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Biomarkers of acute kidney injury in a mixed ICU population. a narrative review

Introduction: Acute kidney injury is a severe complication occurring in critically ill patients treated in the intensive care units, which has a significant impact on morbidity and mortality in these patients. Moreover, despite technological and pharmaceutical advances during the last decades, acute kidney injury is still associated with markedly increased morbidity and mortality. Creatinine, the most common renal dysfunction biomarker in use, has poor predictive accuracy for renal injury due to time delay in its increase and the influence by other factors on its serum concentration such as age, gender, muscle mass etc. Therefore, the need for better renal biomarkers in order to early detect and intervene for acute kidney injury prevention is imperative. During the last years several new acute kidney injury biomarkers have been discovered and validated to improve early detection of patients in risk groups for progressive renal failure, need for renal replacement therapy or death. Aim: The purpose of the present article is to review the studies that have tested the predictive ability of those biomarkers (in urine and/or plasma) for early detection of acute kidney injury in the mixed adult intensive care unit population. Material and Method: We searched Medline (PubMed) using the keywords: acute kidney injury, biomarkers, renal failure, ICU.Results: The most relevant articles were included in this review.Conclusions: During the past decade, multiple AKI biomarkers have been studied and viewed as promising. Further studies are needed to determine their clinical utility.


INTRODUCTION
Acute kidney injury (AKI) consists of a rapid renal function decline or loss which usually increases serum urea and serum creatinine (sCr) levels, and induces uremic symptoms secondary to the accumulation of nitrogenous waste substances in the blood. AKI incidence varies from 1-7% of all hospitalized patients to 30-50% of intensive care unit (ICU) patients with 13,5% of them requiring renal replacement therapy (RRT). Despite many efforts to improve prevention and treatment, AKI is still associated with markedly increased morbidity, mortality and health care costs [1]. Because of the above-exposed reasons, AKI early identification is crucial for its prompt treatment and a better prognosis. The purpose of the present article is to review the studies that have tested the prognostic ability of all novel renal biomarkers for early AKI development in a mixed population of adult critically ill patients during the past decade (excluding cardiac surgery patients).

DIAGNOSIS OF AKI
According to Kidney Disease Improving Global Outcomes (KDIGO) classification in 2012, AKI is diagnosed if sCr increases by 0,3mg/dl (26,5μmol/L) or more in 48h or rises to at least 1.5-fold from baseline within 7 days. AKI stages are defined by the maximum change of either sCr or urine output (UO) (UO<5ml/kg/h x 6-12h, or UO< 3ml/kg/h x 24h or anuria x 12h) [2]. As a result of recent standardization of diagnostic and staging criteria for AKI, our understanding of the epidemiology of AKI has improved in a variety of settings, including outpatient clinics, emergency departments, patient wards and ICUs.

Creatinine limitations
sCr is a metabolite of creatine, a molecule that is synthesized from the amino acids glycine and arginine in liver, pancreas, and kidneys and that serves as a rapidly mobilizable reserve of high energy phosphates in skeletal muscles. Due to its low molecular weight (113 Da), creatinine is freely filtered by the glomeruli. In health, there is a constant production rate matching the rate of renal excretion [3].
The role of creatinine as a marker of renal function is limited by the fact that its half life increases from 4h to 24-72h if the glo-merular filtration rate (GFR) decreases. Thus, the serum concentration may take 24-36h to rise after a definite effect in renal function. Furthermore, a true fall in GFR may not be adequately reflected by sCr in patients with sepsis, liver disease and /or muscle wasting. Drugs also affect sCr concentration, which compete with tubular secretion. In this case, sCr may fluctuate without change in renal function [3][4].
Due to the fact that sCr is measured as a concentration, it is therefore affected by variation in volume status. As a result, the diagnosis of AKI may be delayed or missed in significant fluid shifts or fluid overload. A post-hoc analysis of the fluid and catheter management trial revealed that AKI was classified differently in up to 18% of patients after sCr was adjusted for net fluid balance and estimated total body water. Affected patients had mortality rates similar to those with AKI that was present before the adjustment [4].
In patients with normal kidney function, a rise in sCr by 0.3 mg/dl may be due to an important reduction in GFR. On the contrary, in patients with underlying CKD, rises in sCr represent variable changes in GFR and a rise by 0.3 mg/dl may be within the acceptable daily variation and reflect an inconsequential change in GFR.
Finally, single sCr values do not provide any information about specific stages of the AKI process. The most significant issue is that they do not indicate whether a patient is in a progression phase or if recovery has already begun. Also eGFR formulas are not valid to determine renal function in AKI [5].

