Early detection of acute renal failure by serum cystatin C

Interruption of antiretroviral therapy is associated with increased plasma cystatin C

BACKGROUND

Cystatin C has been proposed as an alternative marker of renal function. We sought to determine whether participants randomized to episodic use of antiretroviral therapy guided by CD4 cell count (drug conservation) had altered cystatin C levels compared with those randomized to continuous antiretroviral therapy (viral suppression) in the Strategies for Management of Antiretroviral Therapy trial, and to identify factors associated with increased cystatin C.

METHODS

Cystatin C was measured in plasma collected at randomization, 1, 2, 4, 8 and 12 months after randomization in a random sample of 249 and 250 participants in the drug conservation and viral suppression groups, respectively. Logistic regression was used to model the odds of at least 0.15 mg/dl increase in cystatin C (1 SD) in the first month after randomization, adjusting for demographic and clinical characteristics.

RESULTS

At randomization, mean (SD) cystatin C level was 0.99 (0.26 mg/dl) and 1.01 (0.28 mg/dl) in the drug conservation and viral suppression arms, respectively (P = 0.29). In the first month after randomization, 21.8 and 10.6% had at least 0.15 mg/dl increase in cystatin C in the drug conservation and viral suppression arms, respectively (P = 0.0008). The difference in cystatin C between the treatment arms was maintained through 1 year after randomization. After adjustment, participants in the viral suppression arm had significantly reduced odds of at least 0.15 mg/dl increase in cystatin C in the first month (odds ratio 0.42; 95% confidence interval 0.23-0.74, P = 0.0023).

CONCLUSION

These results demonstrate that interruption of antiretroviral therapy is associated with an increase in cystatin C, which may reflect worsened renal function.

Epidemiology and diagnosis of acute kidney injury

The development of recent standardized definitions of acute kidney injury (AKI) has allowed us to begin understanding pediatric AKI epidemiology and risk factors and to stratify outcome by AKI severity. AKI incidence will vary with illness severity of the population studied and definition type, ranging from less than 1% when need for dialysis is used to 82% when less conservative definitions (such as > or =1.5 times baseline serum creatinine) are used to define AKI. The most common AKI causes are secondary, such as sepsis, nephrotoxic medication, and ischemia, each leading to acute tubular necrosis (ATN). Children undergoing cardiopulmonary bypass surgery, stem cell transplantation, or with multiple organ dysfunction syndrome are at high risk for these events. A key feature in diagnosis and management includes identifying the presence of ATN versus a reversible hypovolemic state because patients with ATN may quickly develop fluid overload with overaggressive fluid therapy, requiring dialytic removal. Despite advances in acute pediatric dialysis therapy and in overall care of critically ill children, severe AKI still is associated with a high mortality rate, necessitating more research in early AKI identification and therapeutic trials.

Cardiorenal syndrome

The term cardiorenal syndrome (CRS) increasingly has been used without a consistent or well-accepted definition. To include the vast array of interrelated derangements, and to stress the bidirectional nature of heart-kidney interactions, we present a new classification of the CRS with 5 subtypes that reflect the pathophysiology, the time-frame, and the nature of concomitant cardiac and renal dysfunction. CRS can be generally defined as a pathophysiologic disorder of the heart and kidneys whereby acute or chronic dysfunction of 1 organ may induce acute or chronic dysfunction of the other. Type 1 CRS reflects an abrupt worsening of cardiac function (e.g., acute cardiogenic shock or decompensated congestive heart failure) leading to acute kidney injury. Type 2 CRS comprises chronic abnormalities in cardiac function (e.g., chronic congestive heart failure) causing progressive chronic kidney disease. Type 3 CRS consists of an abrupt worsening of renal function (e.g., acute kidney ischemia or glomerulonephritis) causing acute cardiac dysfunction (e.g., heart failure, arrhythmia, ischemia). Type 4 CRS describes a state of chronic kidney disease (e.g., chronic glomerular disease) contributing to decreased cardiac function, cardiac hypertrophy, and/or increased risk of adverse cardiovascular events. Type 5 CRS reflects a systemic condition (e.g., sepsis) causing both cardiac and renal dysfunction. Biomarkers can contribute to an early diagnosis of CRS and to a timely therapeutic intervention. The use of this classification can help physicians characterize groups of patients, provides the rationale for specific management strategies, and allows the design of future clinical trials with more accurate selection and stratification of the population under investigation.

Emerging concepts in acute kidney injury following cardiac surgery

Acute kidney injury (AKI) remains a significant cause of morbidity and mortality following cardiac surgery. Through a more thorough understanding of perioperative genomics and the evolving role of early biomarkers of AKI, the authors seek to improve meaningful outcomes among cardiac surgery patients. In this review, the focus will be on advances in risk stratification, evolving definitions and improving early diagnosis of AKI, identification of effective individualized therapies, and future directions.

Serum cystatin C in mouse models: a reliable and precise marker for renal function and superior to serum creatinine

BACKGROUND

Serum creatinine (SCR) and blood urea nitrogen (BUN) determine the glomerular filtration rate (GFR) improperly in acute renal failure. Serum cystatin C (CYS) has the potential to be a more precise marker for GFR. The aim of this study was to compare the sensitivity of SCR, BUN and CYS with respect to the detection of acute renal failure in mice.

METHODS

In an ischaemia reperfusion (I/R) injury model, mice suffered 60-min left kidney ischaemia and right nephrectomy. In a nephrectomy model, mice were nephrectomized to a different extent: from unilateral (3/6Nx) to bilateral nephrectomy (BiNx). Blood samples were collected 2, 12 or 24 h post-op.

