OBJECTIVES: The DLQI is a 10-item generic dermatological HRQoL measure used widely with psoriasis patients. The PSORIQoL is a 25-item psoriasis-specifi c measure that employs the needs-based model of quality of life (QoL). Both measures have been shown to have good psychometric properties using Classical Test Theory. However, their scaling properties have not been fully assessed using Item Response Theory (IRT). This is the purpose of the study.
METHODS: Psoriasis patients, recruited from an out-patients clinic, completed the DLQI and PSORIQoL. IRT (Rasch analysis) was used to determine each measure’s overall fi t to the Rasch model, individual item fi t, targeting of scales to severity of respondents, functioning of response categories, and the presence of Differential Item Functioning (DIF) by age or gender.
RESULTS: 146 psoriasis patients were included in the study (male 50%, mean age = 44.2, range = 17–83 years). DLQI: There was overall misfi t to the Rasch model (χ2 = 39.85, df = 20, P = 0.005). One item misfi t the Rasch model and another showed borderline misfi t. Four of the 10 items had disordered response thresholds indicating that they did not work in a logical way. Too few DLQI items covered milder levels of HRQoL (meaning that the measure is not sensitive to change in patients with mild disease). DIF by age was found in one item and DIF by gender in another. PSORIQoL: Overall, the measure fi t the Rasch model (χ2 = 56.45, df = 50, P = 0.247), although one item had a high fi t residual suggesting misfi t. The response options worked logically and items were well distributed across the QoL measurement range. No DIF by age or gender was found.
CONCLUSIONS: Application of Rasch analysis indicated that there were several problems with the scaling properties of the DLQI. In contrast, the PSORIQoL fi t the Rasch model and exhibited good measurement properties.
