What are Skin Tests for Drug Allergy?

Picture of Dr Van Nieuwenhuysen
Dr Van Nieuwenhuysen

Dr Christian Van Nieuwenhuysen is an Anaesthetist with additional experience in the investigation of peri-operative drug allergies, including anaphylaxis in response to anaesthetic agents.

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Allergies to medications are potentially serious and need to be investigated. Skin testing is the most accurate way to investigate these types of reactions.

What Is A Skin Test for Drug Allergy?

The most severe kinds of allergic reactions to drugs can be immediately life-threatening (such as anaphylaxis) or can develop into life-threatening reactions over time. Drug allergy skin testing can identify these kinds of reactions in a safe way, prove which drug is causing the reaction, and prove which drugs are safe.

Skin testing is not new – it has been around for decades in its current form, and its origins are even older than that. Many kinds of doctors use it to investigate many different kinds of allergy. It can be used to test for food allergies, allergies to insect venoms, allergies to grasses and pollens, and it can also be used to test for medication allergies.

The principle is aways the same – a very small amount of substance is pricked or injected just under the skin surface, and a small reddened “hive” or “welt” develops at the injection site if the patient does have a real allergy to the substance.

The tested substance stays in the top layer of skin, and does not enter the bloodstream large amounts, therefore the skin testing is not expected to cause systemic or “whole body” symptoms. The test exploits the fact that skin is one of the organs of allergy and is essentially attempting to trigger a reaction in a single patch of skin.

Each skin test is injected, marked, or measured in some way, and then examined again at a later time point, usually 15-20 minutes after testing. Often, clinical photographs of the skin will be taken to make a permanent record of the results.

There are different ways to assess whether a skin test is positive, but some kind of change is required at the injection site to make a skin test positive. For example, some skin tests involve injecting a small amount of substance just under the skin.

To be positive, these skin tests need to show an expansion and reddening of the injection bleb.

Drug Allergy Skin Test

Any positive skin tests may become itchy for a few minutes. While the test is being done, the skin test sites cannot be touched or disturbed, but afterwards no special measures are needed. 

Often a positive control will be used. A positive control is a substance that makes the skin react on purpose, to show what any other positive skin tests should look like on that patient’s skin. Positive controls frequently become itchy for a short time too.

Some kinds of skin tests are not expected to react immediately but may be expected to react over a period of days.

Once a skin test for drug allergy is complete, the results will be interpreted, and the doctor will form a conclusion about whether a drug allergy is present. Often the result is quite certain. In some other cases, further testing may be needed to arrive at an answer. In other cases, a result may be borderline or difficult to interpret.

Any area of intact skin can theoretically be used for skin testing, but common sites include the inside of the forearm or on the back.

Like any medical test, skin tests for drug allergy are not perfect but they are the most accurate test known to identify drug allergy, and the accuracy is very high. They should be considered the gold standard for investigation of drug allergy.