Relationship between serum phosphorus and mortality in non-dialysis chronic kidney disease patients: evidence from NHANES 2001–2018

Background Hyperphosphatemia is common in chronic kidney disease (CKD), associated with higher mortality in dialysis patients. Its impact in non-dialysis patients, especially those with preserved kidney function, remains uncertain. Methods A prospective cohort study was conducted using data from the National Health and Nutrition Examination Survey (2001–2008). Serum phosphorus was analyzed as a continuous variable, or categorized into three groups: < 3.5 mg/dL, 3.5 to < 4.5 mg/dL, and ≥ 4.5 mg/dL. Cox proportional hazards models were used to analyze the association between phosphorus with all-cause and cardiovascular disease (CVD) mortality, with or without adjustment for age, sex, race, hemoglobin, estimated glomerular filtration rate (eGFR), serum albumin, serum calcium, 25(OH)D, obesity, hypertension, diabetes, and CVD. Results A total of 7694 participants were included in the analysis, representing 28 million CKD patients in the United States. During mean 92 months of follow up, 2708 all-cause deaths (including 969 CVD deaths) were observed. Per 1 mg/dL increase in phosphorus was associated with a 13% and 24% increased risk of all-cause mortality (hazard ratio [HR], 1.13; 95%CI, 1.02–1.24) and CVD mortality (HR, 1.24; 95%CI, 1.07–1.45), respectively. Compared with the < 3.5 mg/dL, phosphorus ≥ 4.5 mg/dL was associated with a 28% and 57% increased risk of all-cause mortality (HR, 1.28; 95%CI, 1.05–1.55) and CVD mortality (HR, 1.57; 95CI, 1.19–2.08), respectively. In participants with eGFR < 60 ml/min/1.73m2, elevated phosphorus (≥ 4.5 mg/ dL) were significantly associated with increased risk of all-cause mortality (HR, 1.36; 95%CI, 1.07–1.72). No significant association was observed in eGFR ≥ 60 ml/min/1.73m2 group (HR, 1.31; 95%CI, 0.86–1.99). This correlation does not differ significantly between subgroups defined by eGFR level (P for interaction = 0.889). Conclusion Serum phosphorus above 4.5 mg/dL is significantly associated with a 28% and 57% increased risk of all-cause and CVD death in non-dialysis CKD patients, respectively. This relationship still demonstrated in patients with eGFR < 60 ml/min/1.73m2. However, for population with eGFR ≥ 60 ml/min/1.73m2, further verification is needed. Supplementary Information The online version contains supplementary material available at 10.1186/s12882-024-03525-x.


Introduction
Chronic kidney disease (CKD) is a global health issue with a prevalence of 8%-16% [1].Early recognition and intervention of risk factors are key to improving the prognosis of CKD patients [2,3].In addition to recognized risk factors such as hypertension and proteinuria, research indicates that serum phosphorus can affect the prognosis of CKD by stimulating endothelial cells to cause vascular calcification [4,5].Elevated serum phosphorus is a significant risk factor for all-cause and CVD mortality in CKD patients on maintenance dialysis [6][7][8].However, in non-dialysis CKD patients, especially those with preserved kidney function, there is less research on the correlation between serum phosphorus and mortality, and the conclusions are inconsistent.The Kidney Early Evaluation Program (KEEP) study shows that there is no association between serum phosphorus and all-cause mortality in patients with an estimated glomerular filtration rate (eGFR) < 60 ml/min/1.73m 2 [9].Meanwhile, the European Quality (EQUAL) study reports that serum phosphorus is independently associated with allcause mortality in elderly patients with eGFR < 20 ml/ min/1.73m 2 [10].For patients with an eGFR < 60 ml/ min/1.73m 2 , the target for managing serum phosphorus remains inconsistent.The KDIGO guidelines suggest lowering elevated phosphorus levels toward the normal range in patients with CKD G3a-G5D (Grade 2C, 2017) [11,12].However, most previous studies excluded CKD patients with an eGFR ≥ 60 ml/min/1.73m 2 , so the relationship between serum phosphorus and mortality in this group of patients remains uncertain.Currently, there are no management recommendations for these patients in clinical practice [12,13].
Given the inconsistencies in the literature, further research is needed to better describe the impact of serum phosphorus on patients with CKD, especially those with preserved kidney function.This study uses data from the National Health and Nutrition Examination Survey (NHANES) to investigate the relationship between serum phosphorus and all-cause or CVD mortality in non-dialysis CKD patients.

