The patch test
Contact allergy is diagnosed by a type of provocative test whereby the patient´s disease, eczema, is reactivated albeit in miniature. A century ago, Jadassohn in Vienna was the first to apply a suspected chemical epicutaneously to the skin and thereby evoked an allergic contact dermatitis. Although the method has been meticulously developed and standardized the principle is basically the same.
The suspected chemical is applied in a suitable concentration and solvent (usually petrolatum) on a thin cm-wide plate of aluminum or plastic which in turn is attached to an adhesive plaster together with several other chemicals. Ready-to-use plasters with incorporated allergens are also available. The plaster is applied to the back (Figure 1) for 48 hours and then removed by the patient. After a further 24 hours, ie. on the third day after application, and preferably also on the seventh day, the tests are "read" by an experienced dermatologist.
Figure 1 Applying patch tests with allergens on the back
Eczematous test reactions (Figure 2) are registered and their relevance to the disease considered (some reactions may be an expression of an allergy previously contracted and without present significance). It is very important to inform every patient, also in writing, on his/her contact allergy and how to avoid contact with the causative chemical in order to prevent recurrence of the skin disease.
Figure 2 An itching red eczema elicited by a patch test
The test substances are primarily available as a series of 20-30 standard chemicals known worldwide to induce contact allergy and contact dermatitis. Concentrations and vehicles are carefully selected so as not to result in false positive or false negative test reactions. The patient should also be tested with chemicals and products which might be suspected based on his/her history, working contacts, home conditions, topical treatments, cosmetics etc. It comes without saying that patch testing is extremely important for the disclosure of an occupational dermatitis.
Among most common sensitizers and therefore included in international test series can be mentioned metals (nickel, cobalt, chromium), perfumed compounds (fragrances, balsams), colophony (a resin ), corticosteroids, preservatives and other ingredients in topical products (Kathon CG, thimerosal, lanolin, ethylenediamine), antibiotics (neomycin), dyes (paraphenylenediamine, azo colours), rubber chemicals (thiurams, mercaptobenzothiazole), formaldehyde, plastic chemicals (epoxy, acrylates, phenolic resin), anaesthetics (benzocain, cinchocain), plant allergens (primin) etc. Nickel is the leading contact allergen in all patient materials of the Western world, the cause apparently being the custom of piercing earlobes, primarily in young females.
Patch testing should be carried out in all cases in which a contact allergen can be suspected as a cause, particularly in hand eczema (occupational contacts !) and hypostatic eczema of the legs (topical preparations !). Contact allergy, however, often complicates other forms of eczema (non-allergic contact dermatitis, atopic and seborrhoeic dermatitis) which means that patch testing is a valuable instrument for investigating patients with all forms of eczematous disease. It cannot be used for diagnosing non-allergic contact dermatitis since the causative chemicals (detergents, organic solvents etc) usually do not sensitize.
