A Comparative Study on the Measurement Properties (Criterion Validity, Informativity, and Floor and Ceiling Effects) of Dermatology Life Quality Index (DLQI) and Skindex-29.

Background: The measurement of health-related quality of life (HRQoL) is essential in dermatology, with the DLQI and Skindex-29 being popular tools for assessment. However, the literature comparing the measuring features of these two questionnaires is lacking. Aims: This study aims to provide a comparison of the measurement properties of DLQI and Skindex-29 in patients with common dermatology conditions such as psoriasis vulgaris (PV), vitiligo vulgaris (VV), leprosy, and acne vulgaris (AV). Methods: In this cross-sectional study, the HRQoL of 301 patients with the four dermatological conditions was measured using the DLQI and Skindex-29 questionnaires at a tertiary care centre located in Dehradun, Uttarakhand, India. Statistical analysis included determining ceiling and floor effects, informativity, and criterion validity. Results: The mean DLQI total scores were: PV (11.03 ± 0.89), VV (4.73 ± 0.78), leprosy (8.97 ± 1.09), and AV (7.79 ± 0.63), and the mean Skindex-29 scores were: PV (37.98 ± 2.82), VV (27.92 ± 2.93), leprosy (45.12 ± 3.63), and AV (27.6 ± 1.08), with a 95% confidence interval. Of the patients with a DLQI score of 0 (n = 66), 64 (96%) had a total score greater than 0 on the Skindex-29. The most troubled areas of HRQoL among patients with a DLQI score of 0 according to the Skindex-29 were item 17 (showing affection; 12.1%), item 9 (worry about scars; 9.1%), item 12, item 13, and item 14 (being ashamed, worrying about worsening, and tending to do things oneself; 7.6%). The ceiling effect was observed in 4.7% of patients for DLQI and 0% for Skindex-29. The floor effect was high for the DLQI total score (22%), whereas Skindex-29 observed only 0.7%. The absolute informativity value of Skindex-29 was 1.66, and that of the DLQI was 1.92. The typical relative informativity value for both was 0.83. Conclusion: Skindex-29 showed better sensitivity to clinical severity, less floor effect, higher absolute informativity, and better coverage of the 'emotion' domain. Skindex-29 was found to be more effective in detecting minor HRQoL impairment than DLQI.