Study population
NHANES is a complex, multistage sampling design, nationally representative study to assess the health and nutritional status of non-institutionalized civilians in the United States [14].Since 1999, it has been a continuous project, with each cycle lasting two years [14].NHANES was conducted by the Centers for Disease Control and Prevention (CDC) and and approved by the institutional review board of the National Center of Health Statistics.All participants provided written informed consent.

Measurement of serum phosphorus
The serum phosphorus was measured using a Hitachi model 737 multichannel analyzer (Boehringer Mannheim Diagnostics, Indianapolis, IN), by observing changes in absorbance at 365 nm following phosphomolybdate formation from inorganic phosphorus in an acidic solution, which correlate directly with phosphorus levels [15].

Outcomes
Mortality from any cause and CVD was ascertained by linkage to the National Death Index through 31 December 2019.The ICD-10 was used to determine diseasespecific death.CVD mortality was defined as the primary cause of death being any disease of the circulatory system (ICD-10 codes I00-I09, I11, I13, I20-I51, or I60-I69).

Statistical analyses
Sample weights, clustering, and stratification were incorporated into all analyses because of the complex sampling design of the NHANES [14].Participants were followed up to death or the date of 31 December 2019, whichever comes first.We take 2/9 of the 4-year weight for each individual sampled from 2001 to 2002, and 1/9 of the 2-year weight for each individual sampled from 2003 to 2018.
Serum phosphorus was analyzed as a continuous variable, or categorized into three groups: < 3.5 mg/dL, 3.5 to < 4.5 mg/dL, and ≥ 4.5 mg/dL.Baseline characteristics were summarized as means (standard errors) for continuous variables and numbers (percentages) for categorical variables.Time-to-event data were described using Kaplan-Meier curves, and the between-group difference was compared using log-rank test.The Cox proportional hazards model was used to analyze the association between serum phosphorus and all-cause or CVD mortality, with or without adjustment for confounders, including age, sex, race, hemoglobin, eGFR, serum albumin, serum calcium, 25 (OH) vitamin D, obesity, hypertension, diabetes and CVD.Additionally, a restricted cubic spline Cox regression, with the smallest Akaike information criterion (AIC) of three knots, was performed to test for linearity and explore the shape of the dose-response relation of serum phosphorus concentrations and mortality.
We further performed subgroup analysis stratified by baseline characteristics including eGFR (< 60, ≥ 60 ml/ min/1.73m 2 ), sex, age (< 65, ≥ 65 years), presence/ absence of CVD and obesity.We examined possible effect modification by introducing multiplicative interaction terms between phosphorus and the grouping factor into our primary Cox regression model.
Additional sensitivity analyses were conducted to verify the robustness of the results.Firstly, considering the interaction between vitamin D, blood calcium, parathyroid hormone (PTH), and serum phosphorus.We further adjusted for PTH (only provided in NHANES [2003][2004][2005][2006].Secondly, to reduce potential reverse causality bias, we included participants with a follow-up of more than 2 years.Thirdly, additional adjustments were conducted for co-medications (RAS inhibitors, other anti-hypertension drugs, lipid-lowering drugs, and hypoglycemic) and survey years.Finally, We made multiple imputations for missing values to avoid selection bias.
All analyses were carried out with R version 4.2.0, and a two-tailed P < 0.05 was considered statistically significant.
In the subgroup analysis, the association between serum phosphorus and all-cause mortality remained consistent across subgroups based on age (< 65, ≥ 65 years), sex, and obesity status.This study focused on the effects Fig. 1 The survival probability free from all-cause (Left) and CVD mortality (Right).Abbreviation: CVD, Cardiovascular disease of eGFR (< 60, ≥ 60 ml/min/1.73m2) on serum phosphorus and mortality.In participants with eGFR < 60 ml/ min/1.73m 2 , elevated phosphorus (≥ 4.5 mg/ dL) were significantly associated with increased risk of all-cause mortality (HR, 1.36; 95%CI, 1.07-1.72).Data from subgroup of eGFR ≥ 60 ml/min/1.73m 2 did not show a significant association between elevated serum phosphorus and death (HR, 1.31; 95%CI, 0.86-1.99).However, we did not find that eGFR modifies the relationship between serum phosphorus levels and risk of all-cause death (P for interaction = 0.889).The detrimental association of hyperphosphatemia with all-cause mortality was more pronounced in the subgroup with coexisting CVD (p for interaction = 0.009) (Fig. 3).

