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The development of the thin layer rapid use epicutaneous (TRUE) Test is a history of cooperation between scientists from academia and industry covering several disciplines: medicine, pharmaceutical chemistry, pharmacology, and statistics. The TRUE Test is today a patch test system with documented stability and allergen content. Allergens are incorporated in a dried-in-gel vehicle, which is coated onto a polyester backing to form a patch. Applied to the skin, the allergens are released when the gel becomes moisturized by transepidermal water. This may seem to be a simple technique, but its development required laborious research and solutions to stability and dosage problems. The test has been clinically standardized with serial dilution tests on sensitized patients and validated in comparative multicenter tests. The test is a significant step towards higher reliability of patch testing. Fifteen years of experience and critical investigations are discussed in this article, as are possible improvements such as expansion of the test with new allergens.

It is tacitly assumed that a positive patch test reaction is not affected by adjacent negative tests. However, despite its fundamental importance for the interpretation of test reactions this assumption has not been proven. To test this assumption, special TRUE-test strips were prepared containing placebo, nickel sulphate and fragrance mix as the only allergens, separated by distances of 1 cm and 7 cm and blinded to the investigators. Patients were synchronously tested with two strips. Out of 493 patients tested in 6 centres, the 93 with positive reactions to nickel sulphate only were evaluated. No relevant difference was found between positive nickel reactions in the two different distances to a negative fragrance patch test. We conclude that a positive patch test reaction is not affected by adjacent negative patch tests, which therefore can be neglected for the interpretation of positive reactions.

The development of the caine mix patch of the TRUE TestTM involved extensive formulation work to obtain optimal release, stability and clinical function. Hydroxypropyl cellulose, ordinarily used in the TRUE TestTM, induced crystallization and inadequate release of benzocaine. Addition of the crystal poison glucose caused rapid degradation of the caines. Polyvinylpyrrolidone (PVP) proved to be a vehicle which incorporated the caines in a stable preparation on the patch. Rapid release of the caines from this vehicle when applied on membranes was confirmed by in vivo patch test studies.

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