Study sample
The LASA is an ongoing cohort study on physical, emotional, cognitive, and social functioning in older people in the Netherlands. The LASA cohort (n = 3107) was recruited in 1992 from a random sample of older men and women aged 55–85 years, in the west, north east, and south of the Netherlands. The sample was stratified by age, sex, degree of urbanicity, and expected 5-year mortality. The LASA cohort is representative of the older Dutch population with respect to geographic region and degree of urbanicity. Sampling, data collection, and non-response are described elsewhere.21,22 Since 1992, longitudinal data have been collected every 3 years. Measurements are performed by trained interviewers, who visit responders at home.
For the present study the first six cycles of data collection were used (1992–1993, 1995–1996, 1998–1999, 2001–2002, 2005–2006, and 2008–2009). Responders were included (n = 1712) if they participated in at least three subsequent data collection cycles (starting in 1992–1993) and if there was complete data on their GPs. The time period across which continuity of GP care was determined varied from 7 years (1992–1999) up to 17 years (1992–2009).
Informed consent was obtained from all participants.
Measures
Information about mortality was obtained through linkage with registers of the municipalities in which the responders were living (most recent probing date: 1 November 2013).23 Mortality follow-up was incomplete for four cases. Survival time was computed as the date of death or the most recent probing date minus the interview date of the last data collection cycle for which the GP was known.
Continuity of care (COC) was defined as the duration of the ongoing therapeutic relationship between patient and GP, that is, relational COC. This type of COC is most valued in primary and mental health care.1 To calculate the COC, the Herfindahl–Hirschman Index (HHI) was applied:24,25
The HHI is an economic measure of concentration, defined as the sum of squares of market shares, whereas market shares are expressed as fractions.26 In the present study,
is the ‘market share’ of GPi and N is the number of different GPs for a specific patient. The number of data collection cycles in which a participant provided the name of a specific GP were counted. Fractions were allowed because participants sometimes reported two different GPs per data collection cycle (both GPs were then counted as ½). This count was subsequently divided by the total number of data collection cycles in which the participant provided the name of a least one GP. These ratios were squared and then summed, resulting in the participant’s COC index. The COC index ranges from 1/N (minimal continuity, a different GP at each data collection cycle) to 1 (maximal continuity; the same single GP in all data collection cycles). For example, for a participant reporting GP A (cycle 1 and 2), GP B (cycle 3, 4, 5, and 6), and GP C (cycle 3), the COC index is calculated as: contribution GP A + contribution GP B + contribution GP C = (2/6)^2 + (3.5/6)^2 + (0.5/6)^2 = 0.458.
Based on previous research, the covariates included were age, sex, sociodemographic characteristics, smoking, alcohol use, morbidity, functional limitations, depression, cognition, and personality characteristics.7,12,19,27–38 Covariates were measured at the last cycle for which GP data were available.
Sociodemographic characteristics included housing (independent or care institution), level of education, level of urbanicity, and partner status. Level of education was divided into low (elementary school or less), middle (lower vocational, general intermediate, intermediate vocational, or general secondary school), and high (higher vocational education, college, or university).39 Level of urbanicity was assessed using the number of addresses per square kilometre, distinguishing five categories from low (<500) to very high (>2500).40 Partner status was divided into three categories: no partner, co-residing partner, or partner outside of household. Smoking status was classified as nonsmoker, former smoker, or current smoker..30 Alcohol use was assessed by using the Garretsen indicator of alcohol use, distinguishing three categories: no, light, and moderate to very excessive use of alcohol.41
Morbidity was assessed by self-report of seven major chronic diseases: chronic pulmonary disease, cardiac disease, peripheral arterial disease, diabetes mellitus, stroke, osteoarthritis or rheumatoid arthritis, and cancer. These self-reported answers have been found to correspond well with information from GPs across the full study period.42
Functional limitations were assessed by six self-reported questions about experienced difficulty in doing daily activities, counting the number of items ‘with some difficulty’ or worse (range 0–6; internal reliability 0.85).43 These activities included walking up and down stairs, walking outside the house, use of transportation, dressing oneself, sitting down and rising from a chair, and cutting own toenails.
Depressive symptoms were measured using the 20-item Center for Epidemiologic Studies Depression scale (CES-D; range 0–60; cutoff for depression ≥16).44,45
Cognition was assessed by means of the mini-mental state examination (MMSE; range 0–30; cutoff for cognitive impairment ≤24).46
Personality characteristics were assessed by sense of mastery, self-efficacy, and self-esteem. Sense of mastery is defined as the extent to which one views one’s life as within one’s control as opposed to being ruled by chance or other people. It was measured by the seven-item Pearlin Mastery Scale (range 7–35).47 Self-efficacy is defined as the belief of a person in their ability to organise and execute certain behaviours that are necessary to produce given attainments. It was measured by a 12-item version of the General Self-Efficacy Scale (GSES- 12; range 12–60).48 Self-esteem reflects a person’s overall evaluation or appraisal of their own worth. It was measured by an adapted four-item version of the Rosenberg Self-Esteem Scale (range 4–20).36,49
Analysis
The association between COC and survival time was investigated using Cox regression analysis. In preliminary analyses there was a non-linear association between the COC index and mortality. Therefore, it was divided into four categories. The first category included the COC index value of 1 representing maximum continuity. COC index values <1 were divided into tertiles. Bivariate comparisons were performed to examine the associations of the main outcome mortality with all covariates.
Confounding was investigated by manually introducing all covariates into the basic model. A covariate was considered a confounder if the coefficient of COC changed by more than 10%. Effect modification was investigated by calculating interaction terms of each covariate with COC. Dummies were used to investigate categorical covariates. For COC, the group with maximum COC (index = 1) was used as reference group. Responders participated in three, four, five, or six data collection cycles, corresponding with different periods of data collection. Survival time was calculated starting from the interview date of last data collection. Although this approach made optimal use of available data, responders participating in more data collection cycles may have been younger and healthier, but at the same time their survival time was calculated from a later interview date. In addition, participants who died relatively early may have had a limited number of data collection cycles. To account for this, the patient’s final wave of data collection was used as a stratification variable and allowed baseline hazards to differ between these strata.
The level of statistical significance was set at P<0.05. Data were analysed using SPSS (version 22).