A new serum cystatin C formula for estimating glomerular filtration rate in newborns

Glomerular Filtration Rate Assessment in Children

Glomerular filtration rate (GFR) measurement is a key tool for determining the degree of chronic kidney disease. The assessment of GFR is even more challenging in children than in adults with more variables in the equation than race and sex. Monitoring the progress of the kidney disease can therefore be difficult as in the initial stages of a decline in kidney function, there are no clinical signs. Due to children's growth and development, changes in muscle mass and growth impair GFR estimation based solely on serum creatinine values. More invasive methods of GFR measurement are more reliable, but techniques using ionising agents, requiring large volume blood samples or timed voiding, have limited application in children. This paper reviews the methods of measuring and determining glomerular filtration rate and kidney function in children.

Gestational and Age-Specific Cystatin C Reference Intervals for Newborns

OBJECTIVE

Cystatin C (CysC) references are useful for the monitoring of renal function in neonates. However, the standard CysC references in newborn infants have not been determined. The aim of this study was to establish a useful reference range for CysC in newborns.

STUDY DESIGN

Serum CysC levels were measured in 1,919 blood samples from 1,044 newborns during their first 28 days of life. CysC levels were analyzed for associations between subgroups dichotomized by postnatal age (PA) and gestational age (GA). The serum CysC reference intervals were determined according to the PA and GA. The associations between the serum CysC level and other biochemical parameters as well as perinatal factors were also analyzed.

RESULTS

In this study, the mean GA was 35.8 ± 2.9 weeks and the birth weight (BW) was 2,614 ± 697 g. Reference ranges of serum CysC were determined, and a general decreasing trend of CysC levels was observed as the GA increased. CysC levels differed significantly among the PA and GA categories (p < 0.001). Serum CysC levels were relatively stable throughout the GA range but were impacted by the white blood cell count within the first postnatal 24 hours. Moreover, CysC levels always correlated positively with serum creatinine concentrations (p < 0.001). Serum Cr levels were influenced by multiple factors, including BW, GA, total bilirubin, direct bilirubin, white blood cell count, C-reactive protein, and blood urea nitrogen.

CONCLUSION

Reference levels of serum CysC should be determined according to the PA and GA. In contrast to Cr, serum CysC is a reliable index for assessing renal function in neonates as it is influenced by very few factors. The CysC reference levels will allow neonatologists to accurately evaluate renal function in the neonatal population.

KEY POINTS

· Cystatin C is a useful marker of the glomerular filtration rate in neonates.. · A reference range for cystatin C using 1,919 blood samples of 1,044 newborns was determined.. · In contrast to creatinine, only a few nonrenal factors influence serum cystatin C..

Markers of Kidney Function in Early Childhood and Association With Maternal Comorbidity

IMPORTANCE

Kidney functional capacity is low at birth but doubles during the first 2 weeks of life and reaches near-adult levels at age 1 to 2 years. Existing reference intervals for markers of kidney function in newborns are mostly based on preterm newborns, newborns with illness, or small cohorts of term newborns, and the consequences of maternal comorbidities for newborn kidney function are sparsely described.

OBJECTIVE

To establish robust reference intervals for creatinine and urea in healthy children in early childhood and to assess whether maternal comorbidity is associated with newborn creatinine and urea concentrations.

DESIGN, SETTING, AND PARTICIPANTS

This multicenter, prospective, population-based cohort study assessed data and umbilical cord blood samples from participants in the Copenhagen Baby Heart Study (CBHS) who were born between April 1, 2016, and October 31, 2018, and venous blood samples from a subsample of CBHS participants who were enrolled in the COMPARE study between May 3, 2017, and November 4, 2018. Cord blood samples of 13 354 newborns from the CBHS and corresponding venous blood samples of 444 of those newborns from the COMPARE study were included. Blood samples were collected at birth, age 2 months, and age 14 to 16 months, with follow-up completed on February 12, 2020. Healthy nonadmitted term newborns from maternity wards at 3 hospitals in the Capital Region of Denmark were included.

