A retrospective cohort of elderly Thai population aged 60 years or more from the Fourth Thai National Health Examination Survey (NHES-IV) conducted during 2008-2009 were evaluated in 2016 to determine all-cause mortality. NHES-IV was a nationally representative survey using a multistage, stratified sampling of the Thai population for which the detailed of sampling methods have been described(14). Briefly, the first stage of sampling units was provinces in each region in Thailand, the second stage was district in each province, the third was electoral unit (EUs) or villages, and, the last stage was age and gender-specific individuals. Medical comorbid disease (including eGFR), functional status, and socioeconomic status data were collected at the baseline assessment in NHES-IV. The vital status of each participant was ascertained from the National Civil Registration and Vital Statistics System.
Data collection and measurement
Serum creatinine was measured using modified Jaffe method. Estimated glomerular filtration rate was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation(18). In this study, CKD was defined according to GFR category by Kidney Disease: Improving Global Outcomes (KDIGO) staging, as follows: G1, G2, G3a, G3b, G4 and G5 as GFR >60 (G1 + G2), 45-59, 30-44, 15-29, and <15 ml/min/1.73m2, respectively.(19) Reduced GFR was defined as GFR less than 60 ml/min/1.73m2. In the present study, stages 3-5 CKD were classified as moderately to severely decreased GFR.
Diabetes was defined as fasting plasma glucose of >7.0 mmol/L, diagnosis by a physician, or taking antidiabetic medications.(20) Hypertension was defined as systolic blood pressure >140 mmHg, diastolic blood pressure >90 mmHg, or history of using any antihypertensive medications. Hypercholesterolemia was defined as total serum cholesterol >5.2 mmol/L or use of cholesterol-lowering medication within the previous 2 weeks. According to WHO criteria, anemia was defined as hemoglobin level <13 g/dl in males, and <12 gm/dl in females(21).
Data collected in the NHES-IV, including demographic, socioeconomic, and health data such as smoking history, medication used, and common medical conditions, using semi-structured interview with standardized questionnaires. Medical comorbidities inquired in the interview included history of diabetes, hypertension, high cholesterol, asthma, chronic airway disease, allergy and cerebrovascular disease. Data relating to activities of daily living (ADLs) for both basic ADLs (B-ADLs) and instrumental ADLs (I-ADLs) were also obtained by interviewing enrolled participants and relatives. Impairment in ADLs was defined as the requirement for any assistance in performing ADL-related tasks, which included feeding, dressing, bathing, toileting and transferring from beds or chairs. I-ADL impairment was defined as the requirement for any assistance in complex tasks including using telephone, handling financed, responsibility for own medication, using transportation or driving and housework. The wealth index score was determined by assessing the number of household items owned, and then a standardized score was assigned to each participant(22). Socioeconomic status (SES) was stratified into 5 quintiles, with the lowest quintile indicating the poorest group, and the highest quintile indicating the wealthiest group.
Mortality data was retrieved until May 2016 from the Thai Vital Registration System, Bureau of Registration Administration, Ministry of Interior, which was affirmed to be a reliable source of death statistic in Thailand.(23)
Descriptive statistics were used to compare baseline characteristics of participants. Categorical data are reported as frequency and proportion, and continuous data are given as mean ± standard deviation or median with minimum and maximum depending on the distribution of data. Analysis of variance (ANOVA) test was applied for continuous variables, while chi-square test was used for categorical variables. A p-value less than 0.05 was considered statistically significant. In order to consider for the multistage, stratified sampling survey design of the NHES-IV, the analysis was carried out by using svyset syntax to adjusted for all stage of sampling and finite population correction. The adjusted data was saved for further analysis. Cox regressions were conducted to determine the unadjusted hazard ratios (HRs) for mortality. Potential confounders of all-cause mortality among participants with CKD included in the analysis were age, gender, current smoking status, regular analgesic use, hypertension, diabetes, hypercholesterolemia, history of stroke, anemia, and wealth index score. Variables that had a p <0.1 in univariate analyses were then included in a Cox proportional hazards model. Statistical analyses were performed using STATA 15.0 (StataCorp LP, College Station, TX, USA).