Kidney Injury Rifle Classification System Health And Social Care Essay

Abstract

Early recognition of AKI plays a pivotal role for effective treatment and to prevent mortality and also decrease the human cost and human suffering. Though the serum creatinine is an important indicator for kidney disease management , it only reflects changes in renal function but not injury. Several pre-renal factors like heart problems ,problems with blood pressure and chronic infection ,extreme dehydration etc, influence the serum creatinine levels and it shows delayed sensitivity too.

Since biomarkers represent cellular responses to injury at molecular level, they have the potential to offer not only "first injury information" but also a lesion specific, organ specific information for differential diagnosis of AKI. Several potential AKI biomarkers have been well known as early responders of AKI. Troponina are the superior marker for diagnosis of myocardial injury. The diagnosis of myocardial injury has come a long way from lactate dehydrogenase isoenzymes, total creatine phospho kinase (CPK) levels, isoenzymes of CPK, myoglobin to the troponins and now, the natriuretic peptides (NPs) are being evaluated increasingly for improved sensitivity and positive predictive value.This resulted dramatically decrease in mortality and morbidity in cases like myocardial ischemic or myocardial infraction. Leading contenders for "Renal Troponin" are biomarkers such as NGAL, KIM-1, Cys-C Cys L C-18, NAG, L-FABP and NETRIN-1 and NGAL appears to be the most promising biomarker for earliest prediction of the AKI.

Keywords:NGAL,KIM-1,Cys-C,Netrin,IL-18,L-FABP,AKI.

Introduction

Acute kidney injury which was previously called as acute renal failure (ARF), is one of the most common complications in critically ill patients. The clinical course of AKI was first described by William Heberden in his ‘Commentaries on the History and Cure of Diseases’ in 1802 and the modern study of AKI truly began in 1951 by Homer W Smith, who introduced the term ARF.[1,2]

In 2004, the Acute Dialysis Quality Initiative (ADQI) group proposed the term "acute kidney injury" (AKI) to redefine the entire spectrum of acute renal dysfunction and developed the RIFLE system (Risk of Injury, Failure, Loss of function, and End-stage renal failure)3. More recently in 2007, the AKI-network has suggested a more simplified definition which depends only on a >0.3mg/dL (>25mmol/l) rise or a >50% increase in serum creatinine or development of oliguria (urine output of <0.5ml/kg/h for greater than 6 hrs)[1] (Table-1). These classification criteria for AKI have three grades of severity which includes kidney injury, renal failure and failure of renal function. Serum creatinine and urine output form the basis for RIFLE stratification and this helps in management of acute kidney injury, maintenance of renal perfusion, hydration and limiting exposure to nephrotoxins, design drug protective strategies and use of renal replacement therapies.[4]

Table-1: Acute kidney injury RIFLE classification system

RIFLE

category

Glomerular Filtration Rate (GFR)/

Serum creatinine(sCr)

Urine output Criteria

Risk

sCr increased 1.5 times or GFR decreased >25%

< 0.5mL/kg/h for 6 hours

Injury

sCr increased 2 times or GFR decreased >50%

<0.5mL/kg/h for 12 hours

Failure

sCr increased 3 times or GFR decreased >75% or serum creatinine level greater than 4 mg/dL

<0.5mL/kg/h for 12 hours or An-uria for 12 hours

Loss

Complete loss of renal function for greater than 4 weeks

End-stage

kidney disease

End stage Kidney disease and need for renal replacement therapy for > 3 months

*Adapted from Ref: 4 and modified.

Pathophysiology of AKI:

The concomitant occurrence of ischemia and sepsis (toxins) account for the largest number of cases of AKI.[5] The events which constitute the pathophysiology of AKI could be considered at three levels: vascular disturbances, tubular disturbances and cellular disturbances. Renal vasoconstriction that leads to decreased blood flow and congestion in outer medulla resulting in decreased tissue oxygen delivery to proximal convoluted tubule and ascending loop of Henle constitute the vascular disturbances in AKI. Tubular obstruction, back leak and increased tubule-glomerular feedback constitute the disturbed tubular events during AKI condition. Dramatic decrease in GFR (during AKI) which occurs after AKI can be attributed to these derangements in vascular and tubular compartments of the kidneys. At cellular level, injury leads to loss of cytoskeletal integrity and cell polarity, shedding of the proximal tubule brush borders, as well as apoptosis and necrosis. When the injury is severe enough, viable as well as necrotic and apoptotic cells (non viable cells) are desquamated leaving space where the basement membrane denuded or remains as only barrier between filtrate and the peritubular interstitium which causes interstitial oedema. These cells and debris form casts, appear in the urine and this is the hallmark of AKI.[6] The casts so formed obstruct the tubule, increase the intra tubular pressure and reduce the GFR. When combined with the loss of normal epithelial barrier function, this allows for back-leak of the filtrate. The injury to the epithelium results in inflammation, thus contributing in a critical way to the pathophysiology of AKI. [7]

