Penicillin allergy in children: patch testing

Testing for penicillin allergy

We test for penicillin allergy only if your child has a medical history that suggests a possible allergic reaction.

We use the following steps to test your child for penicillin allergy. 

• A skin prick test for an immediate allergic reaction.
• A patch test for a delayed allergic reaction.
• An oral (by mouth) drug test which we use to exclude or confirm a diagnosis of penicillin allergy.

(If your child needs an antibiotic before we complete patch testing, we will choose a suitable non-penicillin alternative).

About patch testing

We use patch testing after your child has had a negative skin prick test for immediate allergy. (A negative skin prick test means your child is unlikely to have an allergy).

A patch test acts as a second check. It identifies whether your child has a delayed allergy to penicillin. This type of allergic reaction usually happens more than 6 hours after your child takes penicillin.

Patch testing is not painful, but if there is a reaction, your child's skin may become itchy or sore.

If your child becomes distressed because of the patch, you can remove it. If this happens, get in touch with us (see Contact information below).

Accuracy

Patch testing on its own is not 100% accurate. It may give a false negative result (where the test is negative, even though an allergy exists).

However, when we combine it with a third step - an oral drug test (see below) - it gives us an accurate way to exclude or confirm a penicillin allergy.

(An oral drug test means we give your child a single dose of penicillin by mouth, while we carefully observe their reaction). 

What happens during the patch test

The test takes place in Kingston Hospital's Dolphin Ward.

You can expect the following during the test.

• In this test we use amoxicillin, which is a type of penicillin antibiotic. To make the patch, we place a tiny amount of amoxicillin on a paper disc, inside a small metal chamber. The chamber looks like a small metal disc and it measures about 1.5 cm across. 
• We place a control patch (which uses salty water instead of amoxicillin) in a separate chamber next to the amoxicillin chamber. We use this control to check if your child's skin reacts by having something neutral (like salty water) against it.
• We use hypoallergenic tape to fix both chambers to your child's back. (Hypoallergenic tape, designed for sensitive skin, is latex-free and chemical-free.)
• We write some markings on your child's skin, next to the patch.

Recording your child's test reactions

After your child has worn the patch for 48 hours, do the following:

• use your phone to photograph their skin
• remove the patch
• photograph their skin again.

Follow the example timeline below to take the necessary photos of your child's skin.

Example of timeline

If your child's skin appears red on day 5, we will ask you to take another photo 2 days later (on day 7 eg Sunday).

Patch test results

When we receive your photos, we analyse  them. We aim to let you know the test result (by email) within 1 week of receiving your photos.

Oral drug test to confirm patch results

If the patch test is negative

If the patch test is negative (no likely allergy), we ask you to bring your child back to Dolphin Ward for an oral (by mouth) drug test. We use this to exclude or confirm an allergy to penicillin.

We usually schedule this within 1 month of emailing you the negative patch test result.

The oral drug test means we give them a single dose of amoxicillin by mouth while we observe their reaction. This takes a maximum of 1 hour. We will let you know the result before you leave the hospital.

If the patch test is positive 

If the patch test is positive (your child does have a penicillin allergy), we do not proceed with the oral test.  

Follow-up

We will send your child's discharge summary (with their allergy test results) to your GP. We can also give you a copy to take home.