It has been 10 years since the third edition of 'Patch Testing' was published. In this fourth edition, information on new contact allergens has been added as well as updated information on chemicals already included, published between 2008 and March 2018 in the journals 'Contact Dermatitis' and 'Dermatitis'. A large number of (additional) fragrance chemicals investigated by the Research Institute for Fragrance Materials (RIFM) and published up to 2013 is also included with information on their irritancy potential, which may aid in establishing a proper concentration for patch testing them. The data of two of the major relevant textbooks, 'Contact Dermatitis' and 'Kanerva's Occupational Dermatology' have been updated, as new editions were published in 2011 resp. 2012. All available patch test materials from Chemotechnique Diagnostics, SmartPractice CANADA (previously Allergeaze and Brial) and SmartPractice EUROPE (previously Trolab) can be found in the book.

The structure of 'Patch Testing' has remained largely unchanged since the first Edition (which was one of the first books to be printed from 'camera ready copy' supplied by its author) was published in 1986. The number of chemicals included in Table 1 (the 'beating heart' of the book) has increased since then from 2200 to 4900, the number of references has quadrupled to nearly one thousand and the number of pages has doubled in these past 32 years.

The current Edition has several new features and improvements as compared to the previous 2008 3rd Edition. The most important one is that CAS numbers (Chemical Abstracts Service, are provided for each chemical / compound, when available and identifiable. Also, INCI nomenclature is now consistently leading in the choice of the entries names. Prefixes such as cis-, dl-, tert-, p-, bis- and figures such as 2,4-, 1,3,5- etc., that were positioned thus far after the name of the chemical (a relic from a very early and primitive1985 Dbase software) have returned to their proper positions. Also, prefixes are now italicized according to chemistry convention. The systematic search for CAS numbers has revealed many incorrect, imperfect or unusual names, which have now been corrected. However, there are also chemicals that cannot be found in any chemical database or elsewhere on the internet, probably indicating that their names are either incorrect or even non-existent; these can be recognized by the label 'unknown chemical'.

Ten years ago, in the Foreword of the 3rd edition, I wrote: 'Even in this day of digital and on-line information, we think that a printed book ('hard copy') best suits the requirements of patch test departments that information can be looked up quickly, anywhere in a dermatology unit and by everyone needing it'. However, digital sources, I do realize, have important advantages. The current format unfortunately is not very suitable to produce an Ebook, as it is impossible to search for full names. A digital database with separate files for each chemical, which can continuously be updated, would be suitable. An appealing thought and attractive challenge for the author! First, however, the second ('Fragrances') and third ('Topical and systemic drugs') volumes of the recently started 'Monographs in Contact Allergy' Series have to be written.

Anton C. de Groot

Wapserveen, February 2018