A patch test is a method used to check for the cause or causes of irritant or allergic contact dermatitis, also known as an allergic reaction of the skin.
Patch testing is a recognized and well-accepted way for dermatologists to determine which allergen or allergens are causing contact dermatitis. The test is not invasive and is extremely safe, making it a popular allergen-testing method.
Josef Jadassohn, known as the father of patch testing, discovered the possibilities of contact testing in 1895. At this time, Jadassohn was a professor of dermatology in Breslau (currently Wroclaw, Poland). He observed that one of his patients developed a skin rash to mercury bandages. Jadassohn realized looking at the skin's reaction to chemicals applied to it as a potential diagnostic tool.
The use of patch testing was slow to develop. Between its discovery and the 1960s, patch testing was heavily used by some European clinics and not at all by others. During this time, results often varied due to testing methods. The concentration of substances used and how the substances were delivered often changed depending on with who did the test and where it took place. Reading of results was not always done correctly, and at first there was little understanding of the difference between irritant and allergic contact dermatitis. As a result, the Scandinavian Committee for Standardization of Routine Patch Testing was formed in 1952, bringing standardization to patch testing. Shortly after, the North American Contact Dermatitis Group (NACDG) was also formed.
The work done by the NACDG was often informal, and in the 1980s, the Federal Drug Administration (FDA) banned the production and sale of patch test allergens in the United States. This decision forced the NACDG to do serious research on patch testing with the goal of getting the Hermal/Trolab's twenty-allergen patch test kit approved. Through the work of the NACDG and the foundation of other groups, such as the American Contact Dermatitis Society, research and funding for education about patch testing accelerated. As of 2012, patch testing is considered the gold standard for diagnosing contact dermatitis and is a common part of the curriculum in dermatology residency programs.
The purpose of a patch test is to determine what allergen is causing contact dermatitis. There are two types of contact dermatitis. Irritant contact dermatitis is the most common type and occurs when the skin is repeatedly exposed over time to acids or alkaline materials that irritate the skin. Common irritants include soap, rubber gloves, and detergent. People with irritant contact dermatitis may experience red rashes that look like burns, dry skin, cuts on the hands, and pain.
Allergic contact dermatitis occurs when the skin comes in contact with a substance that it is extra sensitive or allergic to. Fabrics, adhesives, fragrances, hair dye, and material found in jewelry, such as nickel, and certain plants (e.g., poison ivy) are common allergens. Allergic contact dermatitis may also be caused by airborne allergies, such as insecticide, but this is much less common than direct contact. Red and patchy rashes are the common allergic reactions to these allergens. These rashes may have red bumps and blisters, feel warm and tender, or become thick and scaly. Often times, a reaction will not take place immediately, but instead appear 24–48 hours after exposure.
Although patch testing is considered the gold standard in contact dermatitis testing, it does have some limitations. Patch testing uses only the most common allergens. Therefore, less common allergies will not be detected through patch testing. Additionally, patch testing can only be used for allergens that cause a reaction due to direct skin contact; it cannot detect allergies due to diet or inhaled substances. Patch testing should not be performed on women who are pregnant and nursing, and although it has been found to be safe, patch testing is not typically performed on children.
Patch testing results can be influenced by the use of creams, lotions, or antihistamines within 48 hours of testing. Sunbathing within four weeks before testing takes place also can influence results. It is important that tested areas remain dry during the test period, because patches that become wet or are removed too early cannot be read for correct results. Additionally, it is possible for false positives and negatives to occur.
During the first visit, small patches are taped to the skin using hypoallergenic tape. Patches are almost always attached to a person's back because this is a large, flat, and out of the way area. Each patch contains a different substance that is a possible allergen. Depending on the number of patches being applied, this process can take 20 minutes or longer. Typically, anywhere from 30 to 160 patches may be used. Once the patches are taped, the position of each patch is marked with a special pen. The patient must keep the area with the patches dry until the final appointment. This means avoiding showering, bathing, or excessive exercise that will cause the person to sweat. Excessive bending and stretching should also be avoided in order to make sure the patches stay in place.
After the patches have been on for 48 hours, the patient must return to the dermatologist's office to have the patches removed. During this visit, the location of each patch may be once again marked as the tape and patches are taken off. The dermatologist will make a first reading of the area; however, no conclusions can be drawn at that time.
The patient must return to the dermatologist a third time to have the area read a second time and to receive results. This third visit typically takes place 48 hours after the patches have been removed (or 72–96 hours after the initial visit). During this visit, the dermatologist will be able to make conclusions about the test results.
The dermatologist will read the area where the patches were located during the second and third visits. There are three possible results for each test site. A negative reaction has taken place when the skin does not show any irritation to the substance on the patch. A positive reaction occurs when there is an irritation. This reaction may range from weak to extreme. An uncertain reaction occurs when the dermatologist is not able to read the site for any clear positive or negative reaction.
A positive reaction signifies that a person is allergic to the substance on the patch. With this knowledge, a person should avoid further contact with this substance and wash away any trace of the irritant that may be left behind. It is possible, but uncommon, for a person to be allergic to a substance that gave a negative result, or for a false positive takes place.
The most common side effect of a patch test is irritation of the area where a positive reading was made. This can range from a small rash to blistering, scarring, or pigmentation of the skin in this area. Water should be used to wash away any trace of the irritant and moisturizers or anti-itch lotions may soothe the irritated area. Most often, the passing of time and the avoidance of the irritant is the best treatment. The irritation should go away on its own within two to three weeks. In extreme cases, a person may be prescribed corticosteroid pills. These pills are started with a high dose, which is then tapered off over 12 days.
Bacterial skin infections are a possible, but extremely rare, complication of patch tests. These infections are typically treated by a dermatologist or family physician.
Lachapelle, Jean Marie and Howard I. Maibach. Patch Testing and Prick Testing. New York: Springer, 2012.
Cambridge University Hospitals. “What Does Patch Testing Involve?” http://www.cuh.org.uk/resources/pdf/patient_information_leaflets/PIN1300_patch_testing.pdf (accessed October 12, 2012).
MedlinePlus. “Allegery Testing—Skin.” http://www.nlm.nih.gov/medlineplus/ency/article/003519.htm (accessed October 12, 2012).
MedlinePlus. “Contact Dermatitis.” http://www.nlm.nih.gov/medlineplus/ency/article/000869.htm (accessed October 12, 2012).
American Academy of Allergy, Asthma, and Immunology (AAAAI), 555 East Wells Street, Suite 1100, Milwaukee, WI, 53202-3823, (414) 272-6071, firstname.lastname@example.org, http://www.aaaai.org .
American Academy of Dermatology, P. O. Box 4014, Schaumburg, IL, 60168, (847) 240-1859, (866) 503-7546, Fax: (847) 240-1859, MRC@aad.org, http://www.aad.org .
American Contact Dermatitis Society, 2323 North State Street #30, Bunnell, FL, 32110, (386) 437-4405, Fax: (386) 437-4427, email@example.com, http://www.contactderm.org .
Tish Davidson, AM