At sub-cellular level during kidney injury, sub-lethally injured cells release uric acid which activates dendritic cells and T cells. Activation of Toll Like Receptor, complement cascade and nuclear factor KB, releases cytokines and chemokines. This promotes intense inflammation in locally injured regions, infiltration by mononuclear cells and activation of macrophages, which promote angiogenesis and sometimes fibrous tissue deposition in cases where the extent of AKI damage in irreparable. Intrinsic renal cells undergo phenotypic transformation by dedifferentiation to progenitor. These dedifferentiated cells lose cell polarity and cell to cell junction, lose their normal phenotype and function, finally acquire characters of myofibroblasts. These have the ability to migrate, engulf, proliferate, secrete cytokines/chemokines and synthesize matrix in the process of repair and re-differentiate to restore cell number.[7]

TIFF_Pathophysiology of AKI

Figure1. Pathophysiology of AKI (Modified from [7 & 8])

Legend: In kidney injury in the proximal epithelial cell brush border, polarity is lost with mis-location of adhesion molecules and viable epithelial cells are shed in the urine. If sufficient nutrients and oxygen were delivered, the kidneys can initiate a repair process. Viable epithelial cells de-differentiate and migrate over the basement membrane. The source of these cells appears to be the kidneys and not the bone marrow. Increasing injury causes cell death due to necrosis or apoptosis.

EPIDEMIOLOGY

Indian data

The incidence of AKI is high in the patient population in India with the outcome being more adverse among children compared to adults. About 36% of the critically ill children developed AKI9 (Indian Pediatr. 2012 Jul; 49(7):537-42. Incidence of acute kidney injury in hospitalized children.Mehta P, Sinha A, Sami A, Hari P, Kalaivani M, Gulati A, Kabra M, Kabra SK, Lodha R, Bagga A) and the mortality was >93%10 (Ren. Fail. 2012;34(10):1217-22. A study of incidence of AKI in Critically Ill Patients. Paudel MS, Wig N, Mahajan S, Pandey RM, Guleria R, Sharma SK). According to another report, the incidence of AKI was 5.2 % in the pediatric wards and 25.1 % in the PICU11 (Indian J Pediatr. 2012 Jun 14. Incidence and Etiology of Acute Kidney Injury in Southern India.Krishnamurthy S, Mondal N, Narayanan P, Biswal N, Srinivasan S, Soundravally R.) The incidence of AKI is spread across medical (87.6%), obstetric (8.9%), and surgical (3.4) cases. Among the medical causes of AKI, acute diarrheal disease was the most common cause and the other causes of medical AKI included drugs (13.4%), glomerulonephritis (9.3%), sepsis (8.8), snake bite (7.8%), leptospirosis (7.5%), malaria (4.4%) and copper sulphate poisoning (4.3%)12. (Ren Fail. 2006; 28(5):405-10.Epidemiologic trend changes in acute renal failure-a tertiary center experience from South India. Jayakumar M, Prabahar MR, Fernando EM, Manorajan R, Venkatraman R, Balaraman V.)

Inspite of the advancements in infrastructure and knowledge, AKI continues to be a common and serious clinical problem in critically ill patients as it results in increased morbidity and mortality.[13] United States and Spain have shown 11% increase yearly on an average of 23.8 cases per 1000 discharges.[14] Every year about 228 AKI cases are reported per 100,000 and about 82000 patients die in the USA. It is estimated that it costs about $ 33 billions per year (2005 estimate) to treat ESKD cases (MMWR 2008;57(12):309 / Clin J Am Soc Nephrol 3: 844-861, 2008). The incidence of AKI varies with the clinical setting and studies have separately addressed community acquired AKI, Sepsis induced AKI, Hospital acquired AKI and ICU associated AKI[1]using renal replacement therapy or serum creatinine levels as criteria.

