The National Health and Nutrition Examination Survey (NHANES), conducted by the National Center for Health Statistics (NCHS), utilizes a complex, stratified probability sampling method to survey the noninstitutionalized US population. Data from NHANES were collected via interviews, physical assessments, and laboratory evaluations of blood and urine specimens collected from participants. This study used 10 cycles of the continuous NHANES from 1999 to 2018. Of the 55,081 participants aged 20 years or older, we excluded those with missing measurements for waist circumference, blood pressure, high-density lipoprotein (HDL), triglycerides (TG), and fasting plasma glucose (FPG) (n = 7,118); those lacking data on urine albumin, urine creatinine, estimated glomerular filtration rate (eGFR), and UACR ≥ 30 mg/g (n = 8,396); those missing information on relevant covariates (n = 12,236); and those with unclear mortality status (n = 33). The final analytical cohort comprised 27,298 participants (see Supplement Figure S1).

Urine samples were collected by trained investigators and frozen at -20 °C before being transported to the laboratory. A solid-phase fluorescence immunoassay measured urine albumin levels. Before 2007, urinary creatinine was measured using the kinetic Jaffe rate method. After 2007, an enzymatic method was employed for creatinine measurement. Comprehensive details of laboratory testing procedures are available on the NHANES website [17]. To minimize discrepancies in urine creatinine measurement techniques, we applied the NHANES-endorsed algorithm to adjust creatinine levels for data collected before 2007 [18]. UACR is calculated by dividing urine albumin by urine creatinine. Participants were classified into tertiles according to their UACR levels within the normal range: low (< 4.62 mg/g), medium (4.62–7.94 mg/g), and high (> 7.94 to < 30 mg/g).

All relevant metabolic factors were measured at the time of the initial NHANES investigation. Metabolic abnormalities were defined based on the National Cholesterol Education Program-Adult Treatment Panel III (NCEP-ATP III) criteria [13], which included the following: central obesity (waist circumference ≥ 102 cm for men or ≥ 88 cm for women), hypertriglyceridemia (serum TG ≥ 150 mg/dL), dyslipidemia (HDL cholesterol < 40 mg/dL for men or < 50 mg/dL for women), hypertension (systolic/diastolic blood pressure ≥ 130/85 mmHg or treatment for hypertension), and hyperglycemia (FPG ≥ 100 mg/dL or treatment for diabetes). According to the NCEP-ATP III definitions of metabolic syndrome (MetS), metabolic abnormalities were divided into three groups based on the number of components: metabolically healthy (0 components), Pre-MetS (1–2 components), and MetS (3–5 components).

The primary outcome was cardiovascular mortality, classified according to the International Classification of Diseases, 10th Edition (I00-I09, I11, I13, and I20-I51). We linked the 1999–2018 NHANES data with National Death Index (NDI) mortality data using probabilistic matching. The cause-specific mortality data in the NDI have been demonstrated to accurately classify deaths with a minimal likelihood of misclassification [19]. Mortality follow-up data were available until December 31, 2019. The follow-up duration was calculated by measuring the interval between each participant’s baseline examination and the last known survival date or censoring date from the mortality file.

The demographic and clinical characteristics of the study participants included age, sex (male or female), ethnic (Non-Hispanic White, Non-Hispanic Black, Mexican American, or Other race/multiracial), education status (less than high school, high school, or more than high school). Smoking status was categorized as never, former, or current based on participants’ reports of having smoked at least 100 cigarettes in their lifetime and their current smoking status. The poverty income ratio reflects the ratio of annual household income to the federal poverty line. Physical activity was classified into three categories: low (< 600 MET-minutes per week), moderate (600–3000 MET-minutes per week), and high (≥ 3000 MET-minutes per week). CVD was defined by self-reported diagnosis of five major cardiovascular events: congestive heart failure, coronary heart disease, angina pectoris, heart attack, and stroke. Cancer status was also self-reported. The eGFR was calculated using the abbreviated Chronic Kidney Disease Epidemiology Collaboration equation [20].

All analyses accounted for the complex sampling strategy of NHANES, including stratification, clustering, and weights [21]. Baseline and metabolic factor distributions were estimated by dividing participants into tertiles (low, medium, high) based on UACR levels. Continuous variables are expressed as mean ± SE, while categorical variables are presented as percentages. Differences between UACR tertiles were evaluated using linear regression for continuous variables and chi-square tests for categorical variables.

The hazard ratio (HR) and 95% confidence interval (CI) for cardiovascular mortality associated with normal UACR were estimated using a multivariate Cox proportional hazards regression model. UACR was analyzed both as a continuous variable (per 10 mg/g increment) and as a categorical variables (UACR tertiles) across the cohort and within subgroups defined by metabolic abnormalities. We adjusted for potential confounders using two multivariate models with progressive degrees of adjustment. Model 1 adjusted for age, sex, ethnicity, education level, poverty income ratio, smoking status, physical activity level, and eGFR. Model 2 was further adjusted for self-reported cancer and CVD. The associations between tertiles of normal UACR and cardiovascular mortality were analyzed within each metabolic abnormality group, with the lowest tertile serving as the reference. Combined effects were assessed using tertile groupings of UACR (low, medium, high) and metabolic abnormality groupings (metabolically healthy, Pre-MetS, and MetS). Trend tests for the 9-categorical groups used the metabolically healthy and low UACR groups as references. Kaplan-Meier survival curves visualized survival rates for the 9-categorical groups, and log-rank tests assessed the significance of associations between groups. Restricted cubic spline regressions with three knots were used to evaluate the dose-response relationship between continuous UACR levels and cardiovascular mortality. The likelihood ratio test evaluated non-linearity.

Subgroup analyses were conducted by age (< 60 years, ≥ 60 years) and sex (male, female) to estimate the effect of continuous normal UACR on cardiovascular mortality within each metabolic abnormality stratum. Effect modification was assessed by including multiplicative interaction term in the models and using the likelihood ratio test. Sensitivity analyses were performed to test the robustness of the findings by: incorporating continuous variables for metabolic factors (waist circumference, systolic blood pressure, diastolic blood pressure, FPG, TG, and HDL) as covariates in the final model; excluding individuals with impaired kidney function (eGFR < 60 mL/min/1.73 m²); and omitting subjects who passed away within the first two years of follow-up to minimize reverse causality bias.

R version 4.2.0 and Empower Stats (http://www.empowerstats.com, X&Y Solutions, Inc., Boston, MA) were used for the analysis. All statistical analyses were two-tailed, and P < 0.05 was considered statistically significant.