The relationship between body weight,cystatin C and serum creatinine in neonates

1 Introduction

Neonatal Acute Kidney Injury (AKI) is characterized by a decrease in glomerular filtration rate (GFR) and derangements in fluid balance and electrolytes [1]. AKI is very common and is associated with poor outcome [2]. Neonates who survive an episode of AKI are at an increased risk of developing chronic kidney disease (CKD) [2]. Perinatal risk factors that predispose neonates to AKI include perinatal hypoxia, sepsis and nephrotoxic medication. As nephron number is determined before birth, prematurity in combination with perinatal insult and nephrotoxic medication will set a neonate on the trajectory for CKD. The modified Kidney Diseases: Improving Global Outcomes (Neonatal Acute Kidney injury (AKI) KDIGO), four stages classification system, is dependent on serum creatinine (SCr) measurement [1]. However, there are limitations to the value of SCr as a measure of renal function in infants.

The glomerular filtration rate (GFR) is the best measure of renal function [3]. An ideal marker for estimating GFR would be produced endogenously in the plasma at a constant rate, freely filtered through the glomerulus; neither reabsorbed nor secreted by the renal tubule, and undergoes no extra-renal metabolism. In clinical practice, serum SCr measurement is used to determine GFR in adults, children and neonates [3]. However, the measurement of SCr, especially in neonates is associated with many pitfalls. There is a significant delay in the rise of serum SCr after renal insult (48–72 hours) [2], and it only starts to rise once the GFR falls by 50% [3]. In newborns, the SCr is high and is influenced by maternal SCr level [4]. In premature infants, the plasma SCr level increases in the first 48 hours followed by a decline [5].

Unlike SCr, Cystatin C (CysC) has the theoretical properties of an ideal biomarker of renal function. CysC is a cysteine proteinase inhibitor and it is produced at a constant rate by a housekeeping gene expressed in all nucleated cells [6]. It is freely filtered through the glomerulus with no excretion from the renal tubules. Furthermore, CysC is completely catabolized by the renal tubules. Hence its plasma level is only dependent on the GFR [6]. Meta-analyses have shown that CysC is superior to SCr for the estimation of the GFR in children [7]. Gender and gestational age have no effect on CysC levels. One study in adults showed that SCr correlated significantly with body weight and lean muscle mass [8]. As no such data is available in neonates, we carried out a study with an aim to determine if infant weight influences SCr or CysC levels. We hypothesized that SCr is correlated significantly with body weight in neonates too.

2 Method

This prospective study was performed at the Department of Neonatology, Townsville Hospital, Australia, which is a tertiary perinatal centre responsible for a region with more than 10,000 births annually. Approximately 2500 of these births occur at the hospital itself. Recruitment of babies for this study was limited to babies admitted to the neonatal intensive care unit. The study commenced in August 2014, and data on patients recruited until May 2016 are presented. The Townsville Health District Human Research Ethics Committee approved this study. Written parental consent was obtained from parents of all infants who participated in this study. Both term [>37 completed weeks of gestation] and preterm babies at term [>37 completed weeks postmenstrual age (PMA)] were recruited for this study. We recruited term and ex-preterm babies at term PMA. This was to ensure we recruited infants of similar maturity but of various different weights. A peripheral venous sample (0.5 mL) was collected for creatinine and CysC measurements simultaneously. To avoid any maternal effect on serum creatinine level, blood specimen in term infants were only collected after 72 hours of life. Term infants were recruited from neonates admitted to the Department of Neonatology for minor conditions (neonatal jaundice, transient tachypnea of newborn and monitoring of blood glucose. Ill term neonates needing respiratory support and neonates with congenital renal abnormalities were excluded.

The normality of the variables was determined by the D’Agostino-Pearson test [9]. The results were expressed as the means±standard deviation (SD) for continuous, normally distributed data and as median [Interquartile range [IQR]] for continuous, non-normally distributed data. Comparisons of means of normally distributed data were made using t-tests, and Mann-Whitney tests were used for non-normally distributed data. P value <0.05 was considered statistically significant. Statistical analyses were performed using MedCalc Version 11.6 [MedCalc Software, Mariakerke, Belgium].

