Contact dermatitis: Patch Testing for Patients, By David Robles, MD, PhD

Contact Dermatitis and Patch Testing 

If your Dermatologist suspects that you may have a contact allergy (i.e., an allergy to something you are coming in contact with) then they may perform patch testing to find out what the culprit is. Patch tests are different from skin prick tests that allergists do, which are used to diagnose hay fever allergy. Skin prick tests or blood tests for allergies have limited value for patients with skin rashes. 

The T.R.U.E. TEST is a skin patch test indicated for use as an aid in the diagnosis of allergic contact dermatitis. The TRUE Test ®, which is approved by the US Food and Drug Administration (FDA), consists of panels with pre-impregnated allergens. 

The appointments

The first appointment will take about 20 minutes. Three panels of patches will be placed on your back and secured with tape. The patches stay in place undisturbed for 48 hours. You are advised not to swim, or exercise, as the patches may come off. The back needs to stay dry, so no baths or showers to avoid the patches from getting wet. 

At the second appointment, two days later, the patches are removed. The back is marked with a felt-tip pen to identify the test sites.

These marks must still be visible at the third appointment (usually two days later) so, although it is ok to shower, the patient has to be careful not to wash off the marker. The back should be checked by the patient and if necessary re-marked with a pen. The reactions will be assessed by the dermatologist. Positive reactions will have a red square of dermatitis coinciding with the test allergen. 

The results

The dermatologist will record any reactions at the second and third appointments (usually 48 and 96 hour readings). The system used is as follows:

▪ Negative Reaction

▪ Irritant reaction 

▪ Equivocal / uncertain 

▪ Weak positive reaction

▪ Strong positive reaction

▪ Extreme positive reaction  

Nickel Allergy

Nickel is one of the most common causes of contact allergy. Nickel is commonly used in jewelry, belt-buckles and fastener snaps, as well as other metal-containing objects. Dermatitis of the mid to lower abdomen is frequently related to a nickel allergy from the belt buckle or snap.  Eyelid dermatitis from metal eyelash curlers may occur and ear lobe dermatitis is often related to the nickel in earrings.  

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