RESULTS

SCR, BUN and CYS increased significantly in the I/R-model in comparison to sham mice and 3/6Nx mice at 12 and 24 h post-op (SCR P = 0.009; BUN P < 0.001 and CYS P < 0.004). There were no significant differences in all three markers between 3/6Nx and sham-operated mice. In graded nephrectomy, BUN and CYS showed already significantly the loss of kidney in 4/6Nx mice 12 h post-op [BUN (mg/dl): sham 26.4 +/- 3.5, 4/6Nx 52.3 +/- 13.4, P < 0.01; CYS (mg/l): sham 0.08 +/- 0.03, 4/6Nx 0.15 +/- 0.04, P < 0.01], whereas SCR was only significantly increased in 5/6Nx and BiNx mice 24 h post-op [SCR (mg/dl): sham 0.39 +/- 0.05, 4/6Nx 0.52 +/- 0.07, P = 0.13, 5/6Nx 1.00 +/- 0.29, P < 0.01]. In the longitudinal experiment, CYS showed the renal damage significantly earlier and to a larger extent (2 h: SCR 57 +/- 15%, BUN 40 +/- 16%, CYS 295 +/- 143%, P <0.001).

CONCLUSIONS

CYS can be used as a reliable and precise marker for renal function in mouse models. CYS is more sensitive than SCR, and it shows renal damage earlier than SCR and BUN.

Screening for kidney diseases: older measures versus novel biomarkers

Biomarkers have been used to screen for kidney disease since creatinine was recognized to be correlated with renal function. The measurement of serum creatinine as a screening test for kidney disease falls short, however, because serum creatinine is not particularly sensitive for the diagnosis of kidney disease. Creatinine reflects renal filtering capacity, which has a lot of reserve and is therefore not sensitive to acute or chronic kidney injury unless the injury is substantial enough to compromise the filtering ability. The sensitivity of serum creatinine is further diminished in certain patient populations that are prone to kidney disease because of the physiology of creatinine. Therefore, researchers are seeking new biomarkers that can aid in the diagnosis of both acute and chronic kidney diseases. The limitations of creatinine in screening for kidney diseases in specific patient populations as well as new potential biomarkers that are actively being researched are discussed in this review.

Cystatin C, Renal Function and Cardiovascular Risk

Cystatin C is a novel serum marker of the glomerular filtration rate (GFR), a critical measure of normal kidney function. Unlike serum creatinine, cystatin C concentrations are independent of gender, age and muscle mass. As cystatin C shown no tubular secretion, it is a much earlier indicator of decreased GFR and allows the detection of mild reductions in GFR, which are not detected by creatinine. Cystatin C has been shown to be associated with future cardiovascular disease and deaths in a dose-dependent relationship that possibly reflects a very early stage of chronic ranal dysfunction. In addition, >sub-clinically< elevated cystatin C concentrations in individuals without chronic kidney disease indicated by creatinine are an independent predictor of progression to chronic kidney disease, heart failure and all-cause mortality.

Urinary interleukin-18 does not predict acute kidney injury after adult cardiac surgery: a prospective observational cohort study

INTRODUCTION

Urinary interleukin-18 (IL-18) measured during the immediate postoperative period could be a promising predictor of acute kidney injury following adult cardiac surgery.

METHODS

In a single-centre prospective observational cohort study, we enrolled 100 adult cardiac surgical patients undergoing cardiopulmonary bypass at a tertiary hospital. We measured the urinary concentration of IL-18 and creatinine preoperatively, on arrival in the intensive care unit, and 24 hours postoperatively. We assessed urinary IL-18 concentration and urinary IL-18/urinary creatinine ratio in relation to the postoperative development of acute kidney injury defined as an increase in serum creatinine of greater than 50% from preoperative to postoperative peak value within 48 hours after surgery.

RESULTS

Twenty patients developed acute kidney injury. On arrival in the intensive care unit and at 24 hours postoperatively, urinary IL-18 (median [interquartile range]) was not different in patients who subsequently developed acute kidney injury compared with those who did not: on arrival in the intensive care unit (168 [717] versus 104 [256] pg/mL; P = 0.70) and at 24 hours (195 [483] versus 165 [246] pg/mL; P = 0.47). On arrival in the intensive care unit (area under the curve for the receiver operating characteristic curve [AUC-ROCC] 0.53, 95% confidence interval [CI] 0.38 to 0.68; P = 0.70) and at 24 hours postoperatively (AUC-ROCC 0.55, 95% CI 0.40 to 0.71; P = 0.48), urinary IL-18 was not better than chance in predicting acute kidney injury. All findings were confirmed when urinary IL-18 was adjusted for urinary creatinine. Urinary IL-18 correlated with duration of cardiopulmonary bypass (P < 0.001).

CONCLUSION

In adults, early postoperative measurement of urinary IL-18 appears not to be valuable in identifying patients who develop acute kidney injury after cardiac surgery, but rather represents a nonspecific marker of cardiopulmonary bypass-associated systemic inflammation.