SCr concentrations were measured using a kinetic modification of the Jaffe procedure on the Beckman DXCi biochemistry analyzer (Beckman Coulter, Australia) using a commercial calibrator which is traceable to an isotope dilution mass spectrometry (IDMS) reference method using the National Institutes of Standards and Technology (NIST) Standard Reference Material 967.

CysC concentrations were measured in serum samples using an immunoturbidimetric assay designed for use on the Beckman AU480 automated autoanalyser platform (Gentian, Australia). The Gentian Cystatin C calibrator is standardized against the international standard ERM-DA471/IFCC [10]. The coefficient of variation for this measurement was 6%.

3 Results

Eighty six infants were recruited into this study out of which complete data were available in 80 infants. Six infants were excluded as blood specimens collected from them were insufficient to measure both SCr and Cys C. The cohort consists of both term and premature infants at term PMA (31 terms and 49 preterms). Table 1 shows the comparison in demography between preterm and term infants. The median gestation at birth for the ex-premature infants was 26.4 [24.7–27.4] weeks but this analysis was carried out at term PMA (>37 weeks). The postnatal age of infants born term was between 3 to 10 days. None of the infants was diagnosed with have AKI. The median weight for the whole cohort was 2478 [2118–3080] g. Both SCr and Cys C measurements were carried out simultaneously using the same peripheral venous sample. The median SCr level was 17 [12–26] umol/L and mean CysC level was 1.64 [0.27] mg/L. SCr had a significant correlation with weight (r = 0.3; P = 0.011) (Fig. 1), whereas serum CysC had no correlation with the infant’s weight (r = 0.01; P = 0.95) (Fig. 2). There were no difference in CysC and SCr levels between male and female infants.

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Fig.1

Relationship between infant weight and serum SCr.

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Fig.2

Relationship between infant weight and serum CysC.

Despite many advances in diagnosis and treatment of neonatal and childhood diseases, the diagnostic assessment of renal function remains challenging. Diagnostic indices are often confounded by maternal influences, diet, treatment and birth weight. The “gold” standard for measuring renal function in neonates has been an Inulin Clearance Test. These tests are cumbersome and invasive. Hence, it is avoided in most routine diagnostic situations [11]. SCr has been used as a marker of GFR since Popper and Mandel’s first proposal in 1937 [12]. Often improved sensitivity and specificity are achieved through the use of estimated GFR (eGFR) calculations [13]. eGFR is derived using SCr and/or Cys C based equations (Bökenkamp, Filler, Grubb, Zapitelli-Cys and, Zapitelli-CysCr equations) [13]. Sharma et al., compared the accuracy of CysC estimated GFR by various CysC equations and found that Zappitelli-Cys equation to have greater sensitivity in estimating eGFR in the neonatal range [14]. Limitations of the use of creatinine as a marker of GFR are well known and include creatinine’s association with muscle mass and therefore low values in children and the low specificity of the alkaline picrate method caused by non-creatinine chromogens [15]. These problems are often exacerbated during the early days of life for neonates and pre-term infants due to low creatinine excretion and tubular reabsorption by immature renal tubules, which may be seen up to 3 weeks of age [4].

The data presented here demonstrate there is a significant correlation between SCr levels and infant weight. Relying on an increase in SCr to diagnose AKI in neonates will likely result in the diagnosis being missed in infants with lower body weight leading to potentially serious delays in treatment. The data presented here show that there is no significant influence on Cys C measurement from body weight. Therefore in infants, Cys C is a better marker for determining renal health and predicting AKI as it is not influenced by body weight.

Our study is limited by the fact that while we examined the effects of weight on CysC and SCr, we did not take into account body composition. Body composition could have an effect on CysC concentrations in adults [16]. Preterm and term infants are likely to have significant difference in body composition despite similar weights. Other limitations of study are that we excluded ill neonates and we did not examine the effects of prematurity.

5 Conclusion

Our data demonstrate SCr level in term neonates is dependent on birth weight. SCr based classification of AKI may cause a delay in diagnosis of AKI in smaller infants. Clinicians managing sick neonates need to consider using an alternate renal function marker such as CysC as the evidence indicate that early recognition of AKI leads to improved outcome.

Disclosure statements

There are no competing interests to declare.

Human research statement

The Townsville Health District Human Research Ethics Committee approved this study.

Financial disclosure statement

Project grant from The National Health and Medical Research Council, Australia.

Conflicts of interest

The authors have no conflicts of interest to declare.