Data
This study involved secondary analysis of cross-sectional data collected through a survey of members of the Alberta Retired Teachers Association (ARTA), in Alberta, Canada. The objective of the ARTA survey was to better understand the factors that affect care and health outcomes in this population of older adults. Participants were recruited by sending an email with a link to the online survey to all members registered with ARTA (N = 14,248). Participants were included if they were retired Alberta Teachers (as described by the Alberta Retired Teachers Association). There was no restriction on gender, race, and health status. Participants were able to understand, read, speak English and they were of sound mind to complete the survey. A total of 6275 (44%) participants responded to the survey, with 2514 (18%) completing the survey. Participant information sheet and consent were completed before participating in the survey. Ethical approval was obtained from the health research ethics board at the University of Alberta.
Measures
The EQ-5D-5 L is a preference-based measure of HRQL [9]. It consists of five dimensions (mobility, self-care, usual activities, pain/discomfort and anxiety/depression) and a visual analogue scale (VAS). Each dimension has 5 levels of problems: 1 “no problems”, 2 “slight problems”, 3 “moderate problems”, 4 “severe problems” and 5 “extreme problems”. Respondents are asked to indicate their level of functioning on each of the five dimensions on that day [9]. The EQ-5D-5 L describes 3125 distinct health states, with 11111 representing the best and 55555 the worst possible health states. Index scores were generated using the Canadian scoring algorithm [14], ranging from − 0.148 for worst (55555) to 0.949 for best (11111) health states [14]. A minimally important difference (MID) for the index score has been suggested as 0.04 [15]. The VAS score ranges from 0 (worst health imaginable) to 100 (best health imaginable).
The SF-12 version 2 is derived from items in the SF-36 health survey [16]. It measures eight domains of functioning and well-being, asking respondents to consider their health status over the past 4 weeks: physical functioning (PF), role limitations due to physical problems (RP), bodily pain (BP), general health perceptions (GH), energy and vitality (VT), social functioning (SF), role limitations due to emotional problems (RE) and mental health (MH) [16]. The eight domains can further be summarized into physical component summary (PCS) and a mental component summary (MCS) scores, which are interpreted as standardized T-scores, with a mean of 50 and SD of 10. SF-12 scales and summary scores were calculated according to Fleishman, Selim and Kazis [14]. Domain scores were also converted into T-scores for this analysis. The SF-6D is derived using 7 items from the SF-12. The SF-6D classification system consists of six domains including PF, RP, BP, VT, SF and MH describing 7500 unique health states [6]. SF-6D index scores were generated based on the UK general population preferences, ranging from 0.345 for worst (345555) to 1.0 for best (111111) health states [6]. The first item of the SF-12 (SF-1) was also used as an indicator of the overall health of participants.
Pain intensity was measured using the 11-point Numeric Pain Rating Scale. Participants indicated their pain intensity over the past 6 months. Pain intensity was subsequently categorized into: 0-no pain, 1–3-slight pain, 4–6-moderate pain, and 7–10-severe pain [17, 18]. Comorbidities were assessed using self-reported chronic condition question. Participants were asked if a health professional has ever told them that they had any of the following 13 conditions: arthritis (e.g. osteoarthritis, rheumatoid), respiratory diseases, cardiovascular diseases, diabetes, high lipids, renal diseases, bowel disorders, musculoskeletal disorders, thyroid dysfunction, eye diseases, cancer, neurological diseases, and mental/psychological conditions. The total number of comorbidities reported by each participant was calculated, and categorized into: 0, 1, 2, and 3 or more chronic conditions. Other data included age, sex, highest level of education, current employment status, total annual household income, ethnicity, and body mass index (BMI), calculated using self-reported weight and height.
Statistical Analysis
Participants with complete EQ-5D-5 L and SF-12 data were included in this analysis (N = 2844). Descriptive statistics were computed for all variables. Ceiling effects for each measure were reported as the percentage of participants in the best possible health states (11111 for EQ-5D-5 L and 111111 for SF-6D).
The discriminative validity of the EQ-5D-5 L and SF-12 was evaluated by first, examining the extent to which each instrument distinguished between participants with or without arthritis, and then, among those with arthritis, based on the level of pain (no, mild, moderate, severe) as a proxy for severity of the condition. The number of other chronic conditions (none, 1, 2 or 3+ chronic conditions), and self-reported health (excellent, very good, good and fair/poor) were used as indicators of the overall health of participants. One-way ANOVA, independent group t-test and chi-square test were used to test differences between examined groups as appropriate. Effect size was used to examine the magnitude of differences and was calculated as the difference in mean scores between participants with and those without arthritis, divided by the pooled standard deviation. Effect size was interpreted as: 0.2–0.49 small; 0.5–0.79 moderate and > =0.8 as large [19].
We hypothesized that EQ-5D-5 L index score, VAS, SF-12 domain and summary scores and SF-6D index score would all be lower in arthritis patients with higher levels of pain severity, while an inverse relationship was expected for the EQ-5D-5 L dimensions. Secondly, EQ-5D-5 L index score, VAS, SF-12 domain and summary scores and SF-6D index score were expected to be lower in arthritis patients with more comorbidities, with an inverse relationship for EQ-5D-5 L dimensions. Finally, EQ-5D-5 L index score, VAS, SF-12 domain and summary scores and SF-6D index score were expected to be lower in arthritis patients with poorer general health status, with an inverse relationship for EQ-5D-5 L dimensions.
The relationships between the EQ-5D-5 L and the SF-12 components were examined using Spearman’s correlation coefficients. Coefficients of < 0.20 were considered absent, 0.20–0.34 as weak, 0.35–0.50 as moderate and > 0.5 was considered strong correlation [20]. We expected mobility, self-care, usual activities and pain/discomfort dimensions of the EQ-5D-5 L to have a strong negative correlation with the SF-12 PCS and PF, RF, BP, and GH. Anxiety/depression dimension of the EQ-5D-5 L was also expected to have a strong negative correlation with the SF-12 MCS, SF, RE, and MH. A negative weak correlation was expected between the mobility, self-care, usual activities and pain/discomfort dimensions of the EQ-5D-5 L and mental health domains and summary scores (MCS, SF, RE, and MH) of the SF-12.