Allergic Contact Dermatitis: Early Recognition and Diagnosis of Important Allergens

Sharon E. Jacob; Tace Steele


Dermatology Nursing. 2006;18(5):443-439, plus 4. 

In This Article

Allergen Evaluation

Dermatology and allergy specialists use patch testing to diagnose patients with allergic contact dermatitis. Patch testing is the gold standard for patients with ACD. The most commonly used patch test is the Thin-layer Rapid Use Epicutaneous (TRUE) Test ® which is a pre-packaged commercially available allergen patch test consisting of 23 common allergens and one negative control. The North American Standard Series formulated by the North American Contact Dermatitis Group consists of a regularly updated series of 65 of the most common allergens and is utilized by contact dermatitis specialists.

The TRUE test, while limited to 23 allergens, is widely available. It can be used as a basic screening tool to ascertain some of the more common allergies in patients in areas where access to a contact dermatitis specialist is limited or unavailable. Over 3,700 antigens exist that can cause reactions in people and the TRUE test only recognizes 23 of these. Research and comparison with the North American Contact Dermatitis Series suggests that the TRUE test is missing clinically important and common allergens (Krob, Fleischer, D'Agostino, Haverstock, & Feldman, 2004). In one study, the TRUE test only succeeded in identifying an allergen in 24.5% of patients with ACD (Saripalli et al., 2003). Furthermore, the test partially evaluated 52% of patients with ACD by not identifying all of their relevant allergens (Saripalli et al., 2003). Patients with more complicated cases or refractory dermatitis following avoidance regimens may need referral to a contact dermatitis clinic.

Patch tests are a multiple-step process. First, the provider must elicit a pertinent and relevant history from the patient in order to determine which of the 3,700 allergens to test. Since the test is cumbersome for the patient in requiring abstinence from washing the test site for the duration of the test and being taped for the initial 48 hours of the test, proper performance of the patch test is imperative. On the first day, the patch is placed on the patient's back. It is important to place the patch in an area where there is no dermatitis, ultraviolet exposure, or topical corticosteroid use (Rakel & Bope, 2005). If the patient already is dermatitic in the area of the patch test, it will be difficult to determine which reaction is due to the patch. Sun damage can blunt the ability of the Langerhans cells to identify an allergen, leading to a false-negative test in the presence of an allergy (Murphy, Sellheyer, & Mihm, 2005). After 48 hours, the patch is professionally removed and a preliminary evaluation is done. At this 48-hour read, severe reactions and irritant reactions can be seen. The final evaluation and interpretation of the test are done between 96 and 120 hours.

At the final reading, all positive reactions must be interpreted by a skilled evaluator to distinguish between, for example, an allergic or irritant response and the clinical relevance of the allergen (Cohen, Brancaccio, Andersen, & Belsito, 1997). Reading and interpretation goes beyond negative and positive. There are several reactions patients can have to an allergen. The allergens are formulated to exact concentrations in order to elicit an allergic reaction in patients who are sensitized, without inducing overwhelming irritation. Additionally, irritant-based contact dermatitis can be induced by any practice that strips the outer protective layer of the skin. Surgeons, with frequent scrubbing of their hands with strong soaps, commonly develop irritant contact dermatitis (Antezana & Parker, 2003). Patients with a history of atopic dermatitis are also at increased risk for this, as well as nonspecific hand dermatitis (Antezana & Parker, 2003).

Irritant reactions will often fade over the subsequent days following patch removal, whereas allergic reactions can worsen over the course of the testing period (Antezana & Parker, 2003). This further underscores the importance of a delayed final reading after 96 hours. This read can be useful in distinguishing between irritants and true allergens. False positives can also occur due to an extremely positive reaction "overflowing" in its neighbor's space or leaking of an antigen into another area, also known as angry back syndrome. False negatives can occur for many reasons including inadequate contact of the patch chemical on the patient's back. Reading a patch too soon, before a patient has reacted, results in a false-negative read. Other allergens such as gold or bacitracin take on average 7 to 14 days to cause a reaction (Rakel & Bope, 2005). The health care provider must warn patients about these delayed reactions in order not to miss a reaction. See Table 2 for reasons for false positives and false negatives.

A critical component of the patch test is patient education. It is imperative that the patient fully understands the test and complies with the procedure and resultant avoidance regimen. Providing the patient with information on specific allergens, synonymous names, and cross reactors in addition to suggestions on how to avoid the allergens is the final step of patch testing. A patient who knows what he/she is allergic to but does not know how to avoid it, is no better able to comply than a patient who has not been patch tested. The American Contact Dermatitis Society has generated a Contact Allergen Replacement Database. This database is a cross-referencing tool which accesses ingredients of products. The allergens are inputted into the database and a personalized product list free of the patients' allergens is generated to help guide the patient towards safe alternatives.

In summary, ACD is an important disease with high impact in terms of patients' well-being and medical economics. The art of patch testing starts with eliciting a pertinent history of exposures from the patient, compiling the allergen list to test, properly performing the patch procedure, and educating the patient on avoidance at the conclusion of the test. This important modality can be used to identify and avoid the culprit allergen and cure the patient of dermatitis. The purpose of this article is to highlight common allergens encountered in our environment on a daily basis and to increase awareness for this important disease. An early index of suspicion can lead to appropriate testing, diagnosis, avoidance, and cure.


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