Patch Testing to Diagnose Contact Dermatitis

Patch testing is a procedure used to identify causes of contact dermatitis such as can happen with exposure to nickel, cosmetics, perfumes, or household chemicals.

Contact allergy does not manifest as an immediate life-threatening reaction but rather is a slower process in which the immune system reacts against certain chemical substances. There are no allergic antibodies involved; instead, the body has an overly sensitive (hypersensitive) reaction to an external stimulus which causes the skin outbreak.

How a Patch Test Is Performed

Before undergoing a patch test, the person will be asked to refrain from taking certain medications (such as topical steroids) or using a tanning bed, especially on the back. The back is usually the area of skin where the patch test is performed.

The patch consists of a variety of different allergens which are applied in little dots on an adhesive sheet. Each patch is applied to the person's back and remains there for 48 hours. During this time, it is important not to get the patches wet, so showers, baths, and excessive sweating need to be avoided.

After 48 hours, the patches will be removed at the doctor's office. Before doing so, the location of each patch will be marked with an indelible surgical marker. This will provide the doctor with a reference when you return to the office for a final evaluation.

At this point, following physician instructions regarding bathing is important, and if permitted to bathe before the final reading, one should avoid scrubbing the pen marks. While itching or rash can occur on the patch site, it's important not to scratch or treat it until after your final doctor's visit is complete.

The final evaluation will be performed anywhere from 72 to 96 hours after the initial placement. Any reaction will be noted, providing insights as to which substances to avoid and the types of treatments to be considered.

Patch testing is painless and, unlike allergy tests, does not involve needles of any sort. Some people, particularly those with several reactions, will get itchy and want to scratch their skin, but you should avoid doing so until after testing is complete. Children can be tested once they are old enough to understand that the patches cannot be removed.

Evaluation and Side Effects

Since the goal of the patch testing is to pinpoint the cause of contact dermatitis, one or more reactions at test sites may occur. A positive test may show redness, bumps, mild swelling, or even form a small blister. Some reactions may be uncomfortable but are typically mild.

Keep in mind that there are limitless numbers of patch tests that can be performed. The standard panels include 30 or more items. A negative test to one patch panel does not mean you do not have contact dermatitis, it means you are not sensitized to any of the reagents tested. If the initial patch test panel is completely negative, additional panels may be recommended based on your history.

Similarly, positive results do not necessarily mean that those particular reagents caused your contact dermatitis; rather, history needs to be correlated with your positive result to ascertain whether it was indeed those chemicals causing the reaction. If the positive result does not correlate with history, again, additional panels may be beneficial in investigating other potential contact allergens.

Once all of the readings of the patch test are completed, a topical steroid may be used to treat any itching or rash.