Sensitivity analyses
We conducted additional sensitivity analyses to validate the robustness of our results.Firstly, we further adjusted for PTH.Due to the limited number of individuals with PTH measurements (available only in NHANES 2003-2006, n = 1,601), the statistical power was insufficient, the between phosphorus and all-cause mortality did not show significant differences (HR, 1.26; 95CI, 0.90-1.76).Secondly, compared with the < 3.5 mg/dL group, phosphorus ≥ 4.5 mg/dL group was associated with a 28% increased risk of all-cause mortality (HR, 1.28; 95%CI, 1.05-1.57)when we included participants with more than 2 years of follow up (n = 6,826) (Table 4).Our results remain stable when further adjusted for comedications (RAS inhibitors, other anti-hypertension drugs, lipid-lowering drugs, and hypoglycemic) and survey years (Supplemental Table 2).Finally, we made multiple imputations for missing values to avoid selection bias in 8363 participants.Our result still shown that elevated serum phosphorus levels (≥ 4.5 mg/dL) was associated with a 17% increased risk of all-cause mortality (HR, 1.17; 95%CI, 1.02-1.34)(Supplemental Table 3).

Discussion
Our study found that elevated serum phosphorus are significantly associated with all-cause and CVD mortality in non-dialysis CKD patients.This association is consistent across subgroups stratified by eGFR (< 60, ≥ 60 ml/min/1.73m 2 ), sex, obesity (no, yes), and age (< 65, ≥ 65 years).Moreover, the harmful association of elevated phosphorus with all-cause mortality is more pronounced in the population with combined CVD.Currently, the results of studies on the relationship between serum phosphorus levels and the risk of mortality are inconsistent.The KEEP study included 10,672 non-dialysis CKD patients with eGFR < 60 ml/ min/1.73m 2 .The results showed that, compared with the phosphorus < 3.3 mg/dLgroup, there was no association between serum phosphorus (levels of 3.3-3.7,3.7-4.1,and ≥ 4.1 mg/dL) and all-cause mortality [9].It is worth noting that although the KEEP study is large in scale, its follow up period is only 2.3 years, which may not be sufficient to capture enough endpoint events (578 patients died).In contrast, our study, which is based on a substantial number of participants and has a follow up period of 92 months, demonstrates that for every 1 mg/dL increase in serum phosphorus, the risk of all-cause mortality increases by 13%, and the risk of CVD death increases by 24%.Compared with serum phosphorus < 3.5 mg/dL, when serum phosphorus exceeds 4.5 mg/dL, it is associated with a 28% increase in the risk of all-cause mortality and a 57% increase in the risk of CVD death.Similarly, a meta-analysis of 12 cohort studies involving 25,546 non-dialysis-dependent CKD patients with 3,089 deaths (13.6%) showed that for every 1 mg/dL increase in serum phosphorus, the risk of all-cause mortality increased by 20% [18].The EQUAL study included elderly non-dialysis CKD patients with eGFR less than 20 ml/min/1.73m 2 from 6 centers, and the results also showed that serum phosphorus is independently associated with all-cause mortality [10].These studies all indicate that serum phosphorus is associated with all-cause and CVD mortality in non-dialysis CKD patients.
Research shows that changes in serum phosphorus occur in the early stages of CKD and progress with the decline of eGFR [19].However, the impact of serum phosphorus on non-renal impairment CKD patients (eGFR ≥ 60 ml/min/1.73m 2 ) is still unknown [12].Our reserch shown that elevated serum phosphorus levels (≥ 4.5 mg/dL) were significantly associated with a 36% increase in the risk of all-cause mortality in subgroup with eGFR < 60 ml/min/1.73m 2 .Although the Framingham Offspring study suggests that higher serum phosphorus levels are associated with an increased risk of CVD in individuals with eGFR ≥ 60 mL/min/1.73m 2 with or without proteinuria [20], our data are not conclusive.However, we also have no evidence that this association differs significantly between subgroups defined by eGFR levels (< 60, ≥ 60 ml/min/1.73m 2 ), suggesting that changes in serum phosphorus may require attention in the early CKD stages.In patients with early CKD, this relationship needs to be validated by subsequent studies involving larger populations.
Furthermore, elevated phosphorus levels are associated with a greater risk of all-cause mortality in individuals with CVD.Compared with serum phosphorus < 3.5 mg/dL, serum phosphorus levels of 3.5 to < 4.5 and ≥ 4.5 mg/dL are associated with a 19% and 61% increase in all-cause mortality risk, respectively.Therefore, more attention should be paid to phosphorus levels in CKD patients with CVD [21].This is a nationally representative sample survey study, and weighted statistics were used, making the conclusions reliable and highly generalizable.In addition, unlike previous studies that mainly focused on dialysis or advanced CKD patients, our analysis covers all stages of CKD, especially in the population with eGFR ≥ 60 ml/min/1.73m 2 , which is rarely included in previous studies.We still found that hyperphosphatemia may increase patient mortality risk.Considering the mutual influence of elevated serum phosphorus on serum calcium, vitamin D, and PTH metabolism [12], we conducted additional analyses.After adjusting for serum calcium, vitamin D, and PTH as confounding factors, the results still showed that elevated serum phosphorus is independently associated with an increased risk of all-cause mortality in CKD patients, further supporting our conclusions.This study also has limitations.Firstly, the NHANES database mainly collects data from the US population.It should be cautious when generalizing the finding to other ethnics.Secondly, our results rely on a single serum phosphorus measurement, rather than repeated measurements.However, previous research has shown that repeated measurements of serum phosphorus may improve exposure estimation and thus strengthen the association with mortality rate [22].Thirdly, despite our extensive adjustment for confounding factors, there may still be significant unmeasured confounders.Finally, recognizing the critical role of serum phosphorus in the mortality risk of CKD patients, therapies to reduce serum phosphorus are expected to reduce the mortality risk of these patients.However, our study design cannot further clarify this issue.
In conclusion, the results of this large-sample cohort study indicate that serum phosphorus above 4.5 mg/ dL is associated with an increased risk of all-cause and CVD mortality in non-dialysis CKD patients.This relationship still demonstrated in patients with eGFR < 60 ml/min/1.73m 2 .However, for population with eGFR ≥ 60 ml/min/1.73m 2 , further verification is needed.