Community acquired AKI: The annual incidence rates reported by a British study that 22per million people using the renal replacement therapy when compared with 175 per million people who using the serum creatinine >5.7mg/dL (<500µ/mol/L).Scotland reported a analogous study of an annual incidence of 50 per million people using life supporting treatment and serum creatinine by 120 per million people and for 620 per million people using serum creatinine cut-off as >3.4mg/dL (>300umol/L). Etiology of community acquired AKI may diverse in tropical settings, this means the infectious infectious diseases, diarrheal illness and snake bites are still common causes of AKI. Natural disasters such as earth quakes contribute to local and regional epidemics of AKI.[1]

Hospital-acquired AKI:A number of studies have attempted on hospital acquired renal insufficiency have recently increased and 5-7% of hospitalized patient occurs AKI revealed by several single center. In a topical investigation from China, acute or chronic kidney injury accounted for nearly 36% of biopsied AKI cases.[1].In some countries including India obstetric cause of hospital acquired renal insufficiency is still prevalent.The increasing perception of hospital admissions has been accompanied by a increased incidence of AKI in patients admitted to hospital.

Sepsis-induced AKI: The incidence of AKI is a common consequence to the incidence of sepsis .AKI occurs in sepsis patients with 19% , severe sepsis with23% and septic shock with positive blood cultures seen to be 51%.[1]

ICU - associated AKI: The mortality is high in AKI in critical ill patients and even if patients survive, they are at risk for End Stage Renal Disease (ESRD). The mortality rate ranges from 23-79% in AKI patients.The prevalence of AKI, in critical care settings requires dialysis is about 6% with exceeding 60% mortality rate. AKI associated mortality and morbidity has considerable increase has been demonstrated in a wide variety of clinical situations, including cardiopulmonary bypass, exposure to radio contrast dye, mechanical ventilation and sepsis. [1]

ETIOLOGY:

AKI is often characterized by an abrupt and sustained inability of the kidney to discharge its normal functions like excretion of metabolic wastes (nitrogenous and non-nitrogenous), maintaining proper fluid and electrolyte balance.[4] The causes of AKI are traditionally divided in to 3 categories, namely, pre-renal, intrinsic renal, post-renal.[6,7,15] The pre-renal azotemia describes physiological response of kidney to hypo perfusion that involves intravascular volume depletion[6,15], there is no tubular injury, however, there is decrease in GFR with changes in serum creatinine levels.[4] Intrinsic renal causes may include ischemia and toxins. The toxins have their direct effect on the vasculature or epithelium of nephron because the toxins are concentrated by the tubule as the filtrate moves down the nephron.[7] . Structures of nephron like glomerules, renal tubules, vessels and interstitium get damage due to intrinsic diseases. The post renal category is a consequence of causing obstruction to urinary track where it cause obstacle to urine flow anywhere from the renal pelvis to the urethra.[7,15]

Figure2: Causes of AKI (Modified from15)

BIOMARKERS FOR AKI

While morbidity is certain, it is also known that even the mildest form of AKI independently increases early as well as long-term mortality, the risk increasing as severity of renal injury increases16,17 (Bihorac A, Yavas S, Subbiah S, Hobson CE, Schold JD, Gabrielli A, Layon AJ, Segal MS: Long-term risk of mortality and acute kidney injury during hospitalization after major surgery. Ann Surg 2009; 249:851– 8. Kheterpal S, Tremper KK, Heung M, Rosenberg AL, Englesbe M, Shanks AM, Campbell DA: Development and validation of an acute kidney injury risk index for patients undergoing general surgery: Results from a national dataset. ANESTHESIOLOGY 2009; 110:505–15)

However, pathophysiology of AKI is very complex which results failure in effective treatment with pharmacological agent and failed to improve outcomes. So, the treatment of AKI has remained a daunting task. In developing effective therapeutic interventions has became a major hurdle to combat AKI and has a limited ability to detect a significant renal injury in timely manner.In critically ill patients the independent association is upto 60% between AKI and mortality rates and our current need is to provide supportive care for patients with AKI, the need for more precise and earlier diagnostic tools is profound. Facilitate smooth the progress

Following characteristics for ideal biomarker for AKI : (i) Non-invasive, easily handy, (ii) Measures rapidly using standard clinical assay platforms (iii) cost effective process with short turnaround time (iv) Aid the early detection of kidney injury, and smooth the progress with a wide dynamic range and cut off values that assist in proper risk stratification (v) High specificity for AKI, should be able to differentiate intrinsic AKI from pre-renal azotemia and chronic kidney disease (vi) guide instigation and monitoring of therapies and (vii) to forecast clinical outcomes such as need for dialysis, length of hospital stay and mortality.[18]

