PROBLEMS OF PREDICTIVE TESTING 355 Table I Parameters Known to Influence Human Skin Reaction Parameter Response Reference Site of application (in sensitization test) Mode of applica- tion (in sensiti- zation test) Type of patch used Dose Trauma (at site of application) Sex 1. Back abdomen extremities 2. Reapplication to same site new site Epicutaneous intradermal Closed open 1. Response depends on amount deposited per unit area, not total dose 2. One large dose is more sensi- tizing than several small doses Chemical trauma mechani- cal trauma Male female Kligman (21), Magnusson (25) Kligman (1, 21) Kligman (21) Magnusson (14, 25, 26) Kligman (21) Lowney (27) Rebello (28), Kligman (21) Lanman (15) Table II Parameters Which May Influence Human Skin Reaction Parameter Comment Reference Age Trauma to skin (at a remote site) Race and heredity Seasonal variation Pregnancy Diet Effect in sensitization not clearly established Probably increases reaction Probably important 1. Response nfinimum occurs during summer 2. Greater reaction during winter 3. Sweating under patch does not increase reaction Not known Not studied Sipos (29) Kligman (21), Forsbeck (22) Hjorth (30) Justice (20), Kligman (19) Bettley (31)

356 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are unequivocal: The product causes adverse effects or it does not. The in-use test still has many important limitations, such as: number of sub- jects their age and sex period of use geographical locale and possible lack of sensitivity to misuse or deliberate abuse. These problems can sometimes be resolved by careful consideration of the product's intended use and the manufacturer's recommended directions. A major problem is the reliability of the test subjects who must be able to communicate with the investigator and must be depended on to use the product. This problem has been noted by Maibach and Epstein (12), who feel that such a test is frequently not a test at all. This is, of course, a matter of experi- mental design, but does not detract from the value of properly supervised and controlled in-use tests. CLINICAL EXPERIENCES So far, the discussion has been primarily in terms of the possible chance of error or possible problems in the interpretation of results. In order to illustrate these problems and to re-emphasize the need for care- fully controlled in-use tests, two examples from the authors' experiences will be discussed. Approximately six years ago a patch test study of a make-up product elicited no evidence of either primary irritation or of sensitization via the repeated insult technique. More recently, this composition was re- examined for primary irritation and sensitivity by the same technique at a different test locality. Surprisingly, the product now caused primary ir- ritation and could not even be subjected to the repeated insult test be- cause of the high incidence of irritation. Our approach to this riddle was a pragmatic one: A series of break-down products, in which one or more ingredients were deleted, was subjected to patch and sensitization tests. After much effort, the offending ingredient was identified, and the product was then reformulated to yield a new, hopefully safer composi- tion. Although a definitive explanation cannot be offered, several things could have taken place during the intervening five or six years: There may have been subtle changes in the raw material due to a different process of manufacture. The time of year during which the tests were conducted was different. The selection of test subjects could have played a role in causing the second test to show positive results, whereas six years ago the product was considered safe and ready for marketing. Last,