The patch test is a bioassay that, when properly performed, approximates the rigorous approach required by the Henle-Kock postulates in the field of microbiology. Steps must be taken to avoid irritant reactions, but weak reactions continue to challenge the physician in terms of interpretation. Clinical relevance has been reported to weak or doubtful patch test reactions, but this low level of reactivity can also represent a false positive reaction. A positive patch test may provoke reactivation of dormant dermatitis, thus confirming the relevance of the test. Undesired reactions to patch tests include prolonged persistent patch test responses, alterations of pigmenta¬tion, anaphylactoid reactions, pustular reaction, infections, and scarring.

The inability to confirm a patch test reaction, which occurs in the setting of multiple positive tests, led to the concept of an "angry back." This concept envisioned a spillover effect modifying nearby tests, leading to false or unverifiable reactions when the allergens were individually retested. However, more recent studies have found the same degree of non-reproducibility 'when tests are applied simultaneously or sequentially. Further- more, the reactive threshold is now known to vary by as much as 250 fold when allergens are retested. This has led to the term "twinkling back" to reflect the biologic fluctuations inherent in patch testing.

Traditionally, groups of allergens that investigate similar environmental hazards are referred to as "trays” or "series”. A basic screening series is commonly supplemented by these more specialized trays. The size of the basic screening series has increased as our knowledge of environmental sources of dermatitis and the complex­ity of our environment have evolved. Currently, the North American Contact Dermatitis Group screening series comprises 65 items. The current sources of patch test materials are included in this chapter.

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