Conventional biomarkers of AKI:

The traditional blood (creatinine, blood urea nitrogen) and urine markers (casts, fractional excretion of sodium, urinary concentrating ability) of kidney injury, which have been used for decades in clinical studies for diagnosis and early detection of AKI, are insensitive and unclear and do not directly reflect injury to kidney cells. Most of these markers reflect functional consequences of the injury. Serum creatinine is considered the gold standard for any kidney disease though it is only a functional (GFR) marker and its level increases only 48h after the injury (Star RA. Treatment of acute renal failure. Kidney Int 1998; 54:1817–1831). The mortality rate associated with AKI has not decreased for many decades. [6] The main reason for this is the delayed diagnosis of the disease as there are only a few conventional diagnostic parameters like urine output and serum creatinine levels which themselves manifest after several hours of injury. [7]

The diagnosis of AKI utilizing the changes in the serum creatinine levels though affordable and easy to perform, has always been a delayed and unreliable indicator for a variety of reasons:

(a) Serum creatinine levels are also influenced by non-renal factors like age, gender, muscle mass, muscle metabolism, protein intake, hydration status, tubular secretion etc.

(b) Certain renal conditions do not involve any change in serum creatinine levels owing to the concept of renal reserve. It is estimated that serum creatinine rises after 50% of kidney function is lost.

(c) Serum creatinine concentrations do not reflect true decrease in GFR in AKI and therefore lie far behind the structural changes that occur during the early stages of renal injury. [19,20]

Thus serum creatinine is not a reliable marker for kidney injury. In view of the seriousness of complications caused by AKI, there is an urgent need for biomarkers for early detection and risk stratification of AKI [4, 19] that permit more timely diagnosis, predict severity of injury and monitor recovery after treatment. Further, such markers could also be used to assess renal toxicity of drugs under development. Previously, urinary biomarkers like casts, fractional excretion of sodium, high molecular weight proteins, tubular proteins or enzymes were investigated to help in early recognition of AKI, but they lacked specificity and sensitivity. [19]

Biomarkers under development for management of AKI:

Acknowledging the inherent deficiencies of serum creatinine to diagnose AKI, the American Society of Nephrology

in 2005 designated identification, characterization, and development of new AKI biomarkers as a key research area for the next 5 yr21 (Berl T: American Society of Nephrology Renal Research Report. J Am Soc Nephrol 2005; 16:1886 –903) which encouraged intense investigation on various prospective protein biomarkers identified by proteomics study. Four biomarkers for AKI are Neutrophil gelatinase associated lipocalcin (NGAL)(Urine and Plasma), Interleukin 18 (IL-18) (urine) , Kidney injury Molecule-1 (KIM-1) (Urine) and liver fatty acid binding protein (L-FABP) (Urine) had been tested for various grades in ongoing clinical trails22 [Devarajan P: Emerging urinary biomarkers in the diagnosis of acute kidney injury. Expert Opin Med Diagn 2008, 2:387-398]. The most promising markers of AKI are Neutrophil Gelatinase Associated Lipocalin-2 (NGAL-2), human Kidney Injury Molecule-1(KIM-I), Cystatin C, Liver type Fatty Acid Binding Protein (L-FABP), Interleukin 18 (IL-18), and Netrin-1, which are reviewed here.

Neutrophil Gelatinase-Associated Lipocalin-2 (NGAL-2):

Neutrophil gelatinase-associated lipocalin -2 was identified as one of the most up-regulated transcripts in the early post-ischemic or nephrotoxic mouse kidney23 (Supavekin S, Zhang W, Kucherlapati R, et al. Differential gene expression following early renal ischemia-reperfusion. Kidney Int 2003; 63:1714–1724), and this finding has since been confirmed in several other transcriptome profiling studies. It is a barrel shaped 25 kDa protein, bound to gelatinase granules of neutrophil. It consists of hydrophobic calyx which binds to iron.[24,25,27] It is produced in a variety of human tissues like bone marrow, uterus, prostate, salivary gland, stomach, lung, liver and kidney.[20] NGAL is generally produced in low quantities in normal conditions, but it is highly induced during epithelial damage.[24] The gene ngal has binding sites for a number of transcription factors including NF-kB which is known to be activated after acute injury of kidney tubules. It acts as a bacteriostatic agent [20] by depriving bacteria of iron. The siderophores produced by eukaryotes function in NGAL-mediated iron shuttling which is critical for various cellular activities like proliferation and differentiation. It promotes epithelial differentiation of the mesenchymal progenitors, which leads to development of glomeruli and proximal tubules. A number of studies have implicated urinary and serum NGAL as an early diagnostic marker of AKI.[24, 27, 28, 29] NGAL levels in urine increase at least 36-48 hours earlier than the serum creatinine and hence NGAL could be a highly discriminatory biomarker with a wide dynamic range and cut off levels that allow for easy medical decision making which means it is used to conclude a persons risk of suffering for a particular disease ( risk stratification.)[28] Significantly, ≥ 10-fold or more increase was recorded within 2-6 hrs of major surgery and within 1-3 h after cardiac surgery in those who developed AKI.[20, 29] 