Urinary cystatin C as an early biomarker of acute kidney injury following adult cardiothoracic surgery

There is a need to develop early biomarkers of acute kidney injury following cardiac surgery, where morbidity and mortality are increased by its presence. Plasma cystatin C (CyC) and plasma and urine Neutrophil Gelatinase Associated Lipocalin (NGAL) have been shown to detect kidney injury earlier than changes in plasma creatinine in critically ill patients. In order to determine the utility of urinary CyC levels as a measure of kidney injury, we prospectively collected plasma and urine from 72 adults undergoing elective cardiac surgery for analysis. Acute kidney injury was defined as a 25% or greater increase in plasma creatinine or renal replacement therapy within the first 72 hours following surgery. Plasma CyC and NGAL were not useful predictors of acute kidney injury within the first 6 hours following surgery. In contrast, both urinary CyC and NGAL were elevated in the 34 patients who later developed acute kidney injury, compared to those with no injury. The urinary NGAL at the time of ICU arrival and the urinary CyC level 6 hours after ICU admission were most useful for predicting acute kidney injury. A composite time point consisting of the maximum urinary CyC achieved in the first 6 hours following surgery outperformed all individual time points. Our study suggests that urinary CyC and NGAL are superior to conventional and novel plasma markers in the early diagnosis of acute kidney injury following adult cardiac surgery.

Acute kidney injury: definitions and new paradigms

Changes in terminology and new consensus definitions of acute kidney injury (AKI) and stages of severity have simplified some of the problems in the clinical approach to this complex syndrome. Nevertheless, new proactive approaches to the diagnosis of kidney injury instead of kidney failure are required to allow clinical translation of successful therapies developed for experimental AKI. The recent development of novel urinary and plasma biomarkers, which predict kidney failure, has allowed the development of new paradigms for detection, prevention, and stage-specific treatment.

Biomarkers of acute kidney injury

The diagnosis of acute kidney injury (AKI) is usually based on measurements of blood urea nitrogen (BUN) and serum creatinine. BUN and serum creatinine are not very sensitive or specific for the diagnosis of AKI because they are affected by many renal and nonrenal factors that are independent of kidney injury or kidney function. Biomarkers of AKI that are made predominantly by the injured kidney have been discovered in preclinical studies. In clinical studies of patients with AKI, some of these biomarkers (eg, interleukin-18, neutrophil gelatinase-associated lipocalin, and kidney injury molecule-1) have been shown to increase in the urine before the increase in serum creatinine. These early biomarkers of AKI are being tested in different types of AKI and in larger clinical studies. Biomarkers of AKI may also predict long-term kidney outcomes and mortality.

Biomarkers in acute and chronic kidney disease

PURPOSE OF REVIEW

The paucity of early, predictive, noninvasive biomarkers has impaired our ability to institute potentially effective therapies for acute kidney injury and chronic kidney disease in a timely manner.

RECENT FINDINGS

Promising novel biomarkers for acute kidney injury include a plasma panel (neutrophil gelatinase-associated lipocalin and cystatin C) and a urine panel (neutrophil gelatinase-associated lipocalin, interleukin-18, and kidney injury molecule-1). For chronic kidney disease, these include a similar plasma panel and a urine panel (neutrophil gelatinase-associated lipocalin, asymetric dimethylarginine, and liver-type fatty acid-binding protein). The biomarker panels will probably be useful for assessing the duration and severity of kidney disease, and for predicting progression and adverse clinical outcomes. It is also likely that the biomarker panels will help to distinguish between the various etiologies of acute kidney injury or chronic kidney disease.

SUMMARY

The tools of functional genomics and proteomics have provided us with promising novel biomarkers for acute kidney injury and chronic kidney disease. It will be important in future studies to validate the sensitivity and specificity of these biomarker panels in clinical samples from large cohorts and in multiple clinical situations. Such studies will be facilitated by the availability of commercial tools for reproducible measurement of these panels.

Sensitivity and specificity of a single emergency department measurement of urinary neutrophil gelatinase-associated lipocalin for diagnosing acute kidney injury

BACKGROUND

A single serum creatinine measurement cannot distinguish acute kidney injury from chronic kidney disease or prerenal azotemia.

OBJECTIVE

To test the sensitivity and specificity of a single measurement of urinary neutrophil gelatinase-associated lipocalin (NGAL) and other urinary proteins to detect acute kidney injury in a spectrum of patients.

DESIGN

Prospective cohort study.

SETTING

Emergency department of Columbia University Medical Center, New York, New York.

PARTICIPANTS

635 patients admitted to the hospital with acute kidney injury, prerenal azotemia, chronic kidney disease, or normal kidney function.

MEASUREMENTS

Diagnosis of acute kidney injury was based on the RIFLE (risk, injury, failure, loss, and end-stage) criteria and assigned by researchers who were blinded to experimental measurements. Urinary NGAL was measured by immunoblot, N-acetyl-beta-d-glucosaminidase (NAG) by enzyme measurement, alpha1-microglobulin and alpha(1)-acid glycoprotein by immunonephelometry, and serum creatinine by Jaffe kinetic reaction. Experimental measurements were not available to treating physicians.

RESULTS

Patients with acute kidney injury had a significantly elevated mean urinary NGAL level compared with the other kidney function groups (416 microg/g creatinine [SD, 387]; P = 0.001). At a cutoff value of 130 microg/g creatinine, sensitivity and specificity of NGAL for detecting acute injury were 0.900 (95% CI, 0.73 to 0.98) and 0.995 (CI, 0.990 to 1.00), respectively, and positive and negative likelihood ratios were 181.5 (CI, 58.33 to 564.71) and 0.10 (CI, 0.03 to 0.29); these values were superior to those for NAG, alpha1-microglobulin, alpha1-acid glycoprotein, fractional excretion of sodium, and serum creatinine. In multiple logistic regression, urinary NGAL level was highly predictive of clinical outcomes, including nephrology consultation, dialysis, and admission to the intensive care unit (odds ratio, 24.71 [CI, 7.69 to 79.42]).

