Standardisation of minimal erythematous dose reading and assessment: a new system

Sunscreen efficacy is usually expressed by the sun protection factor (SPF) which is calculated by establishing the minimum erythematous dose (MED), i.e. the smallest amount of energy required to trigger erythema. Efforts have been made to harmonise SPF testing but in vivo SPF methodology could still be improved to reduce its variability. This article proposes a means of standardising MED evaluations through the development and validation of an MED assessment system based on image analysis.

The MED assessment system comprises a camera combined with a black tube for acquiring pictures of the skin surface. Specific software was then developed to analyse these pictures to determine the MED based on the shape, size and colour of the exposed zones. The MED assessment system was validated through two studies. The first study was designed to assess the correlation between three expert graders who visually determined the MEDs in five subjects on whom three different suncare products (SPF 6, SPF 30 and SPF 50+) were tested. The second study correlated results obtained from one expert grader with those from a grader assisted by the new MED assessment system.

Results of the first study showed substantial variation between graders, with kappa agreement as low as 0.59 (percentage error 19.7%). Results of the study assessing correlation between the expert grader and the grader facilitated by the new MED assessment device showed better correlation, with a kappa value of 0.75 and percentage error of 9.61%.

A high degree of inter-grader variability was seen when MED was assessed by expert graders. The new MED assessment system provides background colour correction and standardisation to enable accurate MED determination. A high level of correlation was seen between the expert grader and the new MED assessment system, thus demonstrating its potential utility in more accurate and homogenous MED evaluations. Future multicentre studies are required to improve and validate the standardised MED determination for suncare products and to further evaluate the role of this new MED assessment system. However, these preliminary results are encouraging and may set the scene for this new MED assessment system to become the standard of the future.