Study design and patient enrollment. This cross-sectional study was conducted to determine the health utility and its relationship with disease activity, HAQ-DI and PASI scores in Thai patients with PsA. All PsA patients diagnosed according to the classification criteria for PsA (CASPAR) criteria12 were invited to participate in this study. These patients had their outpatient regular rheumatology clinics visit at three university affiliated hospitals between January and April 2020. Those who were unable to communicate or denied participating were excluded.
Study protocol. All the subjects received detailed information about the purpose and procedure of the study and agreed to participate in writing. All patients gave their written informed consents before entering the study. Approval to conduct this cross-sectional study was obtained from the Central Research Ethics Committee (CREC) of Thailand in 2019 (certificate number: COA-CREC004/2020). All the methods were performed in accordance with the Declaration of Helsinki, the relevant guidelines and regulations. Sample size estimation. A previous study on health utility in PsA patients reported mean and SD of 0.5 and 0.3, respectively13. Using the above mean and SD, and the 95%CI of true mean of 0.5 ± 0.05 (error = 0.05), the sample size of this study was calculated to be 138.
Data collection. All eligible patients were invited to participate in this study. Baseline characteristics of participants were collected by a medical record review performed by rheumatologists. Participants were face-to-face interviewed for their current clinical status including their health utility, physical disability, and skin lesion activity by a trained research nurse or a rheumatologist. A structured data collection and interview form was developed. It consisted of four parts including 1) baseline characteristics; sex, age, health insurance, types of PsA, current disease activity (measured by clinical Disease Activity Index for Psoriatic Arthritis (cDAPSA))14, deformity, co-morbidity, and PsA treatment, 2) the Thai version of the EQ-5D-5L for patients’ health utility, 3) the Thai version of HAQ (Thai HAQ) for physical disability, and 4) PASI for the severity of psoriatic skin lesions. All collected data was verified by RS, PD, and UP. Rheumatologists who were responsible for data collection were asked for any incomplete data to ensure the data validity.
Outcome Measures. Health utility and physical disability scores were determined by the Thai version of the EQ-5D-5L, and Thai HAQ, respectively15. The PASI questionnaire was used to determine the severity of the psoriatic skin lesions, the PsA disease activity, and the global assessment of the disease activity by the physicians, respectively.
The EQ-5D-5L16 was used for assessing the patients’ health-related utility. It consists of five dimensions including mobility, self-care, usual activities, pain/ discomfort, and anxiety/depression. Each dimension has one question with five response levels. Patients’ responses from the EQ-5D-5L questionnaire were converted to the utility scores based on the Thai algorithms, which had been elicited from 1,207 general population living in 12 provinces from all regions of Thailand17.
The Thai HAQ was used for determining functional disability. It consists of eight domains including dressing and grooming, arising, eating, walking, hygiene, reach, grip, and activities15. The 4-point difficulty level of the individual items from each domain is ranked from 0 (no difଁculty) to 3 (unable to do). The highest score from each domain was then summed and averaged to a single disability index. The HAQ-DI scores range from 0–3, with higher score reflects greater disability.
The PASI questionnaire18 was used for determining the severity of psoriatic skin lesions in such patients. It consisted of four domains including percentages of skin involvement and the severity of skin lesions assessed by 3 clinical signs: erythema score, infiltration score, and desquamation score18. The PASI score was calculated using an equation reported in the original study. For each body section; (head, upper limb, lower limb and trunk), the percentage of affected skin area by psoriasis was estimated on a scale from 0 to 4 according to erythema score, infiltration score and desquamation score, then converted to a grade, ranged from 0 to 618.
Statistical analyses. Baseline characteristics were presented as frequencies for categorical variables and means ± standard deviation (SD) for continuous variables. Kruskal-Wallis test was used to test the differences of the HAQ-DI, PASI, and health utility among disease activity (remission, low, and moderate-to-severe). Dunn’s pairwise comparison with Bonferroni correction was used to test the differences of each comparison when significant difference was observed among disease severity. Spearman rho correlation was performed to relate utility score and VAS.
Two steps regression approach was applied to determine relationship of health utility and it related factors. Univariate linear regression was performed to relate patients’ health utility and its factors. Factors with p-value < 0.10 in the univariate analysis and variables of interests from the literature relating to health utility (age, cDAPSA, HAQ-DI, and PASI) were selected for multivariate linear regression using a forward selection. Factors with p-value < 0.05 in the multivariate analysis indicated significant relationship with patients’ health utility. Variance inflation factor (VIF) was used to assess multi-collinearity among independent variables, while adjusted R2 was used to determine model’s goodness-of-fit. All analyses were performed using STATA version 15.0.