Urine output limitations
Oliguria is one of the oldest ''biomarkers'' of renal injury. Its association with AKI appeared more than 200 years ago as described by the English physician Heberden. However, the utility of oliguria as a predictor of AKI in hospitalized patients is limited by its relative sensitivity but poor specificity [6]. In fact, urine output may persist until renal function almost ceases.
Therefore, the presence of oliguria may be misleading in that it may reflect a normal physiological response or may be indicative of transient haemodynamic disturbance and not necessarily indicative of significant tubular injury. Indeed, severe tubular injury may not be apparent initially as oliguria given that conhttp://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 19/09/2020 04:04:50 | https://ejournals.epublishing.ekt.gr/index.php/HealthResJ centrating ability is disturbed. Under such circumstances, urine flow reflects GFR and oliguria will not be apparent until GFR has fallen significantly [7]. The KDIGO criteria for AKI are based on the presence of oliguria for a minimum 6h [2]. Several experts used a longer minimum period of oliguria (e.g. 12h) in order to provide a significant prediction of the development of AKI [8]. Others have found a correlation between the volume of urinary output and the duration of oliguria and recommended a lower threshold for urinary output (e.g. 0.3 ml/ kg/h instead of 0.5 ml/kg/h) for the definition of stage of AKI [9].
Given the increased risk for mortality associated with AKI, it is easy to understand why those patients who fulfill both urine output and sCr criteria for higher AKI stages do worse, suggesting renal impairment coupled with significant fall in GFR [9].

AKI BIOMARKERS
A biomarker is defined as a structural, biochemical, physiologic or genetic parameter change, which indicates the presence, severity, or progress of a disease. Regarding kidney injury, it begins by inducing biological and molecular changes, which evolve to cellular damage and can be early detected by biomarkers, since they represent the early stress response of the kidney tubules injury [10].
The 10 th Acute Dialysis Quality Initiative (ADQI) Consensus Conference suggested making use of both function and damage biomarkers in combination with traditional markers of renal function in order to define AKI [10]. The purpose was to transform the way clinicians diagnose and manage patients with AKI, by outlining its spectrum better than sCr and urine output alone. Several biomarkers of AKI in plasma or urine have been tested so far for their ability to predict AKI and need for RRT.
The availability of these new markers has allowed the detection of subtle changes in renal function before sCr rises and the identification of evidenced kidney injury, without a change in sCr (sub-clinical AKI) [11]. It is worth mentioning that biomarker positive, creatinine negative patients appear to have a greater risks of complications, longer hospital stay and higher mortality compared to patients without a biomarker rise. How-ever in certain situations, these events reflect higher severity of illness rather than degree of AKI [10][11]. The main biomarkers are analyzed below.

Neutrophil gelatinase -associated lipocalin (NGAL)
NGAL is a novel 25-KDa protein expressed by neutrophils and various epithelial cells. It is a member of the lipocalin family of proteins, which transport small hydrophobic molecules like steroids, retinoids, and lipids. NGAL concentrations are elevated in the serum of patients with acute bacterial infections, which is consistent with NGAL's proposed function as an endogenous bacteriostatic protein that scavenges bacterial siderophores (small molecules which bind iron). By binding iron, NGAL could mediate various physiologic functions, such as bacteriostatic and antioxidant effects. Additionally, there are data supporting a role of NGAL as a growth factor, regulating apoptosis or cell differentiation and as a regulator of the epithelial phenotype, inducing the formation of kidney epithelia in embryo and adults [12]. Mishra et al, initially proposed NGAL as an indicator for early renal injury secondary to renal hypoperfusion [13], and since then it has been studied in a variety of clinical contexts, both in children and adults. NGAL was found to be a useful early AKI predictor, operating its urine or serum levels equally well, having even prognostic value for initiation of dialysis and mortality. Comparing NGAL performance with sCr-derived glomerular filtration rate (eGFR), serum NGAL (sNGAL) and urine NGAL (uNGAL) had diagnostic superiority over sCr-based eGFR for predicting AKI. However, an increase in sNGAL generation does not seem specific to AKI, since it has been described in many pathologies such as chronic obstructive disease (COPD), severe pancreatitis, inflammatory bowel disease, chronic renal disease, as well as in ICU patients without AKI [12].
In a large prospective study of De Geus et al, 632 consecutive patients were included of whom 16% had AKI on entry. The percentage of patients with more severe AKI (RIFLE F) was 9%.