LIMITATIONS

All patients came from a single center. Few kidney biopsies were performed.

CONCLUSION

A single measurement of urinary NGAL helps to distinguish acute injury from normal function, prerenal azotemia, and chronic kidney disease and predicts poor inpatient outcomes.

Utility of N-acetylcysteine to prevent acute kidney injury after cardiac surgery: a randomized controlled trial

BACKGROUND

Acute kidney injury (AKI) after heart surgery is associated with increased mortality. We sought to determine whether prophylactic perioperative administration of N-acetylcysteine (NAC) prevents postoperative AKI in patients with chronic kidney disease undergoing cardiac surgery (clinical trials.gov identifier NCT00211653).

METHODS

In this prospective, randomized, placebo-controlled, double-blinded clinical trial, 102 patients with chronic kidney disease who underwent heart surgery at the Minneapolis Veterans Affairs Medical Center were randomized to either NAC (n = 50) 600 mg PO twice daily or placebo (n = 52) for a total of 14 doses (3 preoperative). The primary outcome was maximum change in creatinine from baseline within 7 days after surgery. Secondary outcome was AKI (ie, >0.5 mg/dL or >or=25% increase in creatinine from baseline).

RESULTS

Creatinine increased in both groups (0.45 +/- 0.7 mg/dL in NAC vs 0.55 +/- 0.9 mg/dL in placebo, P = .53) and peaked on postoperative day 5. Acute kidney injury occurred in 41 patients (22 NAC vs 19 placebo, P = .44) by postoperative day 5, but persisted in only 14 (7 NAC vs 7 placebo, P = .94) by day 30. In multivariable analysis, perioperative NAC was unassociated with AKI (relative risk 1.2, 95% CI, 0.8-1.9, P = .34). Five patients (3 NAC vs 2 placebo, P = .68) underwent hemodialysis, and 5 (2 NAC vs 3 placebo, P = 1.0) died perioperatively. There was no difference in lengths of stay in the intensive care unit (4.9 +/- 7 days in NAC vs 6.5 +/- 9 days in placebo, P = .06) and the hospital (13.2 +/- 13 days in NAC vs 16.7 +/- 17 days in placebo, P = .12).

CONCLUSION

Prophylactic perioperative NAC administration does not prevent AKI after cardiac surgery.

Utility of Serum Cystatin C as a Clinical Measure of Renal Function in Dogs

A human kit for cystatin C determination was evaluated for use with canine sera. A reference range was also established. The association between cystatin C and glomerular filtration rate (GFR) was evaluated in 60 dogs with various diseases, by using exogenous creatinine plasma clearance (ECPC) as a measure of GFR. The correlation between cystatin C and ECPC (correlation coefficient [r] = −0.630; P&lt;0.001) was stronger than the correlation between serum creatinine and ECPC (r = −0.572; P&lt;0.001). Nonrenal diseases (e.g., neoplasia, infection) did not influence serum cystatin C concentration. Test sensitivity was significantly better (P&lt;0.001) for cystatin C (76%) than for creatinine (65%). Specificities for the two tests were 87% and 91%, respectively.

Emerging urinary biomarkers in the diagnosis of acute kidney injury

BACKGROUND: Acute kidney injury (AKI) represents a common and devastating problem in clinical medicine. The lack of early biomarkers for AKI has led to a delay in initiating potentially effective therapies. OBJECTIVE: Identification of novel urinary biomarkers for AKI that have progressed to the clinical phase of the biomarker discovery process. METHODS: A literature review (PubMed, MedLine) from 2000 to the present. RESULTS/CONCLUSIONS;: The most promising AKI biomarkers include neutrophil gelatinase-associated lipocalin, IL-18, kidney injury molecule-1 and liver-type fatty acid binding protein. Studies to validate the sensitivity and specificity of these biomarkers in clinical samples from large cohorts and from multiple clinical situations are in progress, facilitated by the development of commercial tools for their measurement.

Early detection of acute kidney injury: emerging new biomarkers

Acute kidney injury (AKI) has recently become the preferred term to describe the syndrome of acute renal failure (ARF) with 'failure' or 'ARF' restricted to patients who have AKI and need renal replacement therapy.(1) This allows capture of the broader clinical spectrum of modest reductions in creatinine, which are themselves known to be associated with major increases in both short- and long-term mortality risk.(2-5) It is hoped that this change in nomenclature will facilitate an expansion of our understanding of the underlying pathophysiology and also facilitate definitions of AKI, which allow comparisons among clinical trials of patients with similar duration and severity of illness. This review will cover the need for early detection of AKI and the role of urinary and plasma biomarkers, including enzymuria. The primary message is that use of existing criteria to diagnose AKI, namely elevation of the serum creatinine with or without oliguria, results in identification that is too late to allow successful intervention. New biomarkers are essential to change the dire prognosis of this common condition.

Urine neutrophil gelatinase-associated lipocalin is an early marker of acute kidney injury in critically ill children: a prospective cohort study

Introduction

Serum creatinine is a late marker of acute kidney injury (AKI). Urine neutrophil gelatinase-associated lipocalin (uNGAL) is an early marker of AKI, where the timing of kidney injury is known. It is unknown whether uNGAL predicts AKI in the general critical care setting. We assessed the ability of uNGAL to predict AKI development and severity in critically ill children.

