Study site
This study was conducted in Rizhao City, Shandong Province, China. Rizhao City is located in the southeast of Shandong Province, and it's a coastal city in the east near the Pacific Ocean. It mainly includes 4 districts, Donggang, Lanshan, Juxian and Wulian, with altogether 55 townships. In this study, according to the principle of multi-stage stratified random sampling, two districts of Lanshan and Wulian were selected randomly considering the GDP level (high and low) and location (coastal and inland). Then, 4 communities and 4 villages were selected from each sampling district. Thus, we selected 8 urban communities and 8 rural villages in total. Figure 1 shows the location of the two study sites.
Data collection
The survey was conducted from March to April in 2019, a set of standardized questionnaires was used to collect data in the community health service institutions where the respondents were located. The questionnaires included sociodemographic information, SRHMS status, health-related behaviors, and blood pressure values. All participants (over 18 years of age) were interviewed face to face by trained nurses in isolated rooms. To ensure the quality, the completed questionnaires were carefully checked by the quality supervisor after the interview. In total, 4390 eligible participants were included in this study.
Trained nurses performed blood pressure measurements, and the participants measured the blood pressure of the right arm using a sphygmomanometer after sitting for 15 minutes. Each subject's blood pressure was measured three times, and the average of the last two times was taken as the blood pressure value. Hypertension was defined based on the JNC 7 guidelines (SBP ≥ 140 or DBP ≥ 90 mmHg). Considering other diseases that may affect SRH (cancer, mental illness, metabolic disorders, etc.), nurses asked respondents about their past disease history and excluded them from the study.
Variables
Body mass index
Body mass index (BMI) was calculated as weight in kilograms divided by height in centimeters squared, height and weight were measured by trained nurses. The interviewees were asked to remove their shoes for the measurement of height. Further, it was categorized as < 18.5 kg/m2 (underweight), 18.5–25 kg/m2 (normal weight), 25.0–30 kg/m2 (overweight), ≥ 30.0 + kg/m2 (obesity)[26].
Self-rate health
In this study, self-rated health (SRH) was measured by the multi-item self-rated health measurement scale (SRHMS) developed by Xu Jun et al[21, 22]. The scale is designed according to WHO's definition of health, health is regarded as a multidimensional concept including physical health, mental health and social well-being. The scale reflects the change of health measurement from a single dimension to multi-dimension and from group to individual. Previous studies have shown that SRHMS could accurately and comprehensively reflect the individual's health status, reliability and validity are acceptable, and have strong adaptability in China[27, 28].The SRHMS consisted of 48 items, the items are divided into 9 dimensions: physical symptom and organic function, daily physical activities, physical mobility, psychosocial symptom and negative emotion, positive emotion, cognitive function, role activity and social adaptability, social resource and social contact, and social support. The 9 dimensions were also categorized into 3 subscales: physical health, mental health and social health. Each of the 48 items has a maximum possible score of 10 and a minimum possible score of 0, the subscales score and total score maximum possible score of 100, and the minimum possible score of 0. The SRHMS is divided into five grades according to the scores: very bad [0, 62.48), bad [62.48, 68.49), fair [68.49, 80.50), good [80.50, 86.50), very good [86.50, 100].
Other variables
Social demographic factors include gender (male, female), age (< 40, 40-, 60-), marriage status (married, others), education (illiterate or semiliterate, primary school, junior school, senior school or above), residence (rural, urban), income (< 15000, 15000-, 25000- Year Yuan), health insurance (yes, no).
Health related factors include smoking (yes, no), drinking (yes, no), physical exercise (yes, no), reduce salt intake (yes, no).
Data analysis
Descriptive statistic methods were used to analyze the characteristics of the study subjects; Chi-square test or t-test were used to compare the difference in categorical and continuous variables; Logistic regression analysis was used to explore the association between BMI and SRHMS among gender differences in hypertensive patients. The data were double entered and verified using EPI Data 6.04, with all data analyzed using SPSS version 16.0 (SPSS, Chicago, Illinois, USA). The level of significance was set at P-values < 0.05.