Should patch testing be restricted to dermatologists only?

Abstract

Calnan (1) wrote that greatest abuse of patchtesting is failure to use and that number ofdermatologists have never patch tested a patient.However, equally or more serious than this maybe to patch test incorrectly and thus provide er-roneous conclusions (2).Patch testing is, at present, the only practicalscientific procedure for the diagnosis of allergiccontact dermatitis (3). The ease of application ofthe test can mislead the unwary into overlookingthe fact that patch testing is a biological assay witha number of pitfalls. Confidence in results canbe achieved only when appropriate experience hasbeen gained in the indication for the test, its correctapplication, and the reading and interpretation ofits results.Patch testing is indicated when allergic contactdermatitis is suspected and the relevant allergenmust be identified or confirmed. As the only objec-tive assessment of any eczema, it may also serve toexclude a relevant contact sensitivity. Testing isnot a substitute for taking the patient's history orexamination (4). It requires considerable dermatol-ogical experience even to suspect contact allergy orcontact allergy as an aggravating factor in otherskin diseases. Allergic contact dermatitis may beimpossible to differentiate clinically from otherforms of eczema, such as atopic, seborrhoeic, num-mular or dyshidrotic eczema, and these conditionsmay coexist. Taking an appropriate history re-quires time, experience and knowledge of potentialallergens that the patient might have come into* On behalf of the Council of the European Society ofContact Dermatitis (W. Aberer, Austria; K. E. Andersen,Denmark; G. Angeiini, Italy; F. M. Brandao, Portugal;D. P. Bruynzeel, The Netheriands; D. Burrows, NorthernIreland; J. G. Camarasa, Spain; G. Ducombs, France;P. J. Frosch, Germany; N. Hunziker, Switzerland; L.Kanerva, Finland; J.-M. Lachapelle, Belgium; J. E.Wahlberg, Sweden; P. Thune, Norway; I. R. White, Eng-land).contact with at home, in the workplace and duringleisure time. Changing consumer habits, produc-tion procedures, allergen prevalence geographicallyand temporally, and other factors must be con-sidered.The optimal choice of test materials and theirapplication may appear simple. Adequate infor-mation should be given to patients before applyingthe test for legal and practical reasons. The occur-rence of irritant patch test reactions, focal flare ofeczema, active sensitization and other rare side-effects of patch testing need to be dealt with appro-priately (3, 5). The skin ofthe back is the preferredsite for testing; it should be free from eczema andnot have been recently exposed to topical cortico-steroids or UV-radiation. The selection of test sub-stances must be based on the patient's history andwill normally include the current version of thestandard (6). This series must be supple-mented with those substances indicated by the pa-tient's history of exposure. The test de-tects only 50-80% of relevant contact allergens.Only standard concentrations should be used;the potential risk of testing with industrial sub-stances of unknown toxieity must be kept in mindand testing with such materials must be cautious.Test readings should be performed according tothe criteria of the ICDRG (3). Negative, doubtful,positive and irritant well-defined terms. Butthe criteria are of absolutely no value when thephysician is confronted with a relatively weak reac-tion (3); this is frequently the case. The criticalpoint in evaluating a positive test response is nothow severe it is, but the determination of whetherit is a true positive caused by allergy or a non-specific irritant reaction.Accurate interpretation is most important: posi-tive reactions can be positive (allergic) or false-positive (irritant), not relevant to the present der-matitis, relevant to a preceding and/or unrelatedproblem, or be the primary or aggravating cause of