Many, maybe most, cases of allergic contact dermatitis cannot clinically be distinguished from other types of eczematous disease. Patch testing is of help in identifying such cases and in confirming clinically suspected cases. However, because of Bayes’ theorem, patch testing (like other diagnostic tests) is of more use in confirming a suspected clinical diagnosis than in indiscriminate screening.
Patch testing can be carried out with a suspected contactant itself (“use test”) or with the chemical constituents of the contactant. The former is usually more practical, since it does not require the purchase and periodic updating of a patch test kit. Use tests are suitable for most nonindustrial contactants such as clothing, cosmetics, and medications where the item in question has been designed for direct application to the skin. Use tests may be considered inappropriate, when evaluating industrial and laboratory chemicals, because of the possible development of an unexpectedly severe reaction.
A use test is performed by taping the suspected contactant tightly against the skin for 48 hours. At the end of 48 hours (or earlier if the patient experiences severe itching) the bandages or tapes are removed, and the site of application is examined. Positive reactions will be red, raised, and pruritic. Flat, red reactions are interpreted as indeterminate and are not ordinarily considered clinically important. Negative reactions, of course, show no visible change at all. Care should be used so that irritant reactions to the tape are not misinterpreted as reactions to the contactant.
Patch test kits are available for more sophisticated types of testing. These kits contain appropriately diluted concentrations of the most commonly encountered contact antigens. They are generally used to screen either for possible contactants in eczematous disease of unknown etiology or for identification of a single, specific antigen in patients with positive use tests. The complexity of this type of testing together with the need to continually replace outdated material makes the use of patch test kits impractical for most generalists.
A positive patch test does not automatically confirm a diagnosis of contact dermatitis any more than a positive tuberculin skin test proves that a patient’s pulmonary disease is due to tuberculosis. Proof that a patient has allergic contact dermatitis requires both the presence of a positive patch test and improvement of the patient’s condition when the suspected contactant is removed from the patient’s environment.
Tags:chemical constituents, contact dermatitis, irritant reactions, laboratory chemicals sophisticated types
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