EXPOSURES

Maternal comorbidity.

MAIN OUTCOMES AND MEASURES

Creatinine and urea concentrations.

RESULTS

Among 13 354 newborns in the CBHS cohort, characteristics of 12 938 children were stratified by sex and gestational age (GA). Of those, 6567 children (50.8%) were male; 5259 children (40.6%) were born at 37 to 39 weeks' GA, and 7679 children (59.4%) were born at 40 to 42 weeks' GA. Compared with children born at 40 to 42 weeks' GA, those born at 37 to 39 weeks' GA had lower birth weight, Apgar scores at 5 minutes, placental weight, and placental-fetal weight ratio. Children born at 37 to 39 weeks' GA vs those born at 40 to 42 weeks' GA were more frequently small for GA at birth and more likely to have placental insufficiency and exposure to maternal preeclampsia, maternal diabetes, maternal kidney disease, and maternal hypertension. Among children born at 37 to 39 weeks' GA, reference intervals were 0.54 to 1.08 mg/dL for creatinine and 5.32 to 14.67 mg/dL for urea; among children born at 40 to 42 weeks' GA, reference intervals were 0.57 to 1.19 mg/dL for creatinine and 5.60 to 14.85 mg/dL for urea. At birth, multifactorially adjusted odds ratios among children exposed to preeclampsia were 9.40 (95% CI, 1.68-52.54) for a venous creatinine concentration higher than the upper reference limit, 4.29 (95% CI, 1.32-13.93) for a venous creatinine concentration higher than the 90th percentile, and 3.10 (95% CI, 1.14-8.46) for a venous creatinine concentration higher than the 80th percentile.

CONCLUSIONS AND RELEVANCE

In this study, improved reference intervals for creatinine and urea concentrations were generated. Preeclampsia was associated with an increased risk of high newborn creatinine concentrations, suggesting that newborns of mothers with preeclampsia need closer observation of their kidney function.

Filtering Down to Risks and Solutions: Risk Factors and Stratification After Pediatric Cardiac Surgery

Acute kidney injury after cardiac surgery (CS-AKI) is common in neonatal and pediatric populations and is a risk factor for poor outcomes, such as mortality and increased hospital resource utilization. This review presents a summary of CS-AKI risk factors, integration of biomarkers, and the need to improve risk stratification for targeting future clinical trials. To date, studies examining CS-AKI risk factors cannot be generalized easily owing to variability in patient age, surgical complexity or population, AKI definition, and center-specific practices. However, certain risk associations, such as younger age at surgery, history of prematurity, cardiopulmonary bypass time, and surgical complexity, have been identified across multiple, but not all, studies. CS-AKI appears to have different severity and duration phenotypes, and serum creatinine is limited in its ability to identify CS-AKI early and predict CS-AKI course. Treatment strategies are largely supportive, and efforts are ongoing to use biomarkers and clinical features to risk-stratify patients, which in turn may facilitate differential CS-AKI phenotyping and management with supportive care bundles, clinical decision support techniques, and modulation of modifiable risk factors.

Advances in Neonatal Acute Kidney Injury

In this state-of-the-art review, we highlight the major advances over the last 5 years in neonatal acute kidney injury (AKI). Large multicenter studies reveal that neonatal AKI is common and independently associated with increased morbidity and mortality. The natural course of neonatal AKI, along with the risk factors, mitigation strategies, and the role of AKI on short- and long-term outcomes, is becoming clearer. Specific progress has been made in identifying potential preventive strategies for AKI, such as the use of caffeine in premature neonates, theophylline in neonates with hypoxic-ischemic encephalopathy, and nephrotoxic medication monitoring programs. New evidence highlights the importance of the kidney in "crosstalk" between other organs and how AKI likely plays a critical role in other organ development and injury, such as intraventricular hemorrhage and lung disease. New technology has resulted in advancement in prevention and improvements in the current management in neonates with severe AKI. With specific continuous renal replacement therapy machines designed for neonates, this therapy is now available and is being used with increasing frequency in NICUs. Moving forward, biomarkers, such as urinary neutrophil gelatinase-associated lipocalin, and other new technologies, such as monitoring of renal tissue oxygenation and nephron counting, will likely play an increased role in identification of AKI and those most vulnerable for chronic kidney disease. Future research needs to be focused on determining the optimal follow-up strategy for neonates with a history of AKI to detect chronic kidney disease.