Methods

This was a prospective cohort study of critically ill children. Children aged between 1 month and 21 years who were mechanically ventilated and had a bladder catheter inserted were eligible. Patients with end-stage renal disease or who had just undergone kidney transplantation were excluded. Patients were enrolled within 24 to 48 hours of initiation of mechanical ventilation. Clinical data and serum creatinine were collected daily for up to 14 days from enrollment, and urine was collected once daily for up to 4 days for uNGAL measurement. AKI was graded using pRIFLE (pediatric modified Risk, Injury, Failure, Loss, End Stage Kidney Disease) criteria. Day 0 was defined as the day on which the AKI initially occurred, and pRIFLEmax was defined as the worst pRIFLE AKI grade recorded during the study period. The χ² test was used to compare associations between categorical variables. Mann-Whitney and Kruskal-Wallis tests were used to compare continuous variables between groups. Diagnostic characteristics were evaluated by calculating sensitivity and specificity, and constructing receiver operating characteristic curves.

Results

A total of 140 patients (54% boys, mean ± standard deviation Pediatric Risk of Mortality II score 15.0 ± 8.0, 23% sepsis) were included. Mean and peak uNGAL concentrations increased with worsening pRIFLEmax status (P < 0.05). uNGAL concentrations rose (at least sixfold higher than in controls) in AKI, 2 days before and after a 50% or greater rise in serum creatinine, without change in control uNGAL. The parameter uNGAL was a good diagnostic marker for AKI development (area under the receiver operating characteristic curve [AUC] 0.78, 95% confidence interval [CI] 0.62 to 0.95) and persistent AKI for 48 hours or longer (AUC 0.79, 95% CI 0.61 to 0.98), but not for AKI severity, when it was recorded after a rise in serum creatinine had occurred (AUC 0.63, 95% CI 0.44 to 0.82).

Conclusion

We found uNGAL to be a useful early AKI marker that predicted development of severe AKI in a heterogeneous group of patients with unknown timing of kidney injury.

Kidney function assessment in the critically ill child: is it time to leave creatinine behind?

The accurate diagnosis of acute kidney injury (AKI) is especially problematic in critically ill patients in whom renal function is in an unsteady state, rendering creatinine-based baseline assessment measures of renal function potentially inadequate. Herrero-Morin and colleagues performed a cross-sectional analysis of the ability of cystatin C and β₂ microglobulin to reflect creatinine clearance in pediatric patients with AKI. The aim of this commentary is to review the current state of AKI clinical and translational research in the light of the results presented in that study.

Cystatin C and beta2-microglobulin: markers of glomerular filtration in critically ill children

Introduction

Parameters allowing regular evaluation of renal function in a paediatric intensive care unit (PICU) are not optimal. The aim of the present study was to analyse the utility of serum cystatin C and beta2-microglobulin (B2M) in detecting decreased glomerular filtration rate in critically ill children.

Methods

This was a prospective, observational study set in an eight-bed PICU. Twenty-five children were included. The inverses of serum creatinine, cystatin C, and B2M were correlated with creatinine clearance (CrC) using a 24-hour urine sample and CrC estimation by Schwartz formula (Schwartz). The diagnostic value of serum creatinine, cystatin C, and B2M to identify a glomerular filtration rate under 80 ml/minute per 1.73 m² was evaluated using receiver operating characteristic (ROC) curve analysis.

Results

Mean age was 2.9 years (range, 0.1 to 13.9 years). CrC was less than 80 ml/minute per 1.73 m² in 14 children, and Schwartz was less than 80 ml/minute per 1.73 m² in 9 children. Correlations between inverse of B2M and CrC (r = 0.477) and between inverse of B2M and Schwartz (r = 0.697) were better than correlations between inverse of cystatin C and CrC (r = 0.390) or Schwartz (r = 0.586) and better than correlations between inverse of creatinine and CrC (r = 0.104) or Schwartz (r = 0.442). The ability of serum cystatin C and B2M to identify a CrC rate and a Schwartz CrC rate under 80 ml/minute per 1.73 m² was better than that of creatinine (areas under the ROC curve: 0.851 and 0.792 for cystatin C, 0.802 and 0.799 for B2M, and 0.633 and 0.625 for creatinine).

Conclusion

Serum cystatin C and B2M were confirmed as easy and useful markers, better than serum creatinine, to detect acute kidney injury in critically ill children.

Diagnosis, epidemiology and outcomes of acute kidney injury

Acute kidney injury is an increasingly common and potentially catastrophic complication in hospitalized patients. Early observational studies from the 1980s and 1990s established the general epidemiologic features of acute kidney injury: the incidence, prognostic significance, and predisposing medical and surgical conditions. Recent multicenter observational cohorts and administrative databases have enhanced our understanding of the overall disease burden of acute kidney injury and trends in its epidemiology. An increasing number of clinical studies focusing on specific types of acute kidney injury (e.g., in the setting of intravenous contrast, sepsis, and major surgery) have provided further details into this heterogeneous syndrome. Despite our sophisticated understanding of the epidemiology and pathobiology of acute kidney injury, current prevention strategies are inadequate and current treatment options outside of renal replacement therapy are nonexistent. This failure to innovate may be due in part to a diagnostic approach that has stagnated for decades and continues to rely on markers of glomerular filtration (blood urea nitrogen and creatinine) that are neither sensitive nor specific. There has been increasing interest in the identification and validation of novel biomarkers of acute kidney injury that may permit earlier and more accurate diagnosis. This review summarizes the major epidemiologic studies of acute kidney injury and efforts to modernize the approach to its diagnosis.

