Spectrophotometric Measurement of Minimal Erythema Dose Sites after Narrowband Ultraviolet B Phototesting: Clinical Implication of Spetrophotometric Values in Phototherapy

BACKGROUND The spectrophotometer is well known to be a useful tool for estimating the objective minimal erythema dose (MED) during planning of phototherapy protocol. However, only a few spectrophotometric values are used to evaluate the erythema and pigmentation of the MED site during phototesting. OBJECTIVE To determinea new meaning of the relationships among spectrophotometric values during phototesting. METHODS Twenty-five patients with psoriasis and 23 patients with vitiligo were selected before undergoing narrowband ultraviolet B phototherapy. We interpreted the gross findings of erythema and measured the L(*)a(*)b(*) values using a spectrophotometer at each phototest spot. We compared MEDs, basic spectrophotometric values (L(*)a(*)b(*)), and b(*)/L(*) values separately according to skin type, and determined the correlation of each spectrophotometric value and the correlation between a(*) and b(*)/L(*) values. RESULTS Among L(*)a(*)b(*) values, only b(*) values showed a statistically significant difference between the type III and IV groups (p=0.003). There was a positive correlation only between MEDs and b(*) values (p<0.05). The average b(*)/L(*)value in the type IV group was significantly higher than the type III group (p<0.05). CONCLUSION The higher b(*) values in type IV skin indicates that skin tanning develops more prominently than type III. The correlation between MEDs and b(*) values may signify that the skin pigmentation status is deepened with the higher MEDs. The difference in b(*)/L(*)values between type III and IV skin reflects that the b(*)/L(*)value is thought to be an index of tanning. The a(*) value, known as an index of erythema, does not influence the degree of tanning.