Table 1
Baseline characteristics of patients wtih non-dialysis CKD stratified by serum phosphorus levels Abbreviation: CKD chronic kidney disease, CVD cardiovascular disease a All estimates accounted for complex survey designs.Continuous variables were expressed as mean (standard error).Categorical variables were expressed as number (percent) b Only provided in NHANES 2003-2006 (n = 1601) c Obesity is defined as body mass index ≥ 30 kg/m 2

Table 2
The association of serum phosphorus as a continuous variable with all-cause and CVD mortality Abbreviation: CVD Cardiovascular disease, HR hazard ratio, CI confidence interval a Adjusted for age, sex, race, eGFR, hemoglobin, serum albumin, serum calcium, 25(OH)D, obesity, hypertension, diabetes, and CVD

Table 3
The association of serum phosphorus levels with allcause and CVD mortality Abbreviation: CVD Cardiovascular disease, HR hazard ratio, CI confidence interval a Adjusted for age, sex, race, eGFR, hemoglobin, serum albumin, serum calcium, 25(OH)D, obesity, hypertension, diabetes, and CVD

Table 4
Sensitivity analyses for the association of serum phosphorus levels with all-cause mortality Abbreviation: CVD Cardiovascular disease, HR hazard ratio, CI confidence interval, PTH parathyroid hormone a Adjusted for PTH, age, sex, race, eGFR, hemoglobin, serum albumin, serum calcium, 25(OH)D, obesity, hypertension, diabetes, and CVD b Adjusted for age, sex, race, eGFR, hemoglobin, serum albumin, serum calcium, 25(OH)D, obesity, hypertension, diabetes, and CVD