Maturation of glomerular filtration rate in neonates and infants: an overview

Glomerular filtration rate (GFR) increases progressively throughout fetal life, matures rapidly after birth according to gestational and post-menstrual age, and reaches adult values by 1-year post-natal age. GFR is considered the best marker of kidney function, and in clinical practice, estimated GFR is useful to anticipate complications, establish prognosis, and facilitate treatment decisions. This review article summarizes the maturation of glomerular filtration and the factors and conditions that modulate and impair developing glomerular filtration, and discusses the techniques available to assess GFR in neonates and infants. We focused on simple, reliable, easily available, and cheap techniques to estimate GFR, which may provide valuable information on the renal aspects of the clinical care of this group of patients.

Cystatin C: A more reliable biomarker of renal function in young infants? A longitudinal cohort study

Aim

We carried out a longitudinal cohort study to measure serial CysC (Cystatin C) in a cohort of neonates born preterm until the age of 2 years. We hypothesised that CysC levels are independent of body weight and would not vary with gestational age.

Methods

This prospective cohort study was conducted from August 2014 until October 2016, and follow‐up was completed in October 2018. Preterm infants at less than 28 weeks of gestation (extremely preterm infants) were recruited and followed up until the age of 24 months. Blood samples for measurement of CysC were collected at regular intervals.

Results

We recruited 58 preterm neonates with mean gestation was 26.2 (1.5) weeks, and a mean birth weight was 917 (140) g. One‐way analysis of variance (ANOVA) did not show any significant difference in CysC levels between 28, 32 and 37 weeks' gestation (P = .09) despite a significant increase in body weight (P < .001). The mean CysC level was higher in the neonatal period and subsequently plateaued by 24 months.

Conclusion

Serum CysC level is independent of body weight and not influenced by postnatal age nor by gender.

Cystatin C compared to serum creatinine as a marker of acute kidney injury in critically ill neonates

BACKGROUND

Acute kidney injury (AKI) is one of the most common causes of neonatal morbidity and mortality. Diagnosing AKI in neonates is challenging as it lacks specific signs, symptoms, and biomarkers. However, detecting AKI in critically ill neonates is crucial to determine appropriate management and prevent complications. Cystatin C (CysC) has been recognized as a superior kidney biomarker reflecting kidney function in neonates. The objective of this study is to evaluate the diagnostic value of CysC as an AKI biomarker in critically ill neonates.

METHODS

We performed a diagnostic test between cystatin C-based estimated glomerular filtration rate (eGFR-CysC) and serum creatinine-based estimated glomerular filtration rate (eGFR-SCr) as the gold standard to diagnose AKI in 135 critically ill neonates treated in Cipto Mangunkusumo National Hospital from July 2017 to January 2018.

RESULTS

Prevalence of AKI was 23.7% predominantly in neonates with a very preterm gestational age, low birthweight, probable sepsis, and those receiving invasive oxygen therapy or nephrotoxic drugs. The proportion of AKI based on neonate RIFLE criteria was 72.7% risk, 18.9% injury, and 9% failure. eGFR-CysC had the following parameters: sensitivity, 84.8%; specificity, 61.8%; PPV, 41.8%; NPV, 89.7%; LR(+), 2.2; LR(-), 0.24; and accuracy, 67.4%. The AUROC for CysC was 84.9%. The optimal cut-off value for CysC was 1.605 mg/l.

CONCLUSIONS

CysC may be used as a screening biomarker of AKI in critically ill neonates; yet, it was not superior to serum creatinine. Graphical abstract.

