Low-density lipoprotein cholesterol to apolipoprotein B ratio is associated with all-cause mortality and cardiovascular events among incident peritoneal dialysis patients

Background: The ratio of low-density lipoprotein cholesterol (LDL-C)/apolipoprotein B (apo B) is associated with all-cause mortality and cardiovascular events in chronic kidney disease patients. The aim of this study was to investigate the association between the LDL-C/apo B ratio (LAR) and all-cause mortality and cardiovascular events in peritoneal dialysis (PD) patients. Methods: A total of 1199 incident PD patients were enrolled from November 1, 2005 to August 31, 2019. The LAR was used to divide the patients into two groups by X-Tile software and restricted cubic splines using 1.04 as the cutoff. The incidence of all-cause mortality and cardiovascular events at follow-up was compared according to LAR. Results: Of the 1199 patients, 58.0% were men, the mean age was 49.3 ± 14.5 years, 225 patients had a history of diabetes, and 117 patients had prior cardiovascular disease. During the follow-up period, 326 patients died, and 178 patients experienced cardiovascular events. After full adjustment, a low LAR was signicantly associated with HRs for all-cause mortality of 1.40 (95% CI: 1.04 to 1.87, P = 0.040) and for cardiovascular events of 1.62 (95% CI: 1.11 to 2.38, P = 0.008). Conclusions: This study suggests that a low LAR is an independent risk factor for all-cause mortality and cardiovascular events in PD patients, indicating that the LAR may provide signicant information when assessing all-cause mortality and cardiovascular risks.

shown that more intensive LDL-C lowering was associated with reductions in total and cardiovascular mortality [14]. In addition, LDL-C is considered to be an independent risk factor for CVD in chronic kidney disease (CKD), which is the most important cause of death in dialysis patients [15]. However, there is a qualitative difference in LDL-C between PD patients and normal controls because of the increased concentration of small and dense particles along with high apolipoprotein B (apo B) [16,17]. LDL-C/apo B (LAR) can be used as an index to predict all-cause mortality and cardiovascular events in patients with nonrenal diseases. As LDL-C can carry only a single apo B, the LAR can serve as a tool to estimate the LDL particle size [18,19]. Drexel H et al. showed that the LAR could independently predict major cardiovascular events in subjects with established atherosclerosis [20]. In addition, LAR has also been shown to be closely associated with metabolic syndrome, coronary heart risk score, neointimal hyperplasia and intimal instability in nonrenal diseases [21][22][23].
However, the predictive value of the LAR in understanding all-cause mortality and cardiovascular events in PD patients remains unclear. Therefore, the present study was conducted to investigate the impact of LAR levels on predicting all-cause mortality and cardiovascular events among PD patients.

Study population and design
A total of 1457 patients who visited the PD Center of the First A liated Hospital of Nanchang University, Jiangxi Province for PD renal replacement therapy from November 1st, 2005 to August 31st, 2019 were enrolled. All PD patients were followed up until PD cessation, switched to hemodialysis, kidney transplantation, transferred to another center, refused additional treatment, died, or were lost to follow-up or December 31, 2020, whichever came rst. The inclusion criteria for the study were older than 18 years old and a dialysis age of at least three months. The exclusion criteria were age younger than 18 years old, failure of hemodialysis or renal transplantation, cessation of peritoneal dialysis within the rst three months, lack of LDL-C or Apo B data, and known malignant tumors. Finally, 1199 patients were included in the study, excluding 15 cases under 18 years old, 8 cases of hemodialysis, 4 cases of failed renal transplantation, 25 cases of dialysis time less than 3 months, 7 cases of malignant tumor and 199 cases without detection of LDL-C or apo B (Figure 1). This study selected the best cutoff value of LAR (Group 1 < 1.04 and Group 2 ≥1.04) by the X-tile procedure (Yale University, New Haven, CT, USA)[24], which is an important tool for outcome-based cutoff point optimization and restricted cubic splines. The association between LAR and all-cause mortality and cardiovascular events was analyzed by using the Cox proportional risk model. Model 1 was adjusted for age, sex, body mass index (BMI), estimated glomerular ltration rate (eGFR), Charlson Comorbidity Index (CCI), diabetes, hypertension, and premorbid CVD. Model 2 was further adjusted for hemoglobin, albumin, alkaline phosphatase, phosphorus, intact parathyroid hormone (iPTH) and ferritin. Model 3 was further adjusted for high-density lipoprotein cholesterol, triglyceride (TG), and lipid-lowering medication. This study was approved by the Human Ethics Committee of Nanchang University (Application ID: [2021] 9-028).

