Study population
In this study, the longitudinal data of 298 subjects who had health examinations from 2008 to 2015 in the health examination center of Xijing hospital in Xi’an City, Shanxi Province were used for analysis. The health examination of participants included a questionnaire survey, physical examination, and clinical physical and chemical index examination, etc. The main indicators included gender, date of birth, age, marital status, education level, height, weight, body mass index, personal history of diseases, family history of diseases, blood pressure, fasting blood glucose, uric acid, TC, TG, and LDL-C, etc.
Quality control measures for anthropometric and laboratory measurements included the participants in the physical examination from the same individual unit and all adopting the same test method. Before the physical examination, the subjects were required to be free of smoking, alcohol, coffee, and have a fasting stomach for more than 12 hours. Biochemical blood samples were collected 3-5ml from an antecubital vein of participants in the morning by the laboratory staff who had received unified training. Then, the samples were detected within 10-30 minutes after extraction.
All the study methods were performed by the Medical Ethics Committee of Weifang Medical University (NO.2021YX116). The participants provided their written informed consent to participate in this study.
Follow-Up and Outcomes
In this study, the trajectory of TG development over time was taken as the main research index. The unit of measurement of TG is mmol/L. The incidence of stroke was observed through follow-up, and the outcome and survival time were recorded, to analyze the relationship between TG trajectory and the occurrence of stroke.
The outcome variable of this study was a new stroke, which was defined as the sudden or rapid onset of a typical neurological deficit caused by vascular causes for the first time, lasting for more than 24 hours or until death [20]. The clinical doctor diagnosed it based on computed tomography (CT) and(or) magnetic resonance imaging (MRI) according to international clinical diagnostic standards.
Inclusion criteria: (1) The number of physical examinations ≥ 3 times; (2) There were no patients with diabetes, cardio-cerebrovascular disease, liver disease, and kidney disease at baseline; (3) No missing baseline diagnosis information. Exclusion criteria: (1) The number of physical examinations < 3 times; (2) Study subjects who already had a stroke at baseline.
The flow chart of cohort establishment in this study is shown in Figure 1. A total of 298 subjects were eventually included according to the inclusion and exclusion criteria of health check-ups, 70 of whom developed strokes during the follow-up period.
Statistical analysis
In our study, we developed joint models to explore the association between longitudinal TG and stroke onset. The standard joint model was used to capture the relationship between TG and stroke, which can be generalized to the case of three parameterization models [11,21,22] including lagged effects parameterization, time-dependent slopes parameterization, and cumulative effects parameterization. To better understand the lagged effects between the event and longitudinal parts in our model, we assumed that the risk of a terminal event in time depended on the value of TG in the previous 3 years [11]. Then, a time-dependent slopes parameterization joint model was also postulated, in which the risk depended on both the trajectory's current TG value and the true trajectory's slope at time t. However, in many cases, it may benefit by allowing the risk to depend on the longitudinal marker history, ie. the cumulative effect of TG.
The structure of the standard joint model included the longitudinal submodel established based on the longitudinal data and the survival submodel established based on the survival data. In this study, we assumed yi(t) to be the follow-up measurements (TG) for patient i ( i=1,…,n ) at time t, and we followed the framework of linear mixed effect model to fit the longitudinal outcomes [23]:
Among them, xiT(t) is the time-varying covariate with corresponding fixed effect term β. ziT(t) is the time-varying covariate with corresponding fixed effect term bi. According to the distribution requirements of longitudinal observed variables, square root transformation of TG index was required to meet the normal distribution [11].
For the survival submodel, we perform the Cox proportional hazards model [24]:
Here, h0(t) donates the unspecified baseline risk function, γT is the time-varying covariate with corresponding fixed effect term ωi. Most importantly, the shared parameter α represents the impact of longitudinal results mi(t) on event risk. In the lag effect joint model, the risk of the terminal event at a time depends on the real value of the longitudinal detection variable at time t-c, where c is the order of time delay. In the time-dependent slopes parameterization, the explanation of the parameter α1 is the same as the shared parameter α in the standard joint model formula. If mi(t) is constant, the parameter α2 measures the correlation between the slope of the longitudinal trajectory at time t and the risk of time end event at the same time point. And in the cumulative effects joint model, for any time point t, α quantifies the correlation between the area under the longitudinal trajectory from baseline to time point t and the terminal event at the same time point.
The main parameter estimation method of the joint model was the maximum likelihood estimation method [25]. EM algorithm [26] or Newton Raphson algorithm [27] can be used to solve the maximum solution of the log-likelihood function. As a general iterative algorithm, the EM algorithm is widely used.
In this study, quantitative data were calculated by means and standard deviation representation (), and qualitative data were expressed both in frequencies and percentages, i.e.. Kaplan-Meier method was used for survival analysis, which was performed by survminer package of R 4.0.3 software. And the joint model was constructed by JM package of R 4.0.3 software. All tests were two-sided, and values of P<0.05 were considered statistically significant.