Kidney-Detrimental Factors and Estimated Glomerular Filtration Rate in Preterm Newborns: The Role of Nutrition

: Background: Kidney function in preterm newborns may be impaired by many factors.

METHODS

71 newborns with gestational age (GA) < 32 weeks were enrolled. Serum creatinine (sCr), cystatin C (CysC), beta-trace protein (BTP) and urea were measured at T0 (3^rd day of life) and T36 (GA 36 weeks), and estimated glomerular filtration rate (eGFR) was calculated according to different formulas at T36. Pre-natal and post-natal kidney injury risk scores were calculated.

RESULTS

Newborns with GA ≤ 28 weeks had higher sCr at T0, and lower sCr, BTP and higher urea levels at T36 (p = 0.007, p = 0.005 and p = 0.029, respectively). eGFR values were not different according to GA when calculated by the formulas using only CysC, but were higher in subjects with GA ≤ 28 weeks according to the other formulas. The post-natal score was positively correlated with eGFR according to sCr-based formulas, but the correlations did not persist when adjusted for urea levels and GA.

CONCLUSIONS

CysC-based eGFR values are not influenced by GA. Post-natal score shows a direct correlation with eGFR according to sCr-based formulas, not persisting after adjustment for GA and urea levels, implying the importance of the nutritional status, since more premature subjects receive a more aggressive nutritional regimen, testified by higher urea levels.

Renal Precision Medicine in Neonates and Acute Kidney Injury: How to Convert a Cloud of Creatinine Observations to Support Clinical Decisions

Renal precision medicine in neonates is useful to support decision making on pharmacotherapy, signal detection of adverse (drug) events, and individual prediction of short- and long-term prognosis. To estimate kidney function or glomerular filtration rate (GFR), the most commonly measured and readily accessible biomarker is serum creatinine (S_cr). However, there is extensive variability in S_cr observations and GFR estimates within the neonatal population, because of developmental physiology and superimposed pathology. Furthermore, assay related differences still matter for S_cr, but also exist for Cystatin C. Observations in extreme low birth weight (ELBW) and term asphyxiated neonates will illustrate how renal precision medicine contributes to neonatal precision medicine. When the Kidney Disease Improving Global Outcome (KDIGO) definition of acute kidney injury (AKI) is used, this results in an incidence up to 50% in ELBW neonates, associated with increased mortality and morbidity. However, urine output criteria needed adaptations to broader time intervals or weight trends, while S_cr and its trends do not provide sufficient detail on kidney function between ELBW neonates. Instead, we suggest to use assay-specific centile S_cr values to better describe postnatal trends and have illustrated its relevance by quantifying an adverse drug event (ibuprofen) and by explaining individual amikacin clearance. Term asphyxiated neonates also commonly display AKI. While oliguria is a specific AKI indicator, the majority of term asphyxiated cases are non-oliguric. Asphyxia results in a clinical significant-commonly transient-mean GFR decrease (-50%) with a lower renal drug elimination. But there is still major (unexplained) inter-individual variability in GFR and subsequent renal drug elimination between these asphyxiated neonates. Recently, the Baby-NINJA (nephrotoxic injury negated by just-in-time action) study provided evidence on the concept that a focus on nephrotoxic injury negation has a significant impact on AKI incidence and severity. It is hereby important to realize that follow-up should not be discontinued at discharge, as there are concerns about long-term renal outcome. These illustrations suggest that integration of renal (patho)physiology into neonatal precision medicine are an important tool to improve contemporary neonatal care, not only for the short-term but also with a positive health impact throughout life.

