Background
Acute kidney injury (AKI) is a common problem in seriously ill patients and is associated with several poor clinical outcomes [
1‐
6]. Numerous studies have proven that chronic kidney disease-mineral and bone disorder (CKD-MBD) is associated with adverse clinical outcomes [
7‐
9]. In contrast, little is known about mineral and bone disorder and its association with outcomes among patients with AKI.
Reduced vitamin D levels, elevated parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) levels and decreased or increased calcium and phosphate levels have been reported in patients with AKI in small studies [
10‐
14], but their mechanisms are defined insufficiently. In the case of chronic kidney disease (CKD), the FGF23 level rises early in the course of the disease, stimulates urinary phosphate excretion and inhibits the activation of 25-hydroxyvitamin D (25D), and contributes to the development of secondary hyperparathyroidism [
15].
In recent studies, the role of markers of bone metabolism in the prediction of mortality in patients with CKD has been prospectively evaluated. These studies have indicated a link between bone metabolism and cardiovascular events in patients with CKD. Markers of bone formation (bone-specific alkaline phosphatase [BAP]) and bone resorption (tartrate-resistant acid phosphatase 5b [TRACP-5b]) can serve as predictors of cardiovascular morbidity and mortality in patients with CKD [
16].
Based on these observations, we hypothesized that the serum markers of mineral and bone metabolism can be used to predict the mortality of patients with AKI. To systematically evaluate the predictive value of the markers of mineral and bone metabolism for poor outcomes in patients with AKI, we designed a prospective cohort study to evaluate the predictive value of the markers of mineral and bone disorder for the mortality of patients who underwent cardiac surgery and developed AKI.
Methods
Study design
We conducted a single-center, prospective cohort study and enrolled patients who underwent cardiac surgery and developed incident AKI at a comprehensive hospital between June 2014 and January 2016. This study was approved by the hospital ethics committee of Shanghai Ninth People’s Hospital, School of Medicine, Shanghai Jiaotong University (approval number: [2014]45). All of the experimental protocols were implemented in accordance with the relevant guidelines and regulations. In accordance with the Declaration of Helsinki, patients or their legal guardians had obtained written informed consent prior to participation,. This study was registered at
www.clinicaltrials.gov with the identification number NCT00953992.
Study patients
The inclusion criteria were an age > 18 years and the development of incident AKI following cardiac surgery. AKI was defined as an increase in serum creatinine of ≥0.3 mg/dL within 48 h or that of ≥50% within 7 days, which conforme with the criteria established by the Kidney Disease Improving Global Outcomes (KDIGO) work group [
17].
The exclusion criteria were: (1) preoperative AKI (defined as a 0.3 mg/dL rise in the serum creatinine level over 24 h or a 0.5 mg/dL rise over 48 h) [
18], (2) pre-existing CKD (defined as an estimated glomerular filtration rate (eGFR) of < 30 mL/min per 1.73 m
2) or ESRD requiring dialysis, (3) post-renal obstruction or rapid progressive glomerulonephritis as the main cause of AKI, (4) renal transplantation, (5) malignancy, and (6) pregnancy.
Study procedures
The serum creatinine data of the participants were monitored daily in the hospital’s information system (HIS) to rapidly identify patients with incident AKI. We collected and stored the serum samples within 12 h after establishing the clinical diagnosis. The samples were centrifuged, aliquoted, and stored at − 80 °C within 2 h of collection.
Clinical outcomes
We adjudicated all of the outcomes by reviewing the electronic medical records from the HIS. The pre-specified primary endpoint was the 28-day all-cause mortality.
The secondary endpoints were Renal Replacement Therapy or in-hospital mortality (RRT/death). Additional endpoints included the ventilator-free days and hospital-free days in 28 days. The patients who died before 28 days were graded a score of zero.
Laboratory assessment
The bone-specific alkaline phosphatase and tartrate-resistant acid phosphatase 5b levels were measured using a standard ELISA kit (Immunodiagnostic Systems Limited, UK). The C-terminal FGF-23 levels were measured using a standard ELISA kit (Immutopics Inc., USA). The serum calcium and phosphate levels were measured on an automated chemistry analyzer (AU5800, Beckman Coulter, USA). The iPTH level was measured on an automated immunoassay analyzer (UniCel DxI800, Beckman Coulter, USA). The 25-hydroxyvitamin D level was measured on an automated immunoassay analyzer (LIAISON, DiaSorin, Italy).
Statistical analyses
The data are reported as the median and interquartile range (IQR: 25th percentile–75th percentile) for continuous variables and as the n (%) for categorical variables. The baseline and operative characteristics were compared in patients with different stages of AKI using the Kruskal-Wallis test for continuous variables with a non-normal distribution and using the chi-square test for category variables. The correlation between the enrollment mineral metabolite levels and the other variables was analyzed using the Spearman rank correlation coefficient. The comparison of the mineral metabolite levels among the patients who died and those of the patients who did not die was assessed using the Mann-Whitney U test.