No specific, reliable biomarker is in use other than serum creatinine for assessing delayed graft function in kidney transplant patients. NGAL has been evaluated as a biomarker for this application and the elevated levels of urine NGAL on the day of transplant indicate high risk of subsequent development of delayed graft function (which may occur 2-4 days later) with an AUC of 0.9 [20, 29, 30]. Further, NGAL has also shown a reasonably good prediction of nephrotoxicity following contrast administration with a an AUC of 0.90-0.92.[26, 29] In view of the high predictive capability of NGAL for AKI, it is used as an outcome variable in clinical trials to demonstrate improved efficacy of a modern hydroxyl ethyl starch preparation over albumin or gelatin in maintaining renal function in elderly cardiac surgery patients.[29,31,32] Peter A McCollough demonstrated that NGAL could be used as the differentiator for renal involvement with concurrent increase in serum creatinine from those who do not show any increase in serum creatinine, as these conditions require totally different therapeutic approaches.[25] It is conceivable that NGAL levels could be used to initiate and monitor many novel therapies in near future.

There are different assay formats available for measuring NGAL and they are listed below in the Table. Majority of the NGAL values described so far in the literature utilized ELISA assays which are currently available from Bioporto (Gentofte, Denmark) and R&D Systems (MN, USA).[6,20] A point-to-care kit for the clinical measurement of plasma NGAL (Triage NGAL device, Biosite Inc., CA, USA)with good performance characteristics and also, a urine NGAL immunoassay in a clinical platform (ARCHITECT Analyzer, Abott Diagnostics, Abott Park, IL, USA) are available. [6,20] 

Clearly, NGAL represents a novel predictive biomarker of AKI, however it has few limitations. It seems to have high predictivity in children than in adults and the plasma NGAL values may be influenced by a number of co-morbidities like chronic hypertension, chronic kidney disease33 (Mitsnefes M, Kathman T, Mishra J, et al. Serum NGAL as a marker of renal function in children with chronic kidney disease. Pediatr Nephrol 2007; 22:101–108) and systemic or urinary tract infections34 (Xu S, Venge P. Lipocalins as biochemical markers of disease. Biochim Biophys Acta 2000; 482:298–307), inflammatory conditions, other malignancies. However, the rise in NGAL levels is these conditions are much less when compared to AKI.[20] NGAL is existing as center-stage player in the field of AKI as a novel predictive biomarker.

Kidney injury molecules-1(KIM-I):

Kidney Injury molecule is called as KIM-1 in humans and in rodents as Kim-1.[5] In 2002 the first human studies with KIM-1 were published.[35] Kidney injury molecules-1(KIM-I) is a type-1 trans-membrane glycoprotein (90kDa) which is not expressed in normal kidney tissue or urine, but it is over expressed quite significantly in proximal tubular cells  following ischemic or nephrotoxic AKI[4,37] KIM-1 is a trans membrane tubular protein might make it an ideal biomarker if kidney injury whuch is sensitive and specific amd also predictor of prognosis .Its major role takes place in regeneration of tubular epithelial layer [36] until the cell has recovered completely and also has ability to recognize dead cell and remove it from tubular lumen by a prowess called phagocytosis. [37] The KIM-1 expression is unnoticeable in normal kidney and no other orgams can express KIM-1 [35,36] and hence in the kidney injury the expression of KIM-1 in urine is highly specific.When compared to adult cardiac surgery patients who developed AKI ,the levels of urinary KIM-1 reported to be high in children with better sensitivity and specificity who undergone cardiac surgery.[37] and also in CKD patients these KIM-1 levels are found to be high.[18]. In transplant biopsies early tubular necrosis can be diagnosed by KIM-1 expression which was not detected through histological examination and KIM-1 also facilitate to discriminate acute tubular necrosis from other allograft dysfunction. [36]

