Results of the validation study of the Psodisk instrument, and determination of the cut‐off scores for varying degrees of impairment

BACKGROUND: The Psodisk is a 10-item visual instrument, aimed at measuring the burden of psoriasis on patients. OBJECTIVES: To validate the Psodisk in a large sample of patients with psoriasis, and to define categories for the interpretation of the scores. METHODS: Data were collected in 21 dermatological centres. The Psodisk was administered at baseline (t0), after 2 or 3 days (t1) and about 3 months (t2) after baseline, and data were used to assess validity and reliability of the instrument. The cut-off scores were determined using the perception of the severity of the disease by the patient as anchor point. RESULTS: The evaluable population consisted of 320 patients at baseline, with a mean Psodisk score of 36.9. The concurrent validity of the instrument was confirmed by the high correlation with Skindex-29 and DLQI. Factor analyses selected a single factor, which alone explained almost 60% of the variance. Cronbach's coefficient alpha was 0.927, suggesting a good reliability. Test-retest reliability was verified by a Pearson's correlation coefficient between the Psodisk scores at baseline and t1 of 0.924. Five categories of disease burden were defined: 1. minimal (<9); 2. mild (9-15); 3. moderate (16-30); 4. marked (31-50); 5. severe (>50). CONCLUSION: The Psodisk showed good psychometric properties. The definition of the cut-off scores will be useful to evaluate the burden of psoriasis on patients.