Patch testing in hypersensitivity skin reactions

Mohannad Abu-Hilal, MD, is an associate professor of Medicine and head and of the Dermatology Division at McMaster University.

What is the role of patch testing in finding the culprit drug in hypersensitivity skin reactions?

The role of patch testing is, first of all, very controversial. Even when it’s sometimes useful, it’s not useful all the time. So, what do I mean by this?

Sometimes doing the patch testing… it needs to be done where the skin was affected. So, if the patient has a drug reaction and this test was done—the patch testing was done—in an area that was not affected, sometimes it might not give you positive results.

The other thing is… There is something we call a false positive and false negative, which is basically if you do the test, it might be positive while the patient is not reacting to this medication. And also it could be the opposite—the patient could be reacting to the metabolites of the drug and when you do the patch testing, it might just end up being false negative. So, clinically or practically, it is not very much useful.

And the last thing is a bit ethical. For those severe reactions, there is a very small theoretical risk of inducing that kind of reaction again. So we usually, you know, elect not to do patch testing in patients with severe reactions in fear of reactivating that severe reaction again.