A controlled trial of objective measures of sunscreen and moisturizing lotion.

Taking an alcohol swab of a person's forearm and analyzing it using a spectrophotometer has been shown to be a reliable method for detecting the presence of sunscreen. The aims of this study were to determine if moisturizing lotions or other non-sunscreen products influence the absorbance readings from skin swabs in a controlled setting, and to establish the cutoff point in determining the presence or absence of sunscreen using a crystal cuvette instead of a plastic one. In a controlled trial of 30 volunteer office workers, absorbance readings from two popular brands of sunscreen with sun-protection factors (SPF) of 30 and 45 were compared with absorbance readings from two different moisturizing lotions, one with an SPF of 15 and another with no stated SPF. Moisturizers with SPF 15 tested positive for sunscreen, with absorbance readings (mean, 3.77; min, 3.30) comparable to sunblock with SPF 30 or 45 (mean, 3.51; min, 2.02). Moisturizers with no stated SPF factor tested negative for the presence of sunscreen, with extremely low absorbance readings (mean, 0.06; max, 0.19) similar to control readings. The skin swabbing technique remains a valid and useful method for detecting the presence of sunscreen and does not result in false positives when moisturizers with no stated SPF are present. Using a conservative cutoff point of 0.30 with a crystal cuvette reduces any chance of false-positive readings and remains robust when sunscreen of SPF 15 or higher is present.