Study Population
The sample was drawn from the 2006-2016 waves of the New Zealand HWR study, a prospective cohort study of community-dwelling older adults. The HWR commenced in 2006 as a biennial postal survey of a sample aged 55-70, randomly selected from the New Zealand electoral roll. An over-sample of adults of Māori decent was undertaken to ensure adequate representation of this section of the older population. The core questionnaire assesses domains of health and wellbeing; family and social support; work and retirement; financial wellbeing; and cultural identity. Cases included in the current analyses participated in the baseline survey and at least one subsequent follow up period and provided information on health-related quality of life at two or more time points. The original survey had a response rate of 53% (n = 6662), and of these, 2632 consented to participate in subsequent follow up. Compared to those who dropped out, those who completed at least one wave of follow up were older (60.8±4.5 vs. 61.1±4.6, p = 0.033), non-Māori (50.6% vs. 58.2%, p < 0.001), and more male (45.1% vs. 46.6%, p = 0.211). Of those willing to participate in longtidunal follow up, 1609 (41%) were lost to follow up over the five biennial follow up waves (212 to death, and remaining unknown). For the current analysis, participants with a diagnosis of diabetes at baseline who responded to at least one subsequent survey were selected, A comparison group of those without diabetes at the baseline who remained free from DM in all follow- up surveys (2008-2016) were selected. We excluded incident cases of diabetes (n=192). Figure one presents a flow chart illustrating inclusion criteria and attrition 2006-2016.
Measures
Demographic variables
Age, provided as a continuous variable, was categorised as 55-64 years and 65 and over. Marital status was considered as three groups: married/living with a partner, divorced/ separated/single, and widowed. Ethnicity was classified using the priority ethnic groups in NZ: Māori (indigenous New Zealanders) and non-Māori (Europeans, Asians, Pacific people and others). Socioeconomic status indicators included educational qualifications categorised as no secondary, secondary, post-secondary and tertiary. Annual personal income was grouped as 0-25000, 25001-50000, 50001-70000 and >70000 NZ$.
Clinical variables
Medical conditions were defined as a self-report answer to a list of doctor-diagnosed physical and mental conditions including heart disease, stroke and other neurologic diseases (epilepsy, Alzheimer/dementia, Parkinson, migraine headache), musculoskeletal (arthritis, osteoporosis, hip/knee replacement), respiratory (asthma, Chronic Obstructive Pulmonary Diseases (COPD)), chronic liver diseases (cirrhosis), mental (depression and other mental illness), cancer, and one unspecified category. The number of chronic conditions was calculated by the sum of ‘yes' responses as none, one, two and more conditions. Other clinical variables, including hypertension, hearing and eye problems, or sleep disorder, were considered as dichotomous variables (yes/no) based on participant responses to a question about diagnosed health problems. BMI (weight in kg/ height in meter squared) was measured in the 2008 survey wave and was categorised as normal weight (<25), overweight (25-29.9), and obese (≥30).
Health behaviours
Smoking was defined by a question that asked respondents to identify themselves as a regular smoker or not. Alcohol consumption was assessed as the frequency of drinking which was subsequently classified into two categories: regular alcohol consumption (2 or more drinks per week), and non-regular alcohol drinkers (≤ 1 drink per week). Physical activity was measured by the frequency of moderate/brisk walking or vigorous activity in the last seven days that were categorised into two levels: two or more times per week (sufficient), once per week/none (insufficient).
Health Related Quality of Life
Physical HRQOL was assessed using the SF12v2 (22). Ten items of the SF12v2 are rated and a scale of 1-5 and two items on a scale of 1-3. Standardised norm based orthogonal factor weights are used to calculate a PCS
[positive weights for physical functioning (2 items), role physical (2 items), pain and general health] and a MCS [positive weights for vitality, social functioning, relationships (2 items) and mental health (2 items)] with reference a New Zealand population mean of 50 and standard deviaiton of 10 (23).
Statistical analysis
Data were extensively screened. Missing data for chronic conditions such as hypertension, stroke, and so on in some waves were replaced with available data from the preceding or the subsequent waves over the study period. All possible comparisons of the age/year at diagnosis in each wave were checked to ensure that the most reliable list was utilised.
Descriptive analyses were usd to describe the characteristics of the sample. The absolute changes in PCS and MCS were calculated by subtracting the follow-up score from the baseline score, which was the first SF12 completed for each person. For those with multiple scores, the average over time was considered.
A Generalised Estimating Equation (GEE) model was performed to examine the association between demographic, clinical and health behaviours variables with and HRQOL over time to account for longitudinal within-subject correlations. Variables were introduced as fixed (sex, ethnicity, education, personal income, BMI) or time-varying (other variables) into the models based on availability and completeness of the data in each wave. Two models were fitted separately for the physical and mental dimensions of the SF12; the comparison group for each model was people without diabetes. As a sensitivity analysis, the difference between SF12 scores for DM and non-DM participants was also estimated for its baseline values in both crude and adjusted models. Data were analysed using the STATA statistical package version 14; all estimates were reported with 95% confidence interval and a significance level of 0.05.