De nition of cardiovascular events
According to the guidelines and related literature [27,28], this study de ned cardiovascular events in patients as follows: myocardial infarction, angina pectoris, heart failure, cerebral infarction, cerebral hemorrhage, peripheral vascular events, aneurysm dissection or ruptured death, other (metabolic cardiomyopathy, Adams-Stokes syndrome, arrhythmia) or unexplained death.
Statistical analyses SPSS version 22.0 (SPSS, Inc., Chicago, IL) was used for statistical analysis, and R version 3.6.0 (http://www.r-project.org) was used for survival analysis. The t-test, Chi-square test, and Kruskal-Wallis test were used to test for differences between the LAR groups. Frequencies and percentages are presented for categorical variables, medians and interquartile ranges are presented for nonnormally distributed data, and means and standard deviations (SDs) are presented for normally distributed data. P < 0.05 represents statistical signi cance.

Baseline characteristics
A total of 1199 subjects, including 696 men and 503 women, were included: the mean ± SD age was 49.3 ± 14.5 years, 225 patients had a history of diabetes, and 117 patients had prior CVD in this study. During the 35-month (interquartile range=20-57 months) follow-up period, 326 patients died, and 178 patients experienced CV events ( Figure 1). The baseline characteristics of all patients are shown in Table 1. Age, sex, hypertension, CVD, albumin, TC, TG, LDL-C, HDL-C, Apo B, and ferritin were signi cantly different (P< 0.05). No signi cant differences among body mass index, diabetes, eGFR, hemoglobin, phosphorus or iPTH were observed (Table 1). In addition, PD patients with a low LAR were older and had less hypertension; decreased levels of albumin, TC, HDL-C, and LDL-C; and increased levels of TG, apo B and ferritin (P< 0.05).
The correlations among LAR, all-cause mortality, and cardiovascular events At the end of the follow-up, the cumulative survival rate of all patients in the low LAR group decreased signi cantly according to the Kaplan-Meier analysis (P<0.0001, Figure 2A). Similarly, the cumulative rate of cardiovascular events increased signi cantly (P<0.0001, Figure 2B). Using Cox regression analysis, we determined the association between the LAR and all-cause mortality and cardiovascular events, as shown in Table 2. There were signi cant differences in the association between LAR and all-cause mortality and cardiovascular events in Models 1, 2 and 3. In Model 3, the HRs and 95% CIs for LAR<1.04 versus LAR≥1.04 were HR, 1.40 (95% CI: 1.04 to 1.87), and HR, 1.62 (95% CI: 1.11 to 2.38), for all-cause mortality and cardiovascular events, respectively. In addition, the restricted cubic splines showed that the risk of allcause mortality and cardiovascular events decreased gradually when the LAR was >1.04 ( Figure 3).

