Allergy Patch Testing

Patch testing is a procedure doctors and dermatologists use to find out what is causing a particular kind of skin rash called contact allergy—also called contact dermatitis, contact eczema, or allergic contact dermatitis (ACD). During patch testing, adhesive panels holding small plastic or aluminum chambers filled with different test allergens are applied to clean, dry, healthy skin on your back. These panels are left on your skin for 48 hours. If you are allergic to any of the substances tested, a reaction will be visible on your skin when the patch test is removed. Patch testing is a safe and reliable way to identify the substance you are allergic to.

Patch testing is only one kind of allergy test. Along with blood tests for allergies, there is another kind of allergy skin test called a “prick test” or “scratch test,” which is used to detect immediate allergic reactions to substances such as peanuts, pollen, mold, pet dander, or dust mites. In contrast, patch testing is used to detect delayed allergic reactions that are a sign of contact dermatitis. This rash-like skin condition results from touching something you have previously been sensitized to. It may be a substance you were exposed to only minimally, such as a single henna tattoo or a medicine, or an ingredient in something you have used frequently, such as a favorite perfume or piece of jewelry. In either case, the rash may develop days or even weeks or months after your first contact with the substance.

Patch testing can identify many allergens. Allergens may be plants and foods; metals; rubber and plastics; fragrances and cosmetics; preservatives and additives; and more. Common allergens are nickel (often in jewelry or buckles), latex, fragrances, and preservatives such as parabens and methylisothiazolinone. Even gold is allergenic to some people. To simplify the patch testing procedure, standard series of these allergens have been manufactured. There are also series for specific sites on the body, such as the face or feet, or series designed for particular occupations, such as a dental series or hair stylist series. Depending on your history and personal situation, your dermatologist will determine which series or single allergens you should be tested for.

Patch testing requires at least 2 visits to your dermatologist. On the first visit, a nurse or clinician will apply the patch test panels to your back; this takes less than an hour. On the second visit, typically 2 days later, the patch tests are removed, and the dermatologist or doctor “reads” the results by looking for reactions to the test substances at the patch test sites. Your doctor may recommend a third visit for a second reading to ensure an accurate diagnosis and discuss your treatment and management. Although most reactions to the test substances occur within 5 days, some can take as long as 10 days. In such cases, your doctor will suggest an additional reading at day 7 to 10.

A positive patch test result ranges from a small skin rash with a little swelling to red bumps, blisters, and wheals. The more severe the reaction, the more sensitized you are to the allergen. It is also possible to get an irritant reaction to a test substance, which can be misidentified as an allergic reaction. A skilled and experienced dermatologist or patch testing practitioner will be able to read your result accurately. 

Negative results from a patch test are possible. Usually a negative result means you are not allergic to the allergens that were tested, so your doctor may have to test you again with different substances. This process will continue until the allergen causing your contact dermatitis is identified. Fortunately, 35 of the most common allergens cause 66-75% of all allergic contact dermatitis.

As with any kind of skin test, patch testing is not 100% accurate. A patch test may return a “false positive” result, indicating a contact allergy when you do not have one, or a “false negative” result, not triggering a reaction to a substance that you are allergic to. Nonetheless, patch testing is the gold standard of diagnosing contact allergies. A dermatologist who is trained in patch testing is your best bet for an accurate diagnosis.

Manufacturers of patch tests have made many design improvements in recent years that reduce discomfort. You will likely be aware of the patch test panels while wearing them, but the small size of most panels and chambers, as well as the flexible materials, provide for a more comfortable experience.

Because your skin is being exposed to a variety of different chemicals, you may feel itching and/or slight burning at some of the sites while wearing the patch test panels. While this is normal, if the sensation becomes severe, you should contact your doctor immediately. If you do experience itching or burning, you should try not to scratch the patch test area as scratching may further irritate your skin and make the itching worse. Scratching can also make it harder for the doctor to read the results of the patch test.

Other side effects are uncommon. Although some patients have had skin reactions to the adhesive or to the aluminum chamber in some patch test systems, such reactions are very rare. Moreover, patch testing should not make you feel sick.

In addition to fees for 2 or 3 office visits, the patch test allergens cost about $9 each. A typical patch test involves anywhere from 35 to 80 allergens, so an average test may cost around $1000. However, patients often pay less than that out of pocket. Experts agree that for many patients with persistent eczema and rashes, patch testing is likely to save time, money, and trouble in the long-run. Because contact dermatitis is often a life-long disease, over time the cost of patch testing will be much less that that of multiple specialist doctor visits, monthly prescriptions, and lost work time. Without a proper diagnosis—which is best achieved through patch testing—patients may suffer needlessly or even pay for unnecessary treatments for decades.

Patch testing is the most reliable way of identifying contact allergens because it can quickly narrow down the allergens that are potentially causing your dermatitis. However, another test commonly used is the Open Application Test, also called the Repeat Open Application Test (ROAT).  Whereas a patch test typically tests from 35 to 85 allergens at the same time, this test is done with only one allergen at a time. It is recommended when there is a good chance a specific substance is responsible for the skin rash, for example, products you leave on your skin such as cosmetics, face creams, or fragrances. As the name of the test suggests, the substance is simply applied to a small section of skin on several occasions over several days to see if it causes a reaction. ROAT is useful in testing products you use at home that may have ingredients that are not included in patch tests.

There really is no other way to identify a contact allergen, except for a hit-and-miss process of elimination. This would require a very long period of time and is not likely to give accurate results.

Because the patch test panels are usually applied to your back, you must remove your shirt or blouse and put on a kind of short medical gown that leaves your back open. You do not have to remove clothing, such as pants, shorts, or skirts, on the lower part of your body. Female patients may keep their bra on but must unfasten it during patch testing. The bra may be fastened again and worn for the duration (48 hours) of the patch test.

Patch testing interferes very little with your daily routine. However, you should avoid the following activities:

• Exercise and activities that make you sweat;
• Showering, tub baths, swimming, or getting the patch test site wet in any way;
• Moving in a way that could cause the patches to come off;
• Exposure to sunlight.

There is no “cure” for contact allergy, but with your dermatologist’s help you can learn how to manage it and maintain a good quality of life. The first step forward is accurate diagnosis through patch testing. When the allergen or allergens causing the reaction are identified, your dermatologist can then recommend an appropriate therapy and management strategy.