2.1 Study design and participants
A total of 7268 individuals aged 45 years and older from CHARLS 2011 were included. Figure 1 shows the flowchart of the recruitment process for this study.
2.2 CKD
CKD is defined as meeting one of the following diagnostic criteria:
i eGFR < 60 mL/min/1.73m2 (calculated using the CKD Epidemiology Collaboration (CKD-EPI) equation). [7]
ii eGFR ≥ 60ml/min/1.73 m² but self-reported CKD.
The CKD-EPI equation was as follows:
eGFR (mL/min/1.73m2) = 135 × min(SCr/k, 1)α × max(SCr/k, 1)−0.601 × min(Cys/0.8, 1)−0.375 × max(Cys/0.8, 1)−0.711 × 0.995age ×0.969[if female]
SCr refers to serum creatinine (mg/dL), Cys refers to serum cystatin C (mg/liter), age refers to the patient's age (years), α is − 0.248 for females and − 0.207 for males, k is 0.7 for females and 0.9 for males.
CKD was then divided into stages 1–2: eGFR ≥ 60ml/min/1.73 m², but previously diagnosed with CKD, stage 3a: 45 ≤ eGFR < 60ml/min/1.73 m², stage 3b: 30 ≤ eGFR < 45ml/min/1.73 m², stage 4: 15 ≤ eGFR < 30ml/min/1.73 m², and stage 5: eGFR < 15ml/min/1.73 m².
2.3 CVD
Participants who reported a heart attack or stroke were defined as having cardiovascular disease[8], and cardiovascular disease events were assessed with the following questions:
i "Have you ever been told by your doctor that you have been diagnosed with a heart attack, angina, coronary artery disease, heart failure, or other heart problem?"
ii "Have you ever been told by your doctor that you have been diagnosed with a stroke?
2.4 Covariates
Previous literature and clinical considerations have identified several covariates that may serve as potential confounders. These include demographic characteristics (age, gender, household registration, education, marital status)[9, 10], lifestyle factors (smoking, drinking)[11], physiological indicators (BMI, blood pressure, blood sugar, blood lipids, serum uric acid)[12, 13].
The baseline age was calculated by subtracting the date of birth from the year 2011, and then divided into three categories (Q1 [45,60), Q2 [60,70), Q3 [70,∞)) to demonstrate the prevalence of comorbidities, CKD alone, and CVD alone in different age groups. Household registration was divided into agricultual household registration and non-agricultual household registration. Education was classified as “below elementary school”, “elementary to middle school” and “high school or above”. Marital status was recorded as either "yes" (‘married with spouse present’/ married but not living with spouse temporarily) or "no" (‘widowed/divorced/separated/never married’)
Smoking was categorized as yes or no, and drinking was categorized as “drinking more than once a month”, “drinking less than once a month”, “none of the above”.
BMI was calculated using the following formula: BMI = Weight (kg)/[Height(m) ]2. Then we divided BMI into 4 groups according to previous studies: [−∞,18.5) kg/m2 (underweight), [18.5, 25) kg/m2 (normal weight), [25, 30) kg/m2 (overweight), and [30, ∞) kg/m2 (obesity).[14] Hypertension was defined as systolic blood pressure ≥ 140mmHg or diastolic blood pressure ≥ 90mmHg or use of antihypertensive medication.[15] Diabetes was defined as a fasting blood glucose level of 126 mg/dL or higher (to convert to mmol/L, multiply by 0.0555 [i.e., ≥ 7.0mmol/L]), a glycated hemoglobin A1c level of 6.5% or higher (to convert to mmol/mol, multiply by 10.93 and subtract 23.5 [i.e., ≥ 48mmol/mol]), or the use of diabetes medications or insulin.[15] Hyperlipidemia was defined as total cholesterol above 200 mg/dL, or low-density lipoprotein cholesterol above 130 mg/dL, or high-density lipoprotein cholesterol below 40 mg/dL in men or below 50 mg/dL in women, or triglycerides above 150 mg/dL, or use of lipid-lowering medication.[16] Hyperuricemia is defined as a serum uric acid level > 7.0 mg/dL in men or > 6.0 mg/dL in women.[17]
2.6 Statistical analyses
The data are presented as the median (interquartile range (IQR)) for continuous variables and as number (percentage (%)) for categorical variables. The Kruskal-Wallis(K-W) rank sum test was employed for continuous variables, while the chi-square test was utilized for categorical variables. In the event that the data does not satisfy the assumptions of the chi-square test, the Fisher's Exact Test for Count Data with simulated p-value (Fisher's Exact Test (simulated)) is employed.
The prevalence of CVD in the general population was examined, and the data were stratified based on levels of CKD, demographic characteristics, lifestyle factors, and physiological indicators. To further investigate the association between levels of CKD and CVD, Logistic regression models were employed to compute the odds ratios (ORs) and 95% confidence intervals (95% CIs) in contrast with the non-CKD group. Trend testing was performed based on the median value of eGFR of each group.
In the above Logistic regressions, we created four models to eliminate the effects of various covariates. Specifically, the crude model made no adjustments, model 1 adjusted for demographic covariates, model 2 adjusted for lifestyle factors and demographic covariates, and model 3 adjusted for physiological indicator covariates, lifestyle factors and demographic covariates (unless otherwise specified). A restricted cubic spline analysis was employed to explore the dose–response relationship between eGFR and the specified outcomes, and linear associations were assessed using the sensitivity analysis.
Once the association between CKD and CVD had been established, a logistic regression model was employed to calculate the OR and 95% CI for each influencing factor in the only-CKD group in comparison with the normal group. The same method was then used to analyse the influencing factors of the comorbidity group and the only-CKD group.
All statistical analyses were conducted using the R software (R version 2024.04.1 + 748). Visualization was achieved with the "ggplot2", "crosstable" and "forestplot" packages. Statistically significant results were determined at a two-tailed p < 0.05.