Glomerular Filtration Rate Estimation Formulas for Pediatric and Neonatal Use

Renal function assessment is of the utmost importance in predicting drug clearance and in ensuring safe and effective drug therapy in neonates. The challenges to making this prediction relate not only to the extreme vulnerability and rapid maturation of this pediatric subgroup but also to the choice of renal biomarker, covariates, and glomerular filtration rate (GFR) estimating formula. In order to avoid burdensome administration of exogenous markers and/or urine collection in vulnerable pediatric patients, estimation of GFR utilizing endogenous markers has become a useful tool in clinical practice. Several estimation methods have been developed over recent decades, exploiting various endogenous biomarkers (serum creatinine, cystatin C, blood urea nitrogen) and anthropometric measures (body length/height, weight, muscle mass). This article reviews pediatric GFR estimation methods with a focus on their suitability for use in the neonatal population.

Reference values for serum cystatin C in very low-birthweight infants: From two centres of China

AIM

To determine the level of cystatin C (Cys-C) values in preterm babies for the purpose of becoming a good endogenous marker of renal function.

METHODS

A total of 366 very low-birthweight infants (including 70 extremely low-birthweight babies) with gestational age <37 weeks born in two centres were studied.

RESULTS

In very low-birthweight infants, the mean level of Cys-C was 1.96 ± 0.44 mg/L in blood samples taken on day 1, 1.78 ± 0.49 mg/L on day 7 and 1.71 ± 0.47 mg/L on day 28. In extremely low-birthweight infants, the mean level of Cys-C was 2.00 ± 0.49 mg/L on day 1, 1.63 ± 0.38 mg/L on day 7 and 1.62 ± 0.55 mg/L on day 28, respectively. Compared to serum creatinine and blood urea nitrogen, Cys-C level was independent of birthweight and gestational age.

CONCLUSION

Cys-C is regarded as an alternative for assessing renal function in very low-birthweight infants, but its advantages over serum creatinine and blood urea nitrogen has not been fully proved yet. Hence, larger sample study is still necessary.

Association between cord blood cystatin C levels and early mortality of neonates with congenital abnormalities of the kidney and urinary tract: a single-center, retrospective cohort study

BACKGROUND

Some fetuses with congenital abnormalities of the kidney and urinary tract (CAKUT) have severe renal dysfunction during the prenatal period that can result in oligohydramnios, pulmonary hypoplasia, and death following birth. We hypothesized that cord blood cystatin C (CysC) levels are elevated in neonates who have life-threatening pulmonary hypoplasia and oligohydramnios due to severe renal dysfunction. In this study we compared cord blood CysC levels between a non-survivor group with CAKUT and a survivor group.

METHODS

This was a single-center, retrospective cohort study conducted between January 2007 and December 2015. Eighty-seven neonates who were prenatally diagnosed with CAKUT were included in the study. Cord blood CysC and creatinine levels were compared between the survivor and non-survivor groups at discharge from hospital.

RESULTS

Of the 87 neonates enrolled in the study, 67 survived and 21 died before discharge. Median cord blood CysC levels were higher in the non-survivor group than in the survivor group (4.28 vs. 1.96 mg/L, respectively; p < 0.001). Cord blood creatinine levels were not significantly different between the two groups. In patients with oligohydramnios (n = 28), cord blood CysC levels were significantly higher in the non-survivor group than in the survivor group (4.28 vs. 2.23 mg/L, respectively; p = 0.002).

CONCLUSIONS

In this study population, cord blood CysC levels were significantly higher in the non-survivor group with CAKUT than in the survivor group. These results suggest that cord blood CysC levels may be a good marker of the severity of renal dysfunction at birth.

Renal function in the fetus and neonate - the creatinine enigma

The use of serum creatinine levels to estimate glomerular function in infants is admittedly fraught with inherent inaccuracies which are both physiological and methodological in nature. This characteristic can understandably reduce the neonatal clinician's confidence in the ability of serum creatinine levels to provide useful information relevant to their patients' medical care. The aim of this review is to provide further insight into the peculiarities of serum creatinine trends in both premature and term infants with special focus on the maturational and developmental changes occurring in the kidney during this crucial time-period. Though newer markers of glomerular function are gaining increasing traction in the clinical realm, the most prominent of which is currently cystatin C, creatinine nonetheless remains an important player in the scientific evolution of glomerular filtration rate (GFR) estimation. Not only do its limitations provide a level of distinction for newer markers of GFR, but its advantages persist in refining the precision of newer GFR formulae which incorporate multiple patient characteristics.

