For decades, the different editions of "Patch Testing" by Anton de Groot have reliably accompanied researchers and particularly clinicians involved in patch testing world-wide. Especially those colleagues who not only apply the baseline series, or perhaps some other, special test series which is commercially available, but proceed to the second level of diagnostic work-up, which could be called "detective work" or "investigative dermatology". It can be quite complex to identify the adequate set of patch test haptens from the range of commercial preparations offered by the patch test material manufacturers - at least one can here rely on the fact that the chosen test concentration and vehicle are correct and stability is proven. However, if no such preparations exist, one regular issue is to identify the correct concentration and/or vehicle for a substance which shall be patch tested, and needs to be prepared on site - either from samples a manufacturer of a product has hopefully sent (with concentration, vehicle and substance name given!), or from chemicals obtained from sellers of chemicals.

Already the first edition has provided such information on about 2500 haptens, in addition to substance identifiers such as preferred names, synonyms, and, now added to the scope of information, the very important CAS numbers, along with references enabling further in-depth research. In fact, thinking back almost 30 years, when I was working with an enthusiastic team in Göttingen to help set up the Information Network of Departments of Dermatology (IVDK) founded by Axel Schnuch, I remember typing all these substances as a catalogue into the database of the patch test software, which took some time. I surely learnt a lot about the intricacies of nomenclature which transpired, and still transpire "Patch Testing".

The importance of a reliable (or often: as reliable as possible) starting point for determining the adequate patch test concentration and vehicle (which is of course often related) cannot be overestimated: both for the patient, who experiences a valid diagnostic procedure and consequently, receives valid advice, and for the researcher, who perhaps ventures to submit a case report on a (relatively) novel allergen, or a special allergen which he or she found in a new exposure context. Hence, this book supports the quality of such evidence published in journals such as "Contact Dermatitis", and thus has a multiplication effect.

While the first set of substances from the 1st edition is still there, both in the 4th edition and in the intestines of the IVDK (and ESSCA) databases, it is clear that the ever expanding scope of substances, or also extracts (natural mixtures), which have been identified as contact allergens, necessitates constant update and expansion of this invaluable source of information. I am very glad that Anton has now summarized the collective information on how to patch test haptens, presenting the 4th edition of "Patch Testing".

Prof. Dr. Wolfgang Uter
University of Erlangen / Nürnberg
Department of Medical Informatics, Biometry and Epidemiology
Erlangen, Germany

March 2018