Excimer light is a targeted phototherapy that uses 308-nanometer wavelength ultraviolet radiation to treat photoresponsive dermatoses such as psoriasis, vitiligo, and alopecia areata. Ideally, the minimal erythema dose (MED) should be determined to guide the initial treatment dose. However, due to convenience, estimation based on skin phototype is more commonly used.

To determine the MED to excimer light in adult Filipinos using a visual erythema scale.

A prospective interventional, cross-sectional study was conducted among adult Filipinos in a tertiary government hospital in the Philippines. Participants underwent phototesting with six incremental doses of excimer light. MED was defined as the lowest dose that produced uniform, well-defined erythema over the exposed area. Descriptive statistics summarized clinicodemographic data, MED, and adverse events. Fisher’s exact test assessed associations between MED, sex, and skin type.

A total of 149 adult Filipinos with Fitzpatrick skin types III to V were enrolled. MED values ranged from 150 to 400 mJ/cm2, with 200 mJ/cm² being the median and mode (33.56%). No association was found between MED and skin phototype. Sex correlated with MED, with females having higher MEDs than males.

The median MED was 200 mJ/cm2 in Filipino patients with skin types III to V. This may serve as the initial starting dose for phototherapy (except in vitiliginous skin). With the variation in the MED within the population, MED determination is still the ideal method to identify the most appropriate initial treatment dose.