Discussion
In this study, we found that LAR was associated with all-cause mortality and cardiovascular events in PD patients. PD patients with low LAR were older, had less hypertension; decreased levels of albumin, TC, HDL-C, and LDL-C; and increased levels of TG, apo B and ferritin. Moreover, low LAR patients had higher all-cause mortality and cardiovascular events. Therefore, the LAR can be used as a signi cant important risk indicator for PD patients. LDL-C is a strong predictor of coronary heart disease. Decreasing LDL-C levels is considered to be the main therapeutic target for the treatment of hypercholesterolemia and this approach can reduce the incidence of major atherosclerotic events in patients with advanced CKD [33,34]. Related studies have shown that reducing serum LDL-C levels can effectively reduce the risk of ischemic stroke, myocardial infarction and coronary artery death [35]. Massy ZA et al. have shown that half of CKD patients have an LDL-C that exceeds the recommended target, and a signi cant proportion do not receive treatment, thus increasing the morbidity and mortality of CKD patients due to coronary heart disease and stroke [36]. However, other studies have found that there is no signi cant correlation between the level of LDL-C and the prognosis of atherosclerotic CVD in CKD [37]. Tani S et al. found that a low LDL-C level in patients with coronary artery disease and advanced kidney disease was associated with a poor prognosis [38]. Our study showed that after adjusting for age, sex, BMI, eGFR, CCI, diabetes, hypertension, premorbid CVD, hemoglobin, albumin, alkaline phosphatase, phosphorus, iPTH, ferritin, HDL-C, TC, and lipid-lowering medication, the LDL-C levels were not signi cantly associated with all-cause mortality or cardiovascular events. Therefore, further research is required to resolve this controversy.
Apo B is the main structural lipoprotein of LDL, VLDL, medium density lipoprotein and lipoprotein A. Each particle has one apo B, so the measurement of apo B is used to determine the exact number of atherogenic lipoprotein particles [18,39]. The trapping of apo B lipoprotein particles within the arterial wall plays an important role in initiating and driving the atherosclerotic process [40]. A study found that low apo B levels may be an effective marker of left ventricular hypertrophy in PD patients [41]. In addition, other studies have shown that apo B levels are higher in PD patients than in hemodialysis and normal subjects and that a high apo B level increases the risk of coronary heart disease in PD patients [42]. Current evidence suggests that apo B measures the risk of atherosclerosis caused by apolipoprotein more accurately than LDL-C or non-HDL-C [43]. However, the predictive ability of apolipoprotein B is not always better than that of non-HDL-C. Di Angelantonio E [44] and Benn M [45] showed that non-HDL-C and apo B have the same predictive ability. A cross-sectional study showed that higher apo B is associated with the prevalence of coronary heart disease[46]. Lamprea-Montealegre JA et al. showed that higher apo B is associated with an increased risk of atherosclerotic vascular events in patients with CKD [47]. In the fully adjusted model, apo B was positively correlated with CVD-related mortality [48]. However, our study showed that apo B was not signi cantly associated with all-cause mortality or cardiovascular events.
LDL particles can be divided into large and low-density LDL particles (diameter >25 nm) and small and dense LDL (diameter <25 nm) [49,50]. Small and dense low-density lipoprotein particles have higher arterial wall permeability, lower LDL receptor binding a nity, a longer plasma half-life and lower oxidative stress resistance [11,[51][52][53]. St-Pierre AC et al. showed that in patients with LDL particle sizes of 25.5 nm or smaller, the incidence of coronary heart disease increased signi cantly with increasing serum LDL-C levels [54]. Similarly, some patients may have the same LDL-C level, but patients may have more or smaller LDL particles, and their risk may be higher [55]. The LAR indirectly determines the size of the LDL particles. Some studies have shown that the LAR can distinguish between large loose LDL and small dense LDL, and the latter leads to a signi cant increase in the incidence of coronary heart disease[56-58]. Therefore, LAR is an independent predictor of coronary heart disease [13, 59, [19]. This nding is similar to our ndings, which show that all-cause mortality and cardiovascular events increased signi cantly when LAR<1.04. The reason may be that low LAR is mainly characterized by small and dense LDL particles. Since each particle can only contain one apo B, an increase in serum apo B may indicate that there are a large number of LDL particles in the blood circulation, thus increasing the risk of cardiovascular events. This has been con rmed in several studies [56,57,62]. Previous studies have shown that the level of plasma apo B-cholesterol may better re ect the number of atherogenic granules than the concentration of LDL-C[63]. Although our study shows that apo B levels alone do not predict all-cause mortality and cardiovascular events, as an alternative to LDL-C particle size alone, a lower LAR in PD patients may play an important role in predicting all-cause mortality and cardiovascular events.
Our study also has some limitations. For example, this is a retrospective study with a small sample size, followed by no assessment of the effects of in ammatory cells and in ammatory factors, as well as the impact of some unknown factors on blood lipid levels in PD patients. Although lipid-lowering drugs were adjusted in the risk model, this factor was not taken into account in the grouping of LAR, indicating improvements are necessary in future studies.

Conclusion
In summary, LAR can be used as a new risk marker for all-cause mortality and cardiovascular events in patients with PD. Low LAR is associated with higher all-cause mortality and cardiovascular events, along with increased age; decreased levels of albumin, TC, HDL-C, and LDL-C; and increased levels of TG, apo B and ferritin.

Availability of data and materials
All datasets about this study available from the corresponding author.  Figure 1 Enrollment ow chart. PD, peritoneal dialysis; HD, hemodialysis; LAR, Low-density lipoprotein cholesterol to apolipoprotein B ratio; IQR, interquartile range; CV, cardiovascular.

Figure 2
Survival curves of patients strati ed by LAR. A. Cumulative patient survival rate. B. Cumulative rate of cardiovascular events.