The data that support the findings of this study are available from the corresponding author on reasonable request.
Data and participants
The sample sizes of survey in 2010, 2013, 2015 and 2018 were calculated as following:
The confidence level (CI) is 95% (two-side), u=1.96; The prevalence of diabetes 10.4% was selected as P, The design effect (deff ) was 3, Relative error r=20%, d=20%×10.4%. Considering the non-response rate is 15% in the survey.
For each survey, we conducted a complex multistage stratified sampling method to recruit participants. At the first sampling stage, two townships/streets were selected in each surveillance point, which is a district in a city or a county, by probability proportionate to size sampling (PPS ) method. At the second stage, four administrative villages/community were random selected in each township/street. One group was then randomly selected in each administrative village/community at the third stage. In each group, we randomly selected 40 households. KISH table was used to select participants in each household who had lived in their current residence for at least 6 months within a year in survey 2004, 2007, 2010 and 2013, whereas all the family members were selected from each household in 2015 and 2018. Survey in 2004 was conducted in two surveillance points in Shaanxi Province. We expanded the number of points in Shaanxi from 2 to 5 in 2007 and 2010, further to 10 in 2013, 2015 and 2018, respectively. Details of the establishment, design, sampling method and quality control had been reported elsewhere [17-21]. The surveillance was approved by the ethical review committee of the Chinese Center for Disease Control and Prevention and written informed consent was obtained from all participants at enrollment. The study was conducted in accordance with the Declaration of Helsinki.
A face-to-face interview was conducted by trained staff from local CDC (Centers for Disease Control and Prevention) and health care institutes using standardized questionnaires. Information of each participant on demographic characteristics, medical history and lifestyle factors was collected. Height, weight and blood pressure was measured on-site health examinations.
The blood pressure of each participant was measured at the non-dominant arm 3 times in succession with 1 minute interval in a separate examination room after a 5-minute rest, except 2 times were measured in 2004. We adopted an electronic upper arm blood pressure monitor (HBP-1300, Omron Healthcare, Inc., Kyoto, Japan) to measure it, except for a Standard mercury column mercury meter in 2004. The mean of the last two readings was used for analysis from 2007 to 2018. The second readings were used in 2004.
According to the Chinese Guidelines for the Prevention and Treatment of Hypertension 2018, hypertension is defined as systolic blood pressure (≥140 mmHg) and/or diastolic blood pressure (≥90 mmHg), or those with hypertension and takes anti-hypertension drugs in the past. The proportion of participants with hypertension in all the participants was defined as prevalence of hypertension. Participants who had been diagnosed with hypertension before the survey were deemed as awareness of hypertension. The rate of hypertension treatment is the proportion of participants with hypertension taking antihypertensive drugs in the past two weeks before the survey. The rate of hypertension control is the proportion of hypertensive patients whose systolic blood pressure <140 mmHg and diastolic blood pressure < 90 mmHg.
Current smoking is defined as smokers used cigarettes every days or some day during the survey. Alcohol drinking is defined as consumed alcohol in the past 12 months. According to the standard of Chinese guidelines for the prevention and control of overweight and obesity in adults, BMI < 24kg/m2 is considered normal or low weight, 24kg/m2 ≤ BMI < 28kg/m2 was considered overweight, and BMI ≥ 28kg/m2 was considered obesity .
In each survey, investigators had to be trained in both national and provincial level and those who were passed the test after training could conduct the survey. Anthropometric meters were standardized before the survey. A checking system including checking in the field by interviewers, checking by supervisors in the work group was adopted during the survey. Subjects were re-interviewed when logical questions and missing values were found.
Since all the surveys were conducted by a complex multistage stratified sampling, we calculated weights of each survey, which was the product of sampling weights and post-stratified weights. We reported all the prevalence and confidence intervals (CI ) with weights. Cochran-Armitage trend test was used to analyze the trend of year. We also estimated unadjusted changes in hypertension prevalence and its management rates across survey years. To explore the demographics associated with hypertension and its management in a robust way, we conducted multivariable logistic regression with stepwise approach using pooled data from 2004 to 2018. Prevalence, awareness, treatment and control were the outcome in the models. Sex (male/female), age (18-44/45-59/≥60), education level (≤6,7-8, ≥9 years), marital status (single/ (married/co-habitat) /( divorce/ widow/separation)), occupation (employed/ unemployed/ retired), residency (urban/ rural), BMI (<24/24-28/≥28), cigarettes smoking (never/ever) and drinking (never/ever) were the co-variates in the model. Since there are differences in sex, age, education level and other characteristics throughout surveys, we also standardized the prevalence by using the 6th national population census in 2010.
The age range in 2004 and 2007 was 15-69 year old and the rest of surveys were all above 18 years. In order to verify the reliability of the results, we also did sensitivity analysis. The main prevalence and rates including participants at 18-69 years old at all five survey was recalculated in the study. SAS 9.4 was used for statistical analysis in our study. All p values were two sided and <0.05 was considered statistically significant.