The contrasting characteristics of acute kidney injury in developed and developing countries

Acute kidney injury (AKI) has become increasingly prevalent in both developed and developing countries, and is associated with severe morbidity and mortality, especially in children. Uncertainty regarding the true incidence of AKI limits awareness of the problem, thereby reducing political visibility of the disorder and hampering efforts to prevent its occurrence. In developed countries, AKI occurs predominantly in urban intensive care units and is associated with multiorgan failure and sepsis, high mortality, and occurrence in older populations. While cases of AKI in urban areas of the developing world have similar characteristics to those in the developed world, AKI in rural regions commonly develops in response to a single disease and specific conditions (e.g. gastroenteritis) or infections (e.g. severe malaria, leptospirosis, or hemolytic-uremic syndrome) and in younger otherwise healthy individuals. Many causes of AKI in rural settings, such as diarrhea, poisoning, malaria, or septic abortion, can be prevented by interventions at the individual, community, and regional levels. Treatment with dialysis is often unavailable or too costly in developing regions, so there must be community-wide efforts to eradicate causes of AKI, expedite diagnosis, and aggressively manage prerenal conditions and specific infections. We have reviewed recent literature on AKI, identified differences and similarities in the condition between developed and developing areas, analyzed the practical implications of the identified differences, and made evidence-based recommendations for study and management.

Timing of renal replacement therapy in critically ill patients with acute kidney injury

PURPOSE OF REVIEW

Timing of renal replacement therapy in critically ill patients with acute kidney injury is highly subjective, and may influence outcome. We discuss renal and nonrenal criteria for timing considering the recent literature.

RECENT FINDINGS

Two randomized and four nonrandomized controlled trials investigated the effects of timing on patient outcome. All but one randomized controlled trial indicated better outcome with early renal replacement therapy but had poor methodological quality. The heterogeneity of timing definition, study population and mode of therapy, however, hampered comparison among studies.

SUMMARY

In the absence of large randomized controlled trials we can make no firm recommendations for timing of renal replacement therapy in acute kidney injury. Since rapid recovery of renal function is unlikely when other organ failure persists and the consequences of acute kidney injury may be more severe in critically ill patients, we suggest other organ failure is also considered. Patients with acute kidney injury, persisting shock and poorly recovering functions of other organs may benefit from early therapy. For future studies, we recommend describing renal replacement therapy timing according to the 'RIFLE' classification, as modified by the Acute Kidney Injury Network, and quantifying the severity of other organ failure. Biomarkers may refine acute kidney injury and timing definitions in the future.

Proteomics for biomarker discovery in acute kidney injury

Acute kidney injury (AKI), previously referred to as acute renal failure, represents a common and devastating problem in clinical medicine. Despite significant improvements in therapeutics, the mortality and morbidity associated with AKI remain high. A major reason for this is the lack of early markers for AKI, and hence an unacceptable delay in initiating therapy. Fortunately, the application of innovative technologies such as functional genomics and proteomics to human and animal models of AKI has uncovered several novel biomarkers and therapeutic targets. The most promising of these are chronicled in this review. These include the identification of biomarker panels in plasma (neutrophil gelatinase-associated lipocalin and cystatin C) and urine (neutrophil gelatinase-associated lipocalin, kidney injury molecule-1, interleukin-18, cystatin C, alpha1-microglobulin, Fetuin-A, Gro-alpha, and meprin). It is likely that the AKI panels will be useful for timing the initial insult, and assessing the duration and severity of AKI. It is also probable that the AKI panels will distinguish between the various etiologies of AKI and predict clinical outcomes. It will be important in future studies to validate the sensitivity and specificity of these biomarker panels in clinical samples from large cohorts and from multiple clinical situations. Such studies will be facilitated markedly by the development of commercial tools for the reproducible measurement of biomarkers across different laboratories.

Cystatin C could be a replacement to serum creatinine for diagnosing and monitoring kidney function in children

BACKGROUND

Chronic kidney disease (CKD) is a worldwide public health problem. Glomerular filtration rate (GFR) is accepted as the best way to diagnose and monitor kidney function. Plasma Cystatin C (CysC) has been proposed as a better marker of GFR than serum creatinine (SCr), but it is not widely used because of some drawbacks with CysC assays. Our purpose is to determine the diagnostic accuracy of CysC and SCr for GFR estimation in children, using 99Tc-DTPA clearance (Cl(Tc)) as the reference standard. We also discuss some of the economic implications of these tests, in order to guide clinicians when to use CysC or SCr for the diagnosis or monitoring of CKD.

METHODS

Data were collected from 109 Colombian outpatients aged less than 18 years referred for determination of GFR because of suspected or definite renal insufficiency. The cost of each test was determined in Bogotá, Colombia, and in Madrid, Spain.

RESULTS

Using a GFR of 90 mL/min as a cut-off value, we found: CysC sensitivity 75%, specificity 84%, and area under ROC curve (AUC) 0.84. SCr sensitivity 46%, specificity 100%, and AUC 0.72. Using a GFR of 70 mL/min as a cut-off value, we found: CysC sensitivity 100%, specificity 48%, and AUC 0.94. SCr sensitivity 77%, specificity 91%, and AUC 0.81. In all calculations predictive values behave correspondingly and ranges were narrow at CI 95%. In AUC, p=0.0001. Cost per enzymatic test in Bogotá: CysC U$ 27; SCr U$ 2. Cost per enzymatic test in Madrid: CysC U$ 3; SCr U$ 0.08.

