Study design and sample
Data were obtained from the China Health and Retirement Longitudinal Study (CHARLS, wave 4), which was a survey of middle-aged and elderly people in China, based on a sample of households with members aged 45 years or above, aimed to establish a high quality public micro-database that can provide a wide range of information from socioeconomic status to health conditions [9.29]. In order to ensure the representativeness of the sample, the survey followed strict randomization procedures and used a multi-stage sampling method to ensure sample representativeness. When sampling county and rural administrative units, a probability proportional to size (PPS) sampling method was adopted. In the first stage of sampling, 150 county-level units were randomly selected using the PPS method from a sampling frame containing all county-level units in China (excluding Tibet). In the second stage, three communities (urban resident committees or rural administrative villages) were randomly chose using the PPS method from a sampling frame containing all communities in the county-level units. In the third stage, to create a sampling frame, using the software developed by the CHARLS team which utilized Google Earth map images, all dwelling units in a community were listed following an extensive mapping and listing operation, and then a certain number of dwelling units were randomly chose. The ethical approval was granted by the Institutional Review Board (IRB) of Peking University. The IRB approval number for the main household survey including anthropometrics is IRB00001052-11015, and the IRB approval number for biomarker collection is IRB00001052-11014 [9.29-31]. All data collected in CHARLS were preserved by the Institute of Social Science Survey of Peking University and released on the CHARLS website (http://www. charls.pku.edu.cn/).
CHARLS is a cohort study. A baseline survey was conducted in 2011, and the followed three wave survey were conducted in 2013y,2015y and 2018y. In order to understand the status of multiple chronic diseases and depressive symptoms in the elderly, we used the latest data from the wave 4 (2018y) of the CHARLS, which was conducted between July 2018 and March 2019, involved 19816 respondents in 150 counties/districts and 450 villages/urban communities [9]. According to our purpose, the elderly aged 60 years and over were selected as the research objects. After excluding the data was incomplete (incomplete filling of relevant variables such as depression, chronic diseases and daily activity ability), the final sample included in the analysis was 6360.
Measures
Depressive symptoms
Depressive symptoms were measured using the Center for Epidemiologic Studies Depression Scale (CES-D-10) which has been validated among elderly respondents in China using CHARLS data [32]. The CES-D-10 response scale includes 10 questions regarding the participant’s experience in the past week: feeling bothered, having trouble in concentrating, feeling depressed, feeling as though everything was effortful, feeling hopeful, feeling fearful, having restless sleep, feeling happy, feeling lonely and having difficulty getting going [33]. The total CES-D-10 score ranges from 0 to 30, with higher scores indicating more severe depressive symptoms. In this study, we used a cutoff score of ≥ 10 to distinguish participants with depressive depression from those who were relatively free of depression [34-35]. The CES-D-10 used in CHARLS exhibited good internal consistency, with α = 0.793 in the baseline investigation in 2011y, α = 0.787 in the follow-up investigation in 2013y, α = 0.779 in the second follow-up investigation in 2015y [36], and α = 0.818 in our study.
Chronic disease and multiple chronic conditions (MCCs)
In CHARLS, the elderly with chronic disease is those who had been diagnosed chronic disease by the doctor up to the time of investigation. A total of 14 chronic diseases was investigated, including hypertension, dyslipidemia (high or low), diabetes or hyperglycemia, cancer, malignant tumor, chronic lung disease, liver disease, heart disease (heart infarction, coronary heart disease, angina pectoris, congestive heart failure, etc.), stroke (including cerebral infarction and cerebral hemorrhage), kidney disease (excluding cancer or cancer), diseases of stomach or digestive system (excluding tumor or cancer), memory related diseases (Alzheimer's disease, brain atrophy, Parkinson's disease), arthritis or rheumatism, and asthma. The relevant variables of chronic diseases in this study were respectively defined as: ①Having chronic diseases or not: as long as you have been diagnosed by a doctor with any kind of chronic diseases(14 chronic diseases in CHARLS), you are regarded as a patient with chronic diseases, and the value is 1, otherwise the value is 0; ②Having multiple chronic conditions (MCCs) or not: MCCs are defined as having two or more chronic diseases, and if you have MCCs , it is assigned to 1; otherwise it is assigned to 0; ③The number of MCCs: having two chronic diseases are assigned to 1, having three chronic diseases are assigned to 2, and having four or more chronic diseases are assigned to 3.
Statistical analysis
All the data in our study were obtained from the CHARLS database, which was converted into XLS format by StataV13.0 and analyzed using IBM SPSS Statistics V22.0. Demographic characteristics of the elderly were described using frequencies and percentages (the categorical variables were n (%)), and Chi-square test was used to compare the prevalence of depression between the elderly with MCCs and without MCCs, and OR-unadjusted was calculated. Then a generalized linear model (GLM) was employed to estimate the association between MCCs and depression after controlling other confounding factors, the results were expressed as adjusted Odds ratio (OR-adjusted) and it’s 95% confidence interval(95%CI).
GLM is an extension of the traditional linear model that allows the overall average to depend on a linear predictor through a nonlinear link function and allows the response probability distribution to be any member of the exponential distribution family, including logistic regression model. In our study, a binary logistic regression model was used to analyze the influence of MCCs on depression in the elderly when controlling for other confounding factors. A GLM always includes three components:
In order to explore the association between MCCs and depression in the elderly, three logistic regression models were built, one model for estimating the influence of MCCs on depression after controlling all the confounding factors, and two other models stratified by having MCCs or not to estimate the risk factors for depression in each group. Then binary logistic regression model were used to analyze influencing factors of the occurrence of depression among the elderly with MCCs or not. All statistical analyses were performed using IBM SPSS StatisticsV22.0, and P-value < 0.05 was considered statistically significant.