Urinary KIM-1 can be measured by using ELISA method.[5] or by subtractive hybridization38(Ichimura T, Bonventre JC, Bailly V, et al. Kidney Injury Molecule-1 (KIM-1), a putative epithelial cell adhesion molecule containing a novel immunoglobulin domain, is up-regulated in renal cells after injury. J Biol Chem 1998; 273: 4135–4142.). In kidney biopsies from patients of AKI , the KIM-1 expression is found to be markedly induced in proximal tubules and KIM-1 molecule helps in differentiating the ischemic AKI from pre-renal azotemia and chronic renal disease39,40 ( Han WK, Bailly V, Abichandani R, et al. Kidney injury molecule-1 (KIM-1): a novel biomarker for human renal proximal tubule injury. Kidney Int 2006;62:237–244; Vaidya VS, Ramirez V, Ichimura T, et al. Urinary kidney injury molecule-1: a sensitive quantitative biomarker for early detection of kidney tubular injury. Am J Physiol Renal Physiol 2006;290: F517–F529). Patients with AKI induced by contrast did not have increased urinary KIM-1. Thus, KIM-1 represents a promising candidate for inclusion in the urinary "AKI panel". KIM-1 is more specific to ischemic or nephrotoxic AKI and significantly not affected by pre renal disturbances , urinary track infections or chronic kidney disease. This is regarded as an advantage of KIM-1 over NGAL. Both the NGAL and KIM-1 is found to be emerge or exist as tandem biomarkers of AKI . NGAL is known from earliest time points as most sensitive and KIM-1 is now adding a significant specificity at slightly later time periods.

Cystatin-C:

Cystatin-C (Cys-C) is a small 13kDa, non-glycosylated protein, it was previously called as r-trace, post-r-globulin and gamma-CSF, one of the most extracellular inhibitors of cysteine proteases which does not bind to any other plasma protein.[5, 41,42] It is produced by all nucleated cells[6] and expressed at a constant rate.[42] Because of the positive charge at physiological pH it can be freely filtered at the glomerulus [41], which is the only elimination route for cystatin-C.[42] It is not secreted by renal tubules, but reabsorbed by renal tubules,[6] which undergoes complete catabolism in proximal tubular cells and appear in the urine even if very little in amount. Therefore Cys-C levels can be used as marker of tubules dysfunction, also as a more sensitive, better serum marker of GFR and stronger predictor than serum creatinine.[5, 41,42] 

Cys-C levels are not influenced by gender, muscle mass, age, protein intake, metabolites like bilirubin, ketones, elevated glucose or ascorbic acid and various drugs which interfere with creatinine tests like cyclosporine A, cephalosporins and aspirin.[42] Before 1994 Serum Cystatin- C was measured by an enzyme-amplified single radial immune diffusion technique which takes at least 10-20 hrs and had a relatively high coefficient of variation (>10%).[5] Afterwards, an automated rapid particle enhanced immuno- turbidimetric and immuno-nephlometric methods were developed which are more specific and approved by FDA.[5]

Liver-type fatty acid binding protein (L-FABPs):

Fatty acid binding proteins (FABPs) are small (15kDa) cytoplasmic proteins which are expressed in tissues with active fatty acid metabolism.[37] The primary function of FABP is to facilitate long chain fatty acid transport, reduction of oxidative stress and regulation of gene expression.[37] In human kidney, two types of FABP have been identified, called liver-type FABP (L-FABP) in the proximal tubule and heart type FABP (H-FABP) in the distal tubule.[5] In proximal tubules, free fatty acids are combined with cytoplasmic FABPS and transported to peroxisomes or mitochondria, where they get metabolized by β-oxidation.[6] Urinary L-FABP is  undetectable in healthy urine [37] but it has been identified in clinical and preclinical animal models and also it has been found to be a potential biomarker in a number of pathologic conditions, such as chronic kidney disease (CKD), IgA nephropathy, diabetic nephropathy and contrast nephropathy.[5] In ischemic conditions, the expression of tubular L-FABP gene is induced and in renal disease the re-absorption of L-FABP is reduced.[37] In children who underwent cardio pulmonary bypass and subsequently developed AKI, urine L-FABP concentrations were significantly increased within 4hrs of surgery.[18] The levels of urinary L-FABP increases in 4hrs can be regarded as a independent risk factor for the development of AKI and the AUC was 0.81.In hospitalized patients with AKI, the AUC of urinary L-FABP for prediction of AKI was 0.93. Urinary L-FABP levels were significantly higher in poor outcome patient, defined as renal replacement therapy or the composite end point of death. L-FABP in septic shock and AKI patients reported at admission was significantly higher in the non-survivors than in the survivors with an AUC for mortality prediction of 0.99.[18] Thus it is emerging as a promising urinary biomarker of AKI, even though additional studies are needed to determine the utility of L-FABP in AKI especially in the setting of ischemia, nephrotoxin exposure, and sepsis.[5] 

