Introduction: To assess the measurement properties of the Spanish version of the Psoriasis Disability Index (PDI) quality of life questionnaire for patients with moderate or severe psoriasis in ordinary clinical practice.
Patients and methods: A study was carried out with 294 patients with moderate or severe psoriasis who had received therapeutic treatment. Sociodemographic and clinical variables were collected and the PDI and EuroQoL-5D questionnaires were administered at the baseline visit, and at 3 and 6 months later. The feasibility (percentage of patients responding to the questionnaire), validity of the construct, internal reliability and sensitivity to change were analyzed.
Results: Feasibility: 98.6 % of the patients answered over 80 % of the questions on the questionnaire. Validity of the construct: the PDI scores were correlated with the scores obtained in the Psoriasis Area Disability Index (PASI) (r = 0.33) and the Visual Analogue Scale (VAS) of the EQ-5D (r = -0.41) (p < 0.01). Longitudinal validity: the PDI questionnaire score after 6 months of treatment was correlated with the changes in the PASI index and the VAS (r = 0.39 y -0.51). Reliability: the internal consistency of the dimensions was high (Cronbach's alpha = 0.89). Sensitivity to changes: the size of the effect corresponding to the changes experienced by the patients who noticed an improvement in the severity of their psoriasis during the period between visits was 0.95.
Conclusion: The Spanish version of the PDI has proven to be reliable, valid and sensitive to changes for use with patients with psoriasis in the Spanish population, although the test-retest reliability should be analyzed for clinically stable patients.