Study design and sample
To obtain a better representative sample, this cross-sectional survey was conducted using a multi-stage sampling survey method from January through July 2018. First, we selected three prefecture-level cities from the 16 prefecture-level cities in Anhui Province: Fuyang (North); Hefei (central, the capital of Anhui provincial), and Anqing (South). Second, in each prefecture-level city, one county was randomly selected. Next, in each selected county, two townships were randomly selected. A total of six townships were finally selected. At the last stage, we randomly selected three villages in each selected township, and a total of 18 villages were identified as the survey sites.
According to the data of local household registration system, 50 poor households were randomly selected from each selected village based on the list of poor households, and 75 non-poor households were randomly selected from the neighbors of poor households. The elderly (aged ≥ 60) from poor and non-poor households were selected as the survey subjects. With the assistance of local community workers and village doctors, each subject was investigated in the form of a household survey, and a face-to-face interview was conducted by the graduate students from Anhui Medical University who had received unified training using a structured questionnaire. The purpose and procedures of the study were dictated to all respondents, and informed consent was signed before the survey. Persons aged 60 and over and living in their residence for at least one year participated in the survey. Individuals with cognitive impairment, with language communication disorders, who were deaf, and with lack of access during the survey were excluded.
A total of 3491 older adults participated in the survey, and 155 older adults who were exhausted or occupied and could not complete the questionnaire were excluded. Finally, 3336 older adults were included for further analyses, with an effective response rate of 95.56% (3336/3491).
Assessment instruments
30-item Geriatric Depression Scale(GDS-30)
Depressive symptoms were evaluated using GDS-30, which was specifically designed for screening depressive symptoms in older adults [30]. This scale comprises 30 items; each item is dichotomized into two categories: "Yes" or "No". Among the 30 items, 10 items were scored in reverse order (1 point for "No" and 0 point for "Yes"), and the remaining were scored in positive order (1 point for "Yes" and 0 point for "No"). The total score ranged from 0 to 30 points, and the clinical boundary was 10 points. An individual with a GDS-30 score ≤ 10 was regarded as non-depressive and with a GDS-30 score ≥ 11 as depressive [31]. Studies have confirmed that GDS-30 had a high sensitivity (70.6%) and high specificity (70.1%) in a Chinese population aged 60 and over [32]. In this study, the internal consistency of GDS-30 is as indicated by the Cronbach's α score of 0.890.
WHO Disability Assessment Schedule (WHODAS 2.0)
WHODAS 2.0 was employed to examine the functional disability of the respondents. The scale consists of 36 items, including 6 dimensions: (1) cognition, (2) mobility, (3) self-care, (4) getting along, (5) life activities, and (6) participation. Each item is scored on the Likert 5 scale (1 = no difficulty, 2 = mild difficulty, 3 = moderate difficulty, 4 = severe difficulty, 5 = extreme difficulty). According to the scale instruction manual [33], we recoded the scores of each item, calculated each evaluation dimension and the overall score, and converted the original score to correspond to points 0-100 (0 = no disability, 100 = complete disability). Based on the International Classification of Functioning (ICF) disability and health criteria, each evaluation dimension and overall disability level was evaluated, i.e., <4 points are classified as no problem disability status and >4 points as mild or above disability status [34]. The internal consistency of the schedule is as indicated by the Cronbach's α score of 0.967 in this study.
Demographic Characteristics
The survey also collected demographic characteristic variables. These variables included gender (male, female), age (60-69, 70-79, ≥80), educational level (illiterate and primary school and above), employment status (unemployed, employed), living style (living alone, living with spouse, others refer to living with children or other relatives), region (northern, central, southern), poverty (families whose annual per capita income was lower than the national poverty line need to be recognized by the Poverty Alleviation Office and other departments). Simultaneously, we also investigated the variables related to health status, including whether they had physical discomfort during the previous two weeks (Yes, No), chronic disease diagnosed by doctors (No, Yes), or were hospitalized in the previous year (Yes, No).
Statistical analysis
Descriptive statistics were used to assess the sample demographic characteristics (frequency and percentages). Median and interquartile ranges (IQR) were measured as the data showed skewed distribution. Mann-Whitney U tests were employed to compare the functional disability of subjects with different depressive symptoms. The chi-squared tests were also employed to examine the difference between different depressive symptoms groups.
Binary logistic regression analysis with an enter method was used to explore the relationship between functional disability and depressive symptoms after adjusting for potential covariates (including gender, age, educational level, employment status, living style, region, poverty, physical discomfort, chronic diseases, and hospitalization). According to the criteria of tolerance and variance inflation factor, there was no multi-co-linearity between independent variables (Table S1 and Table S2). For functional disability variables, subjects who had no problem in each dimension were grouped as the reference group in a logistic regression model. For other variables, subjects who were male, aged 60-69 years, illiterate, unemployed, living alone, belonging to the northern region, poor, had physical discomfort, had no chronic disease, hospitalized were grouped as reference. The results of the binary logistic regression analysis were expressed with the adjusted odds ratio (AOR) and associated 95% confidence interval (95% CI).
To further investigate the interactive association between functional disability, relevant factors and depressive symptoms, a classification and regression tree model (CART) was used. This model is a novel analysis method, which can automatically judge and classify according to the significance of the test, optimal segment many types of variables and samples, and show the interaction between different variables through the tree diagram. It can also overcome the co-linearity problem that exists in the traditional analysis method and investigate the complex combination or interaction between the factors and variables neglected in the traditional analysis method [35]. The CHAID algorithm was used to screen variables and include statistically significant variables in univariate analysis. Additionally, to fully mine the interaction between variables and keep the tree structure relatively simple, the minimum cases in the parent node and child node of the model parameters were set 400 and 200, respectively, and the maximum growth depth was 3.
All data were analyzed using SPSS statistics software, version 25.0 (IBM, Armonk, NY, USA). A two-tailed p<0.05 was considered to be statistically significant.