Interleukin – 18:

Interleukin-18 (IL-18) is a cytokine of 23 kDa molecular weight, synthesized as an inactive precursor by several tissues including monocytes, macrophages and proximal tubular epithelial cells.[5,41] It is a pro-inflammatory cytokine produced in the proximal tubule as an important mediator in the process of AKI and easily detected in the urine after ischemic AKI in animal models.[3,37,43] Animal studies reported that IL-18 is a mediator of acute tubular necrosis, inducing both neutrophil and monocyte inflammation of the renal parenchyma.[41] The intracellular protease caspase-1 converts the cytokines interleukin 1b and IL-18 to their active forms. IL-18 then enters the urine. Parikh et al reported that IL-18 was an early marker for AKI in patients with acute respiratory distress syndrome.[4] In AKI patients the cross-sectional study reported that urine IL-18 levels were markedly elevated, but not in patients with urinary tract infection, chronic kidney disease, nephrotic syndrome or perennial azotemia.[43] Urinary IL-18 levels greater than 100pg/mg were predictive of the development of AKI 24 hours before the levels of creatinine increased. [4]. In kidney transplantation, urine IL-18 is a predictive biomarker for delayed graft function.[43]  Raise in IL-18 level in urine is able to predict the complication about 2days prior to rise in serum creatinine in AKI patients in Pediatric intensive care setting. This represent the Urine IL-18 measurements helpful in daignising the AKI patients and also regarded as early biomarkers of AKI . IL-18 levels are influenced by a number of coexisting variables, such as endotoxemia and cisplatin toxicity. IL-18 levels increase in various patho-physiologic states including inflammatory arthritis, inflammatory bowel disease, and systemic lupus erythematosus.[43] In cardiac surgery, IL-18 levels increased within 4 hrs in children with AKI though there was no correlation to AKI in adults. The strong correlation between IL-18 and CPB duration tells that's IL-18 is more a nonspecific inflammatory marker than a specific marker of AKI. IL-18 can be measured by ELISA method [5] 

NETRIN-1:

Netrins are molecules that belong to the laminin-related family of axon-guidance molecules. Apart from axonal guidance they play role in development of mammary gland, lung, pancreas, blood vessels, and inhibition of leukocyte migration during sepsis, mitogenesis and chemo attraction of endothelial cells.[43-45] Four Netrins have been identified till date in different species, expressed in tissues other than nervous system and its highest expression was found in kidneys.[45] Netrin-1, 2, 4 are the few forms found in mouse. Netrins mediate their effects through two receptors namely, DCC (deleted in colon cancer) and UNC5 (…..) all expressed in normal kidney.[45] In one of the early studies conducted by Ganesan et al ( ), it was demonstrated that urinary excretion of netrin-1 is an early prognostic biomarker of human AKI. In patients undergoing CPB, subjects who developed AKI, urinary Netrin-1 levels increased significantly within first 2 hours and peaked at 6 hours after surgery and remained significantly elevated

until 48 hours after injury.[44, 46]

Schematic of the predicted time course of change in biomarker levels in AKI after cardiac surgery in adults. KIM-1: urinary kidney injury molecule-1; NGAL: urinary neutrophil gelatinase–associated lipocalin. (McIlroy et al.41)

Emerging biomarkers for AKI:

Several other candidate biomarkers for AKI have been identified, including pro Atrial Natriuretic Peptide, neutrophil CD11b and IL-6, -8, and -10 in serum as well as matrix metalloproteinase-9, multiple forms of glutathione-S-transferase, microglobulins and retinol binding protein. Hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), chemokine interferon inducible protein 10 (IP-10; CXCL10) also have been reported to be useful in the management of AKI. On the other hand, studies on these biomarkers are small and restricted to limited patient cohorts , and the role of these biomarkers is unknown whether these take part in detecting and monitorinf the AKI in future.