CONCLUSION

CysC is a very interesting option, and could be a replacement to serum creatinine for diagnosing and possibly for monitoring kidney function in children.

Cardiorenal syndrome: refining the definition of a complex symbiosis gone wrong

BACKGROUND

The term "cardiorenal syndrome" has generally been reserved for declining renal function in the setting of advanced congestive heart failure. Considering the complex and bi-directional relationship between the heart and the kidneys, we postulate refining the definition to recognize the symbiotic nature of these organs.

DISCUSSION

We divide the cardiorenal syndrome into five subtypes: type I, acute cardiorenal syndrome; type II, chronic cardiorenal syndrome; type III, acute renocardiac syndrome; type IV, chronic renocardiac syndrome; and type V, secondary cardiorenal syndrome.

CONCLUSIONS

As early recognition of dysfunction in one organ may prove important in mitigating the spiral of co-dysfunction in both, the need for early and treatment-guiding biomarkers, along with their characteristics, are also discussed.

Potential interventions in sepsis-related acute kidney injury

Sepsis is an important cause of morbidity and mortality. Acute kidney injury often complicates sepsis, leading to greater complexity, cost of care, and worsening prognosis. In recent years, a consensus definition of acute kidney injury has been developed, facilitating research into the pathophysiology and epidemiology of this disorder. New and emerging biomarkers to recognize kidney injury before functional abnormalities are manifest may allow early recognition and facilitate prevention or treatment. Furthermore, advances in the clinical management of sepsis may have secondary benefits with respect to renal outcomes. Existing and hybrid extracorporeal therapies are being investigated not only as means to replace lost kidney function but also to modulate the immune response to sepsis. For those who have more advanced forms of kidney injury, strategies to promote renal recovery are being sought to minimize the long-term consequences of impaired kidney function. This review provides an update on the current state of the science and a glimpse toward the future of intervention in sepsis-related acute kidney injury.

Technology Insight: biomarker development in acute kidney injury--what can we anticipate?

Early diagnosis has been the 'Achilles heel' of acute kidney injury (AKI) that has prevented successful implementation of treatment strategies. To date, pharmacological intervention has been largely unsuccessful or equivocal, and morbidity and mortality associated with AKI have remained unacceptably high. Despite their well-known limitations, the most widely used biomarkers for the early diagnosis of AKI are serum creatinine, blood urea nitrogen and urine output. Development of new biomarkers is imperative. A variety of methods have been employed to discover new biomarkers of AKI, including transcriptomics, proteomics, gene arrays, lipidomics and imaging technologies. Clinical trials are underway to establish the validity of the biomarkers discovered using these techniques. This Review summarizes the importance of biomarkers of AKI, from their discovery to clinical practice, from the current perspective and that of what to expect in the future. Great strides forward are being made in breaking down important barriers to the successful prevention and treatment of this devastating disorder.

Biomarkers for the early detection of acute kidney injury

Acute kidney injury (AKI), previously referred to as acute renal failure (ARF), represents a persistent problem in clinical medicine. Despite significant improvements in therapeutics, the mortality and morbidity associated with AKI remain high. A major reason for this is the lack of early markers for AKI, akin to troponins in acute myocardial disease, and hence an unacceptable delay in initiating therapy. Fortunately, the application of innovative technologies such as functional genomics and proteomics to human and animal models of AKI has uncovered several novel genes and gene products that are emerging as biomarkers. The most promising of these are chronicled in this article. These include a plasma panel [neutrophil gelatinase-associated lipocalin (NGAL) and cystatin C] and a urine panel [NGAL, interleukin 18 (IL-18), and kidney injury molecule 1 (KIM)-1]. As they represent sequentially expressed biomarkers, it is likely that the AKI panels will be useful for timing the initial insult and assessing the duration of AKI. Based on the differential expression of the biomarkers, it is also likely that the AKI panels will distinguish between the various types and etiologies of AKI. It will be important in future studies to validate the sensitivity and specificity of these biomarker panels in clinical samples from large cohorts and from multiple clinical situations.

Biomarkers for the diagnosis and risk stratification of acute kidney injury: a systematic review

The diagnosis of acute kidney injury (AKI) is usually based on changes in serum creatinine, but such measurements are a poor marker of acute deterioration in kidney function. We performed a systematic review of publications that evaluated the accuracy and reliability of serum and urinary biomarkers in human subjects when used for the diagnosis of established AKI or early AKI, or to risk stratify patients with AKI. Two reviewers independently searched the MEDLINE and EMBASE databases (January 2000-March 2007) for studies pertaining to biomarkers for AKI. Studies were assessed for methodologic quality. In total, 31 studies evaluated 21 unique serum and urine biomarkers. Twenty-five of the 31 studies were scored as having 'good' quality. The results of the studies indicated that serum cystatin C, urine interleukin-18 (IL-18), and urine kidney injury molecule-1 (KIM-1) performed best for the differential diagnosis of established AKI. Serum cystatin C and urine neutrophil gelatinase-associated lipocalin, IL-18, glutathione-S-transferase-pi, and gamma-glutathione-S-transferase performed best for early diagnosis of AKI. Urine N-acetyl-beta-D-glucosaminidase, KIM-1, and IL-18 performed the best for mortality risk prediction after AKI. In conclusion, published data from studies of serum and urinary biomarkers suggest that biomarkers may have great potential to advance the fields of nephrology and critical care. These biomarkers need validation in larger studies, and the generalizability of biomarkers to different types of AKI as well as the incremental prognostic value over traditional clinical variables needs to be determined.

