Practical problems in the use of patch testing in the evaluation of patients with contact dermatitis

Abstract Patch testing is widely used to establish a specific causal agent(s) causing allergic contact dermatitis (ACD). This test is often helpful in differentiating ACD and irritant contact dermatitis (ICD) which may otherwise be indistinguishable on clinical or histologic grounds. The test has been used since the past century, but only in the last two decades have consistent methods of performing the test in cases of suspect ACD evolved. There are equally well worked-out test procedures at present for the evaluation of cases of photoallergic contact dermatitis. The sensitivity and specificity of a diagnostic test are fundamental characteristics of its accuracy in the categorization of patients as diseased or not, in this case affected with ACD or not. The cut-off patch test concentration used will have an effect on these two parameters, unless the irritancy threshold concentration is above the concentration on the sensitization elicitation curve which would elicit all truepositive allergic responses. As true-positive and false-negative results are usually morphologically indistinguishable, their differentiation rests on clinical judgement and the use of confirmatory tests such as the repeated open application test, usage tests, and environmental exposure analysis. The development of groups of patch test chemicals to screen for contact allergy has evolved in the past 20 years. Such tests have a relatively high risk of leading to false-positive test results. These tests and other collections of test chemicals called “Aimed Trays” should be used only when one is unable to narrow down the list of possible contact allergens with which to test the subject based on the patient's history and clinical findings. Since positive predictive value (PPV) declines as the prevalence of ACD in the population being tested declines, the widespread use of pre-packaged collections of patch test materials, such as the “True” test and “Epiquick,” in an easy-to-use dosage form can be expected to been found in the hands of experts like the North American Contact Dermatitis Group (NACDG) or the European Contact Dermatitis Research Group (ECDRG). This outcome may have serious consequences as patients may be advised they are allergic to environmental agents to which they are not. Clinicians should use these new patch test kits judiciously and exercise great caution in their interpretation. Problems with patch testing of a technical nature (e.g., tape, marking methods, test materials, the scoring of responses) are discussed. Contact dermatitis is an inflammatory skin condition caused by skin contact with an exogenous agent or agents with or without a physical agent (e.g., ultraviolet light). It poses a challenge to the clinician because establishing the cause may prevent the disorder in many cases. In addition to the presenting patient, it offers the opportunity to protect other individuals from experiencing similar skin problems once the cause is identified. Irritant contact dermatitis (ICD) is more common than allergic contact dermatitis (ACD) by about 5 to 1. ICD may be divided into acute, subacute, and chronic forms. The last is the more vexing problem to manage. Even if further contact with the irritant is avoided, these patients often do not fully improve. The differentiation of ICD from ACD is a common clinical problem which often is not easy. Patch testing is often used to assist in this diagnostic challenge. ACD is due to a cell-mediated response to a specific environmental contactant. Sometimes this process involves ultraviolet light. Once induced, the immune recognition tends to be lifelong. Characteristically the affected individual responds to topical challenge concentrations of the allergen far below the concentration that would cause skin irritation in a non-allergic person. Patch testing is based on the premise that if one applies the allergen to the skin at a sub-irritant concentration that only a subject allergic to the test chemical will exhibit a positive response. In practice, this is often not the case. An overlap between the patch test concentration that will induce a response in all allergic persons and the minimum concentration that will cause an irritant response in some normal persons often occurs. These normal people that have a reaction exhibit false-positive patch test reactions which usually cannot be differentiated morphologically from true-positive test reactions. Present patch test systems produce frequent unexplained positive responses, many of which may be false-positive results. Since a conventional sensitivity test for diagnostic tests has not been carried out with the present patch test systems, the number of such false-positive responses is not known. Although the present screening series of the European Contact Dermatitis Research Group (ECDRG) and the North American Contact Dermatitis Group (NACDG) offer a sensitivity rate near 70%, this is in populations where the majority of the test subjects have ACD. With the advent of simple, easy-to-use, pre-packaged patch test kits (e.g., “True” test and “Epiquick”), it is possible that many more physicians will be encouraged to carry out patch tests. Since the prevalence of ACD in these patient populations is likely to be lower than has been reported by these expert panels, it is predictable that more false-positive reactions will be the result. Those who use these tests must be cognizant of this problem and treat positive responses with discretion. The practice of testing many contactants at one time should be avoided to minimize the risk of false-positive results. Only if there is no clue as to a specific substance to which the patient may be allergic should one embark on screening patch test series. As a rule of thumb, on average one may encounter a false-positive patch test response in one of every 20 patches that are applied at the same time. The scoring of patch test responses has been agreed to by international convention. The clinician is well advised to describe the morphology of the patch test reaction which occurs and then compare his description with that of the international convention to arrive at a score. This technique will avoid a confusing diagnosis with morphologic interpretation.