Urinary biomarkers for the differentiation of prerenal and intrinsic pediatric acute kidney injury

BACKGROUND

Urinary calprotectin and neutrophil gelatinase-associated lipocalin (NGAL) have recently been identified as promising biomarkers for the differentiation of prerenal and intrinsic acute kidney injury (AKI) in adults. In the study reported here we examined the diagnostic accuracy of calprotectin, NGAL, and kidney injury molecule 1 (KIM-1) in pediatric patients.

METHODS

Urinary calprotectin, NGAL, and KIM-1 concentrations were assessed in a study population of 139 pediatric subjects including 39 patients with intrinsic AKI, 14 with prerenal AKI, and 86 non-AKI subjects.

RESULTS

Median urinary calprotectin and NGAL concentrations were higher in patients with intrinsic AKI than in those with prerenal AKI (calprotectin by 22-fold, NGAL by 9-fold). Receiver operating characteristic (ROC) curve analyses for the differentiation of intrinsic and prerenal AKI resulted in an area under the curve (AUC) of 0.90 [95 % confidence interval (CI) 0.81-0.98] for calprotectin and 0.73 (95 % CI 0.58-0.87) for NGAL. Median urinary KIM-1 concentrations were not significantly different between patients with prerenal AKI and those with intrinsic disease (P = 0.98; AUC 0.50, 95 % CI, 0.35-0.65). The AUC for the fractional excretion of sodium (FE_Na) and proteinuria was 0.78 (95 % CI 0.63-0.92) and 0.77 (CI 0.65-0.90), respectively.

CONCLUSIONS

Urinary calprotectin outperforms NGAL, KIM-1, FE_Na, and proteinuria as a biomarker for the differentiation of prerenal and intrinsic AKI in pediatric patients with a high diagnostic accuracy.

Assessment of kidney function in preterm infants: lifelong implications

This educational review will highlight the historical and contemporary references that establish a basic understanding of measurements of kidney function in the neonate and its relevance for the life of an individual. Importantly, the differential renal function of preterm infants relative to term infants has become paramount with the increased viability of preterm infants and the realization that kidney function is associated with gestational age. Moreover, neonatal kidney function is primarily associated with absolute renal mass and hemodynamic stability. Neonatal kidney function and its early developmental progression predict lifelong cardiovascular and renal disease risks. Validation of estimation equations of kidney function in this population has provided important reference data for other investigations and a clinical basis for prospective and longitudinal follow-up. Future research should be directed towards a better understanding of surrogate markers of kidney function from infancy through adulthood. Pediatric nephrologists should be aware of the developmental aspects of kidney function including the importance of the congenital nephron endowment and the preservation of kidney function throughout a lifetime. • Nephrogenesis occurs in utero in concert with other organ systems by branching morphogenesis, including the lungs, pancreas, and vascular tree, with over 60 % of nephrons being formed during the last trimester. • Infants born preterm before 36 weeks' gestation are in active nephrogenesis and are at increased risk of having a decreased nephron endowment from prenatal and postnatal genetic and epigenetic hazards that will impact the patient for a lifetime. • Post-natal adaptation of kidney function is directly proportional to the number of perfused nephrons, estimated by total kidney volume (TKV), mean arterial pressure (MAP), and gestational age. • Accurate measurement of glomerular filtration rate (GFR) in infants is problematic due to the unavailability of the gold standard inulin. The traditional use of creatinine to estimate GFR is unreliable in preterm infants due to its tubular reabsorption by immature kidneys and its dependence on muscle mass as an endogenous marker. Alternative endogenous markers to estimate GFR are cystatin C and beta trace protein (BTP). • Long-term follow-up of renal function in those born preterm should be life long and should include assessment of GFR, total kidney volume (TKV) relative to body surface area (BSA), and cardiovascular risks including hypertension and vascular stiffness.