Cystatin C (CysC)
CysC is a 13.3 kDa non-glycosylated cysteine protease inhibitor produced by all nucleated cells of the body, which is released into the intravascular compartment at a constant rate. CysC is freely filtered by the glomerulus, and subsequently reabsorbed by the renal proximal tubule through the endocytic receptor megalin and catabolized [12]. Unlike sCr, CysC is not secreted into the urine by the tubule, and it is not normally found in urine in significant amounts, although urinary CysC (uCys) excretion is increased by albuminuria. Consequently, serum cystatine (sCysC) concentration is a marker of glomerular filtration rate (GFR), whereas urinary CysC (uCysC) can reflect tubular damage [18]. The blood concentration of CysC depends on the individual's GFR, and the correlation between CysC and GFR is evident even in a range where sCr cannot detect changes, namely GFR of 60-90 mL/min; that means that sCr is insensitive for small decreases in GFR. In more severe renal impairment accompanied by a marked reduction of the GFR, CysC could not demonstrate any superiority over sCr. It is important to recognize that the analysis of CysC may be affected by thyroid dysfunction, cigarette smoking, inflammation, cancer, and glucocorticoids but seems to be independent of gender, race, muscle mass, and hydration status [12,18].  Table 2].

Kidney injury molecule -1 (KIM-1)
Kidney injury molecule-1 (KIM-1) is a type I cell membrane glycoprotein containing a unique six cysteine immunoglobulinlike domains and a mucin domain in its extracellular portion.
ΚIM-1 is expressed at low levels in the normal kidney and also in other organs, but its expression is significantly upregulated in the kidney after kidney injury, as it was initially identified using representational difference analysis on kidneys, following ischemia/reperfusion injury. In preclinical and clinical studies, http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 19/09/2020 04:04:50 | https://ejournals.epublishing.ekt.gr/index.php/HealthResJ urinary KIM-1 (uKIM-1) has been used as an early diagnostic indicator of kidney injury. Studies in adults suggested that uKIM-1 was significantly elevated in patients with clinical diagnosis of acute tubular necrosis, and predicted adverse clinical outcomes, including dialysis requirement and mortality [22]. KIM-1 was also useful for stage 3 AKI diagnosis with an AUC of 0.82 [25], [ Table 3].

Interleukin-18 (IL-18)
Interleukin-18, also known as interferon-gamma inducing factor, is a member of the IL-1 family of cytokines which regulates adaptive immunity. As an inactive 23-kDa percursor is synthesized by several tissues including monocytes, macrophages, and proximal tubular epithelial cells, and is processed into an active 18,3 kDa cytokine by a caspase-1. IL-18 levels are enhanced in numerous endogenous inflammatory processes, such as sepsis, inflammatory arthritis, inflammatory bowel diseases, systemic lupus erythematosus, psoriasis, hepatitis and multiple sclerosis, with this property significantly limiting its application due to low sensitivity and specificity. It is a sensitive mediator of ischemic injury in different organs (heart, brain, and kidney) [26].  Table 4].

Liver-type fatty acid-binding protein (L-FABP)
L-FABP, also known as fatty acid-binding protein 1 (FABP-1), is ple logistic regression analysis revealed that uL-FABP was significantly associated with mortality. ROC curve analysis revealed an AUC of 0.99 for death prediction [31]. Likewise, Doi et al found an AUC of 0.90 for 14-day mortality for uL-FABP measured on ICU admission. The AUC of uL-FABP for AKI prediction 1 week post-ICU entry was 0.75 [27]. Finally, findings from a recent study showed that high uL-FABP-1 and uL-FABP-3 levels could serve as markers for AKI patients with risk for dialysis need (p< 0.001) [32], [ Table 5].

TIMP-2 and IGFBP7
Tissue inhibitor of metalloproteinases 2 (TIMP-2) is 21-kDa protein, member of the TIMP family. TIMP-2 is an endogenous inhibitor of metalloproteinase activities. Insulin-like growth factor-binding protein 7 (IGFBP-7), a 29-kDa secreted protein, is known to bind and inhibit signaling via insulin-like growth factor 1 receptors. Both markers are involved in the process of  Table 6].

N-acetyl-β-glucosaminidase (NAG)
NAG is a lysosomal enzyme mainly found in the proximal tubule cells. Increased urinary concentration of NAG is a sensitive marker that may suggest renal injury. NAG enzyme is not filtrated by the glomerulus due to relatively high molecular weight (~ 140 kDa), but in active kidney disease urinary NAG (uNAG) levels are elevated. The utility of NAG still remains limited due to the fact that its urinary excretion is also elevated in chronic glomerular diseases [26].