital rather than at home or in a standard clinic room.

For children: a different approach

In children, the pathway is somewhat different. The evidence review for NICE's drug allergy clinical guideline found no evidence to support the reliability of allergy testing — skin, intradermal, or IgE blood tests — for the diagnosis of antibiotic allergy in children. In addition, these tests are painful and restricted to only a few specialist centres in the UK.

Current clinical experience suggests that most children in the community who are believed to be allergic to an oral antibiotic will be challenge-negative — meaning they are able to tolerate the antibiotic without reaction. NICE has recommended investigating whether a supervised oral antibiotic challenge, without prior skin testing, could be used directly in children — recognising that skin tests offer little diagnostic value in this age group and add unnecessary pain and inconvenience.

For children with a suspected penicillin allergy, it is worth discussing this with your GP, who can refer to a paediatric drug allergy specialist. A supervised oral challenge, without prior skin testing, is likely to be the most appropriate investigative route.

Who should consider getting tested

Penicillin allergy testing is worth considering if you carry an allergy label and:

• You have needed antibiotics for a serious infection and alternatives were less effective or caused problems

• You are facing surgery and your surgeon has been unable to use standard antibiotic prophylaxis because of your label

• You have recurrent infections requiring antibiotic treatment

• You have never had formal testing and simply carry the label from a childhood reaction

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Penicillin allergy testing at The London Allergy Clinic

Penicillin allergy assessment for adults is offered by Dr Gururaj Arumugakani, our Consultant Allergist and Clinical Immunologist. The process begins with an initial consultation (£350), during which Dr Arumugakani will take a detailed history of your reaction, arrange a blood test, and discuss whether further investigation is appropriate. Blood testing is billed separately; the cost depends on which tests are clinically indicated.

When the blood test result comes back — almost always negative, as expected — the next steps are skin testing and a supervised oral challenge, carried out at the Fitzrovia Hospital. Depending on your circumstances, this may be completed in one session or across two appointments.

At the time of writing, an oral drug challenge is £1,750. A challenge with intradermal skin testing is £2,500. These fees are in addition to the initial consultation and blood test costs.

This service is available to adults and adolescents aged 18 and over. No GP referral is needed to book.

To find out more, book a consultation → or contact us by phone or WhatsApp. Mon–Sat 9am–7pm: 0207 637 9711