Comparison between a serum creatinine-and a cystatin C-based glomerular filtration rate equation in patients receiving amphotericin B

Serum cystatin C (Cys C) has a number of advantages over serum creatinine in the evaluation of kidney function. Apart from Cys C level itself, several formulas have also been introduced in different clinical settings for the estimation of glomerular filtration rate (GFR) based upon serum Cys C level. The aim of the present study was to compare a serum Cys C-based equation with Cockcroft-Gault serum creatinine-based formula, both used in the calculation of GFR, in patients receiving amphotericin B. Fifty four adult patients with no history of acute or chronic kidney injury having been planned to receive conventional amphotericin B for an anticipated duration of at least 1 week for any indication were recruited. At three time points during amphotericin B treatment, including days 0, 7, and 14, serum cystatin C as well as creatinine levels were measured. GFR at the above time points was estimated by both creatinine (Cockcroft-Gault) and serum Cys C based equations. There was significant correlation between creatinine-based and Cys C-based GFR values at days 0 (R = 0.606, P = 0.001) and 7 (R = 0.714, P < 0.001). In contrast to GFR estimated by the Cockcroft-Gault equation, the mean (95 % confidence interval) Cys C-based GFR values at different studied time points were comparable within as well as between patients with and without amphotericin B nephrotoxicity. Our results suggested that the Gentian Cys C-based GFR equation correlated significantly with the Cockcroft-Gault formula at least at the early time period of treatment with amphotericin B. Graphical abstract Comparison between a serum creatinine-and a cystatin C-based glomerular filtration rate equation in patients receiving amphotericin B.

Longitudinal analysis of serum cystatin C for estimating the glomerular filtration rate in preterm infants

BACKGROUND

Cystatin C (Cys-C) is a more sensitive marker of renal function than creatinine (Cre) in pediatric and adult populations. However, the reference values of serum Cys-C for estimating glomerular filtration rates (eGFRs) in premature infants during the first year of life have not been sufficiently studied.

METHODS

In this prospective study, 481 blood samples were collected from 261 preterm infants with uncomplicated clinical courses during their first year of life. Infants were divided into three groups according to gestational age at birth: 27-30 weeks, 31-33 weeks, and 34-36 weeks. Serum Cys-C and Cre levels were measured at 6-30 days, 3-5 months, 7-9 months, and 12-14 months after birth and the eGFR was calculated using two previously published equations.

RESULTS

The median serum Cys-C levels were 1.776, 1.248, 1.037, and 0.960 mg/L at the first, second, third, and fourth measurement time-point, respectively, with the value significantly decreasing with age up to 12-14 months. Cys-C levels were independent of gestational age and gender. In contrast to Cys-C, serum Cre values declined rapidly up to 3-5 months, then remained constant up to 12-14 months. Using the Cys-C-based equation, the eGFR significantly increased with increasing age until approximately 1 year after birth; however, no such trend was noted using the equation based on Cys-C + Cre.

CONCLUSIONS

Reference ranges for Cys-C in premature infants decline gradually over the first year after birth. Cys-C appears to be a more reliable marker than Cre for estimating GFR in preterm infants.

Assessment of glomerular filtration rate in the neonate: is creatinine the best tool?

PURPOSE OF REVIEW

This article answers the question of whether creatinine is the best biomarker for monitoring neonatal glomerular filtration rate (GFR) in view of recent advances in measuring neonatal renal function.

RECENT FINDINGS

We rely largely on serum creatinine for the estimation of GFR in the newborn, even though creatinine is freely exchanged through the placenta. During the first few days of life, the serum creatinine reflects maternal renal function or the maternal creatinine. Back filtration of creatinine in preterm newborns is also a serious limitation. This review summarizes current knowledge on the prenatal and postnatal handling of creatinine as well as that of other, more novel biomarkers of GFR, such as cystatin C (CysC) and β-trace protein (BTP). Only small amounts of CysC cross the placenta, whereas BTP does not cross the placenta at all. However, BTP measurements are not widely available. Recent studies on renal volumetry are also discussed.

