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To the Editor:—
Our attention was called to the fallacy of the preliminary tests patients make on themselves, supposedly to assure their safety from irritation of certain substances, often cosmetics and especially hair dyes. Recently, we have seen cases of severe dermatitis of the face, neck, and ears among women who first proudly, and then with disillusionment, stated, "I did the test just as the manufacturer told me to, and now look at me!"This is the fallacy. Apparently, neither patients nor some manufacturers realize that a test done on the normal body will be negative, but that after exposure there is a period during which sensitization develops, and the same test performed a week or so later may prove quite positive. Thus, a woman will apply a hair dye convincingly ballyhooed and obtain no reaction from her first test; then, when she uses the product two or three weeks
Goldsmith NR. SKIN TESTING WITH COSMETICS. JAMA. 1952;148(6):490. doi:10.1001/jama.1952.02930060072023