NGAL is an early predictive biomarker of contrast-induced nephropathy in children

We hypothesized that neutrophil gelatinase-associated lipocalin (NGAL) is an early predictive biomarker of contrast-induced nephropathy (CIN). We prospectively enrolled 91 children (age 0-18 years) with congenital heart disease undergoing elective cardiac catheterization and angiography with contrast administration (CC; Ioversol). Serial urine and plasma samples were analyzed in a double-blind fashion by NGAL enzyme-linked immunosorbent assay (ELISA). CIN, defined as a 50% increase in serum creatinine from baseline, was found in 11 subjects (12%), but detection using increase in serum creatinine was only possible 6-24 h after CC. In contrast, significant elevation of NGAL concentrations in urine (135 +/- 32 vs. 11.6 +/- 2 ng/ml without CIN, p < 0.001) and plasma (151 +/- 34 vs. 36 +/- 4 without CIN, p < 0.001) were noted within 2 h after CC in those subjects. Using a cutoff value of 100 ng/ml, sensitivity, specificity, and area under the receiver-operating characteristic (ROC) curve for prediction of CIN were excellent for the 2-h urine NGAL (73%, 100%, and 0.92, respectively) and 2-h plasma NGAL (73%, 100%, and 0.91, respectively). By multivariate analysis, the 2-h NGAL concentrations in the urine (R (2) = 0.52, p < 0.0001) and plasma (R (2) = 0.72, p < 0.0001) were found to be powerful independent predictors of CIN. Patient demographics and contrast volume were not predictive of CIN.

Beta-trace protein is not superior to cystatin C for the estimation of GFR in patients receiving corticosteroids

OBJECTIVES

Comparison of the effect of corticosteroid therapy on the diagnostic performance of cystatin C (Cys) and beta-trace protein (bTP), two endogenous markers of GFR.

DESIGN AND METHODS

Out of a total of 193 pediatric inulin clearance studies, a random sample of 85 steroid-free studies served to establish GFR prediction equations (eGFR), which were used to compare the remaining 76 steroid-free and 32 steroid-positive studies (median prednisone dose 33.0 mg m(-2) day(-1)).

RESULTS

We found a positive relationship between prednisone dose and eGFR(betaTP) (b=0.414, p=0.0002) and a negative relationship with eGFR(cys) (b=-0.208, p=0.0091). Only Cys independently predicted GFR below 90 mL min(-1) 1.73 m(-2), both in steroid-positives (b=6.260, p=0.010) and steroid-negatives (b=6.845, p=0.012). Glucocorticoid therapy did not affect the accuracy in estimating GFR within 30% of measured GFR for Cys, while accuracy was lower with bTP (65.6% vs. 81.6%, p=0.08).

CONCLUSION

Glucocorticoids have less impact on the diagnostic accuracy of Cys than bTP.

The prognostic importance of a small acute decrement in kidney function in hospitalized patients: a systematic review and meta-analysis

BACKGROUND

Recently, acute kidney injury defined by small changes in serum creatinine levels was associated with worse short-term outcomes; however, the precision and variability of this association was not fully explored.

STUDY DESIGN

Systematic review and meta-analysis.

SETTING & PARTICIPANTS

Hospitalized patients.

SELECTION CRITERIA FOR STUDIES

MEDLINE and EMBASE databases were searched for observational cohort studies and randomized controlled trials published from 1990 through February 2007 that provided information for small changes in serum creatinine levels.

PREDICTOR

Small acute changes in serum creatinine levels by absolute and percentage of changes in serum creatinine levels (lower threshold for increase in serum creatinine <0.5 mg/dL or <25%).

OUTCOME

Short-term mortality (<or=30 days).

RESULTS

Compared with controls, patients with a 10% to 24% increase in creatinine levels had a relative risk (RR) of death of 1.8 (95% confidence interval [CI], 1.3 to 2.5). By comparison, subjects with a 25% to 49% acute change in creatinine levels had an RR of death of 3.0 (95% CI, 1.6 to 5.8), and those with the largest change (>or=50%) had the greatest RR of death (RR, 6.9; 95% CI, 2.0 to 24.5). Results were similar when absolute changes in creatinine levels were considered and when pooled estimates of adjusted RR were used.

LIMITATIONS

Individual patient data were unavailable; thus, only group-level data were pooled for meta-analysis. Results showed a significant degree of statistical heterogeneity that was only partially ameliorated by separating studies into subsets based on clinical setting.

CONCLUSIONS

Short-term mortality and acute decreases in renal function are associated through a graded relationship such that even mild changes in serum creatinine levels portend worse outcome in a variety of clinical settings and patient-types.

Defining acute renal failure: the RIFLE criteria

Acute renal failure is common among critically ill patients and carries significant morbidity and mortality. The reported incidence and the attributed morbidity and mortality of acute renal failure vary widely, largely owing to the use of a wide variety of definitions for acute renal failure. Until recently, no consensus existed about how to best define, characterize, and study acute renal failure. This lack of a standard definition has been a major impediment to the progress of clinical and basic research in this field. This review outlines some of the physiologic principles that may help us better understand and define acute renal failure and describes the RIFLE criteria (an acronym comprising Risk, Injury, and Failure; and Loss, and End-stage kidney disease), a recent consensus method of defining and stratifying acute renal failure. Also discussed are many of the challenges and controversies associated with achieving consensus and developing a classification for acute renal dysfunction.