SUMMARY

Currently, CysC may be the most suitable marker of neonatal renal function, but its availability is still limited, it is more costly, and the best method of reporting acute kidney injury and neonatal estimated GFR remains to be established.

Urinary Tissue Inhibitor of Metalloproteinase-2 (TIMP-2) • Insulin-Like Growth Factor-Binding Protein 7 (IGFBP7) Predicts Adverse Outcome in Pediatric Acute Kidney Injury

Background

The G1 cell cycle inhibitors tissue inhibitor of metalloproteinase-2 (TIMP-2) and insulin-like growth factor-binding protein 7 (IGFBP7) have been identified as promising biomarkers for the prediction of adverse outcomes including renal replacement therapy (RRT) and mortality in critically ill adult patients who develop acute kidney injury (AKI). However, the prognostic value of urinary TIMP-2 and IGFBP7 in neonatal and pediatric AKI for adverse outcome has not been investigated yet.

Methods

The product of the urinary concentration of TIMP-2 and IGFBP7 ([TIMP-2]•[IGFBP7]) was assessed by a commercially available immunoassay (NephroCheck^™) in a prospective cohort study in 133 subjects aged 0–18 years including 46 patients with established AKI according to pRIFLE criteria, 27 patients without AKI (non-AKI group I) and 60 apparently healthy neonates and children (non-AKI group II). AKI etiologies were: dehydration/hypovolemia (n = 7), hemodynamic instability (n = 7), perinatal asphyxia (n = 9), septic shock (n = 7), typical hemolytic-uremic syndrome (HUS; n = 5), interstitial nephritis (n = 5), vasculitis (n = 4), nephrotoxic injury (n = 1) and renal vein thrombosis (n = 1).

Results

When AKI patients were classified into pRIFLE criteria, 6/46 (13%) patients fulfilled the criteria for the category “Risk”, 13/46 (28%) for “Injury”, 26/46 (57%) for “Failure” and 1/46 (2%) for “Loss”. Patients in the “Failure” stage had a median 3.7-fold higher urinary [TIMP-2]•[IGFBP7] compared to non-AKI subjects (P<0.001). When analyzed for AKI etiology, highest [TIMP-2]•[IGFBP7] values were found in patients with septic shock (P<0.001 vs. non-AKI I+II). Receiver operating characteristic (ROC) curve analyses in the AKI group revealed good performance of [TIMP-2]•[IGFBP7] in predicting 30-day (area under the curve (AUC) 0.79; 95% CI, 0.61–0.97) and 3-month mortality (AUC 0.84; 95% CI, 0.67–0.99) and moderate performance in predicting RRT (AUC 0.67; 95% CI, 0.50–0.84).

Conclusions

This study shows that urinary [TIMP-2]•[IGFBP7] has a good diagnostic performance in predicting adverse outcomes in neonatal and pediatric AKI of heterogeneous etiology.

A step forward towards accurately assessing glomerular filtration rate in newborns

In this edition of Pediatric Nephrology, Milena Treiber and colleagues have published a study on cystatin C (CysC) concentrations in relation to renal volumetry in 50 small-for-gestational age (SGA) and 50 appropriate-for-gestational age (AGA) neonates, deriving a new formula for estimating neonatal glomerular filtration rate (GFR). The study builds on previous work which established that renal volumetry together with CysC blood levels is a superior method for establishing GFR in term and pre-term newborns [The Journal of Pediatrics (2014) 164:1026-1031.e2]. Treiber et al. use the expected difference between SGA and AGA renal volumes to document the superiority of their new formula, which is based on total renal volume, CysC and body surface area, but does not incorporate gold-standard inulin clearance. Treiber et al.'s study adds new knowledge to the field that will hopefully improve the safety of renally excreted critical dose drugs in the newborn period. This editorial discusses the strengths and limitations of the current study.