Value of Repeated Tests in a Percutaneous Absorption Study

Abstract

Percutaneous absorption of salicylic acid was measured by blood levels in 20 rabbits, each given four runs at 1-week intervals. Dimethyl sulfoxide was added to the ointment in two runs (Runs 1 and 3 for half the animals, Runs 2 and 4 for the others), thus forming a crossover design with one repetition of each condition. The four successive runs caused a progressive and statistically, highly reliable decrease in percutaneous absorption. This effect of prior exposure increased during successive 1.5-hr, time intervals, from 1.5 to 7.5 hr., and showed no reliable interaction with dimethyl sulfoxide and with pH. A large effect of prior exposure was the increased stability of the salicylate absorption scores and, thereby, greater reliability of the experimental effects, shown by much lower residual terms in the analysis of variance for Runs 3 and 4 than for 1 and 2. Skin dehydration and decreased emotional arousal are possible explanations for the slower and more consistent rate of percutaneous absorption after prior exposure to the test situation.