PSY31 The Relevance of 'not Relevant' Responses on the Dlqi: A Rasch Analysis in Patients with Psoriasis

Abstract

Dermatology Life Quality Index (DLQI) is the most frequently used quality of life questionnaire in psoriasis patients. In eight out of the 10 items of the DLQI, patients may answer that the item does not apply to their life ('not relevant' response). We aim to compare the measurement performance of DLQI in psoriasis patients with and without 'not relevant' responses using an application of item response theory (Rasch analysis). Cross-sectional data of 425 patients with psoriasis were analysed. Patients were divided into two groups: those with (R-group) and without any 'not relevant' responses (NR-group). A partial credit model was applied to the DLQI data. The following model assumptions and measurement properties were tested: dimensionality, item fit, person reliability, order of response options and differential item functioning (DIF). A total of 259 (60.9%) and 166 (39.1%) patients belonged to the R- and NR-group, respectively. The NR-group tended to be, on average, 10 years older, more likely female, have chronic plaque psoriasis, palmoplantar psoriasis or psoriatic arthritis and associated with higher disease severity scores (p<0.05). Both for the R- and NR-group, the DLQI demonstrated a unidimensional construct and an acceptable reliability. Person separation reliability was slightly better in the R-group (0.920 vs. 0.899). In the NR-group, response thresholds were disordered for items 6 (sport), 7 (working/studying) and 9 (sexual difficulties), whereas in the R-group, only item 7 was illogically ordered. Seven of the 10 items exhibited DIF between the R- and NR-group. The Rasch model suggests that 'not relevant' responses lead to measurement bias in the DLQI. Scores of patients with and without 'not relevant' responses should not be compared. Further efforts are needed to effectively revise the DLQI and to standardise quality of life measurement in patients with psoriasis.