S. No

Biomarker Name

M.Weight

Role

Variations

Sample Source

Detection Method

1

NGAL

25 kDa

Up regulated expression in proximal tubules of kidney and urine after ischemia

Mouse, rat, humans

Serum/ Urine

ELISA

2

KIM-1

90 kDa

Detected in urine following acute kidney injury in clinical and preclinical studies

Zebra fish, rat, mouse, dog, monkey, human

Urine

ELISA, Luminex-based assay

3

IL-18

25 kDa

Cytokine with broad properties

Mouse, rat, humans

Urine

ELISA, Luminex-based assay

4

Cys-C

13 kDa

Elevated urinary levels reflect tubular dysfunction, may predict poorer outcome

various anthropometric measures, inflammatory

processes, corticosteroids and changes in thyroid

function

Urine

ELISA, nephlometry

5

L-FABP

15 kDa

Expressed in proximal tubule epithelial cells and a biomarker in CKD and Diabetic nephropathy

Mouse, rat, humans

Urine

ELISA

6

NETRIN

75 kDa

Play role in development of mammary gland, lung, pancreas, blood vessels, inhibition of leukocyte migration during sepsis, mitogenesis and chemo attraction of endothelial cells. Expressed highly in kidney.

Mouse, rat, humans

Urine

Western Blotting, Immuno fluorescence, immuno precipitation, ELISA,

Table 2: Summary of biomarkers for AKI*

CONCLUSION:

The purpose of this review was to emphasize the urgent need for an injury biomarker for the management of AKI, to update the information on biomarkers which are at various stages of development and their relevance to AKI. We have also enlisted the limitations and consequences of using creatinine as a marker for AKI. Early intervention in course of AKI will decrease the need for RRT and also reduce the morbidity and mortality. Highly sensitive and specific biomarkers like the Troponins have enabled diagnosis of myocardial damage within a few hours in patients of MI/myocardial ischemia. One of the most common causes of AKI is renal ischemia and if detected immediately could save the kidneys and the patient. The troponins provide an excellent window for early diagnosis, assessment of severity and also for monitoring the recovery of the myocardium. On the other hand for AKI, we have only serum creatinine which reflects loss of kidney function after 48 h of injury (when > 50% kidney function is already lost) as an approved marker. Is there a putative Troponin for AKI and is there a biomarker which will correlate with the extent of injury and recovery of the tubules? Among the several biomarkers which have been investigated and reported to be useful in the management of AKI, only NGAL in urine and to some extend in serum has been extensively investigated as a biomarker for the total management of AKI. Whether, it will emerge as the ‘troponin of kidneys’ remains to be seen. Incidence of AKI during Cardio Pulmonary Bypass (CPB) is well documented and recently a temporal relationship47 (Krawczeski CD, Goldstein SL, Woo JG, Yang YH, Piyaphanee N, Ma Q,Bennett M, Devarajan P: Temporal relationship and predictive value of urinary acute kidney injury biomarkers after pediatric cardiopulmonary bypass. J Am Coll Cardiol 2011, 58:2301-2309) was noticed among four biomarkers, NGAL elevated at 2 hours, IL-18 and LFABP elevated at 6 hours and KIM-1 elevated at 12 hours in patients who developed AKI after CPB initiation. Considering the structural complexities of kidneys and the pathophysiology of AKI, it seems likely that more than one biomarker will be needed to form a "Kidney Biomarker Panel" consisting of these four biomarkers for AKI, in which depending on the time of sampling the urine levels of any one of them would aid in the clinical diagnosis of AKI prior to changes in kidney function, in timing the injury and also in assessing the duration and thereafter for monitored for recovery of the patient, based on the available performance characteristics so far. Urine NGAL appears to have potential to be a troponin for AKI. NGAL has successfully cleared the preclinical and initial clinical testing stages in the process of the biomarker development for AKI. It is being validated across the world in large cohorts and across different laboratories geographically. This biomarker also has the potential to risk profile kidney transplant patients about "delayed graft function" and hence rejection. The concept of "renal angina" is emerging. We have reports of cases who presented elevated AKI biomarker but with normal serum creatinine with a poor prognosis of classical functional AKI29, suggesting that we may need to re-visit and refine the definition of AKI based on these modern biomarkers.

Since they represent tandem biomarkers, it is likely that the AKI panels will be useful for timing the initial insult and assessing the duration and severity of AKI (analogous to the cardiac panel for assessing chest pain). Depending on the expression of biomarkers , AKI panels helps to distinguish between the various types and etiologies of AKI, and predict clinical outcomes.