Univariate Cox regression was used to identify potential confounding variables, including demographics (sex and age), preoperative renal function (serum creatintine and eGFR), comorbidities (hypertension, chronic heart failure and diabetes mellitus), operative characteristics (operation type, whether cardiopulmonary bypass or not), postoperative characteristics (low blood pressure, applying diuretics), Acute Physiology and Chronic Health Evaluation (APACHE) II score and biochemical parameters. After that, we used multivariate Cox regression to adjust for covariates using two different models: model 1 was adjusted for age, sex, baseline eGFR, hypertension, chronic heart failure (New York Heart Association functional class [NYHA] ≥ 2) and diabetes mellitus; model 2 was further adjusted for the operation type and APACHE II score on enrollment, according to P value in the univariable regression (P < 0.1) or clinical importance. Logistic and linear regression models were used to assess the association between the mineral metabolite levels and secondary endpoints, adjusting for the same covariates as above. Receiver-operating characteristics (ROC) curves and areas under the curves (AUCs) were calculated using the survival and time ROC packages to compare the predictability of the mineral and bone metabolite levels for the 28-day mortality, Furthermore, the C-index was calculated using the survival package. The evaluation criterion of the C-index was similar to that of the AUC.
We used linear mixed models for the repeated measures to test for significant differences in the mineral and bone metabolite levels over time between the patients with different severities of AKI. The comparison of the mineral and bone metabolite levels at individual time points was assessed using the Spearman rank correlation coefficient (for the correlation between the severity of AKI and the mineral and bone metabolite levels).
All of the comparisons were two-tailed, with
P < 0.05 considered as being statistically significant. The statistical analysis was performed using the software package R version 3.4.0 (
www.r-project.org).
Discussion
In this prospective cohort study, we confirmed that higher serum cFGF23 levels obtained within 12 h of a clinical diagnosis for AKI after cardiac surgery are independently associated with a greater 28-day mortality risk, as well as with several other adverse outcomes, including in-hospital mortality and the need for RRT, and fewer ventilator-free and hospital-free days. Additionally, we also observed that other mineral and bone metabolites, such as phosphate, iPTH, and BAP, are marginally associated with poor outcomes. Moreover, we found that the serum cFGF23 levels rose most significantly and were associated with the severity of AKI. These findings suggest that there is a complex interplay between AKI, mineral and bone metabolism, and adverse clinical outcomes after cardiac surgery.
Multiple studies have demonstrated that mineral and bone disorders are prevalent and associated with mortality in patients with CKD and ESRD [
7‐
9]. However, studies on minerals, particularly those on bone metabolite levels in AKI, are limited. In contrast to our prior study, which investigated the levels of serum vitamin D (including 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D) in patients with hospital-acquired AKI only [
14], here, we evaluated both mineral and bone metabolite levels in patients with AKI after cardiac surgery. Thus, the findings from this study not only demonstrate an independent and prospective association between the mineral metabolite levels and poor outcomes of AKI but also represent the comprehensive study of bone metabolites in clinical setting, which is a rare finding in similar AKI cohort studies.
BAP and TRACP-5b are sensitive biomarkers of osteoblastic bone formation and osteoclastic bone resorption [
8,
19,
20]. The few prior studies that measured the serum BAP and TRACP-5b levels were conducted almost exclusively among patients with CKD or ESRD. Astrid et al. measured the levels of bone markers in serum samples from 627 patients with CKD and found that the markers of bone formation (BAP: OR: 1.01, 95% CI: 1.01–1.02) and bone resorption (TRACP-5b: OR: 0.86, 95% CI: 0.75–0.99) can serve as predictors of cardiovascular morbidity and mortality in patients with CKD [
16]. Further contrasting with the results of prior studies, our data indicated that a high BAP serum level, which implies that there is high bone formative activity, is potentially associated with an increased risk of death in patients with AKI.
In addition to the 28-day mortality, we also evaluated the association between the mineral and bone metabolite levels and several secondary endpoints. The serum cFGF23 levels were markedly and independently associated with RRT/Death and longer durations of mechanical ventilation and hospital length of stay. Coincidentally, higher serum BAP levels were also associated with longer durations of mechanical ventilation and hospital length of stay.
Whether mineral and bone metabolite levels are merely an illness severity marker or directly contribute to poor outcomes is a critical question that could not be answered by this study. Higher FGF23 levels are related to pathogenesis of left ventricular hypertrophy [
21‐
23] and inflammation [
24‐
26] in CKD. In fact, a large number of epidemiologic data suggest that decreased vitamin D metabolite levels are associated with adverse outcomes in those with critical illness [
27,
28]. Maybe increased FGF23 levels could lead to adverse outcomes via inhibiting the activation of vitamin D metabolite [
15,
26]. However, we did not find positive or inverse correlations between the serum cFGF23 and 25D levels, and we also did not find a correlation between the 25D level and the 28-mortality risk, which is consistent with the results of our previous study. Thus, the associations between the FGF23 and vitamin D metabolite levels remain unclear.
We acknowledge several limitations of this study. All samples used in our study were from one single centre. Therefore, selection bias in the enrollment was inevitable. 18 patients had died within 28 days, due to the limitation of the sample size in this cohort study, no further subgroup analysis has been done for the cause of death, and so it was difficult to determine the link between mineral and bone disorder and the cause of death. Additionally, urine output was an important criterion, which was not accessed in this study. Thus, the incidence of AKI may be underestimated and the estimation of AKI stages may be affected. Of course, this result was only uncovered in a small sample size and a larger clinical sample is needed to confirm this result.
Acknowledgements
We appreciate the support provided by Shi Peng, MS (Department of Medical Statistics, Children’s Hospital, Center for Evidence-based Medicine, Fudan University, Shanghai), who provided expert assistance with the statistical analysis.
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