Validation of an Optical Pencil Method to Estimate the Affected Body Surface Area in Psoriasis

Body surface area (BSA) affected by psoriasis is one of the most often used measures for assessing severity, but this method has shortcomings.

To validate a new way to estimate BSA.

Prospective, multicenter study in 56 patients with psoriasis. Each patient was evaluated by 2 dermatologists in 2 visits to the same hospital. Each dermatologist used 2 methods for estimating BSA: the traditional visual estimation in which the area of the palm equals 1% of the total body surface and an optical pencil (OP) method in which the affected area is drawn on a touch screen. Software in the application then calculates the BSA.

Overall concordance between the 2 methods was acceptable according to an intraclass correlation coefficient (ICC) of 0.87. However, the limits of agreement were unacceptably large and there was systematic bias: traditional estimates were consistently greater than OP calculations. Concordance between the methods was better (ICC>0.8) on the trunk and lower extremities. Intraobserver reliability was excellent with both methods (ICCs, 0.97 and 0.98 for the traditional and OP estimates, respectively). Interobserver reliability was also high (ICCs, 0.91 and 0.94 for the traditional and OP methods), although the mean BSA differed significantly between observers. The ICCs were much lower for BSA estimates on the head.

This study to validate the OP method for estimating the affected BSA in patients with psoriasis shows good agreement between the OP and traditional approaches. The OP calculations also showed less variance and better interobserver reliability.