Serum and Urinary Biomarkers of Acute Kidney Injury

Acute kidney injury (AKI) is a frequent clinical problem in critically ill patients and the associated mortality is high. Standard serum and urine biomarkers are insensitive and nonspecific for the detection of kidney injury in its early stages which limits the therapeutic options and may compromise the outcome. The study presents new candidates for biochemical markers of AKI, with potentially high sensitivity and specificity, causally related to its pathogenesis and development. Some of these biomarkers measured in serum or urine are well known in laboratory practice but have been used in other tests, while some novel biomarkers have been proposed as a result of experimental and clinical studies. In current clinical practice, identification and classification of AKI is based on elevations in serial serum creatinine concentrations, which are delayed and therefore unreliable in the acute setting. The most promising of the new serum AKI markers are cystatin C, neutrophil gelatinase-associated lipocalin and uric acid. Urinary AKI markers may be classified as enzymes released from damaged tubular cells (alkaline Received: May 28, 2009 Accepted: January 12, 2010 Published online: April 14, 2010 Dr. Barbara Lisowska-Myjak, PhD Department of Biochemistry and Clinical Chemistry Medical University of Warsaw, ul. Banacha 1 PL–02-097 Warsaw (Poland) Tel./Fax +48 22 57 20 735, E-Mail basia.myjak @ interia.pl © 2010 S. Karger AG, Basel 0253–5068/10/0294–0357$26.00/0 Accessible online at: www.karger.com/bpu D ow nl oa de d by : 54 .7 0. 40 .1 1 10 /6 /2 01 7 4: 09 :4 5 A M Lisowska-Myjak Blood Purif 2010;29:357–365 358 tensive care unit patients who develop AKI is 5-fold higher than in patients without such a complication [1–7] . The large variations in the reported incidence and mortality rates probably reflect the lack of a uniform definition for AKI. The Acute Dialysis Quality Initiative Working Group suggested standardizing the definition of AKI in adults, using a new system to classify the clinical symptoms of the condition and laboratory markers of kidney function [2, 8, 9] . The consensus classification RIFLE (risk, injury, failure, loss, and end-stage renal disease) defines three grades of severity [risk (class R), injury (class I) and failure (class F)], and two classes of clinical outcome (loss and end-stage kidney disease). The RIFLE criteria, based upon determinations of serum creatinine, are a useful way of assessing successive stages of insufficiency development, but creatinine measurements have a too low sensitivity for detecting early intrarenal lesions [1, 9, 10] . The devastating course of AKI explains the growing interest of clinicians in effective protection against the consequences of the disease, based upon its early detection and assessment of the clinical course and prognosis, allowing institution of effective treatment [11, 12] . A particularly acute diagnostic problem is the lack of biochemical markers for early kidney injury that are sensitive and easily applicable in clinical practice and may help to predict the development of AKI. Pathogenesis and Development of AKI AKI is characterized by a sudden reduction in the glomerular filtration rate (GFR) as well as slow and permanent loss of the most affected nephrons. The main cause is the acute apoptosis of renal tubular cells. In some 80% of AKI patients, necrosis of tubular cells is found, but not acute disease of glomeruli or parenchyma. In clinical practice, the terms AKI and acute tubular necrosis (ATN) are often mistakenly used as synonyms. The term ATN must be limited to the condition which is confirmed by renal biopsy and that is often impossible in critically ill patients [4, 5, 9, 12, 13] . Epidemiologic studies have revealed a wide variety of etiologic and risk factors for AKI and their association with chronic kidney disease. ATN may result from oxygen deficiency (e.g. shock, extensive surgical procedures on the major blood vessels and heart, kidney transplantation), septic or toxic kidney injury (e.g. by radiocontrast agents or antibacterial, antimycotic or cytotoxic drugs) [1, 4, 13–15] . Ideal Biomarker for Diagnosing AKI Over the last decade, numerous new markers, measured in blood and urine, satisfying specific requirements for diagnosing AKI have been presented [2, 4, 11, 13, 15, 16] . An ideal biomarker for AKI should: (1) be highly organ-specific and allow differentiation between intrarenal, prerenal and postrenal causes of AKI and acute renal glomerular injury. In clinical practice, the best available test as yet for the differentiation of intrarenal AKI from prerenal lesion has been the fractional excretion of filtered sodium and urinary sediment assessment. In prerenal lesion, the sediment is normal while in intrarenal injury, it contains epithelial cells from tubules, and granular, leukocyte and waxy casts; (2) allow recognition of the etiology of AKI (hypoxia, toxins, sepsis, or a combination of these factors); (3) correlate with the histological findings in kidney biopsy specimens; (4) be site-specific to detect early injury and identify pathological changes in various segments of renal tubules (the pathogenesis of AKI involves injuries affecting different sites within renal tubules); (5) correlate with the degree of tubular injury and have high sensitivity for early detection of minor changes and the onset of more severe damage. Such a marker should be detectable throughout the entire course of AKI with defined threshold values to assess progression and regression of renal injury. So far, there has been no good method for differentiating benign, mild, moderate, or severe renal dysfunction; (6) the investigation should be noninvasive, and (7) the relevant laboratory tests should be simple and quick to perform, accurate, reliable, inexpensive and commonly available allowing serial assessment of a large number of samples. Biomarkers of AKI Measured in Serum Nitrogenous compounds accumulated in blood, blood urea nitrogen (BUN) and serum creatinine are routinely used standard markers for early stages of AKI. In clinical practice, AKI is detected when serum creatinine concentrations increase over a short period of time, with or without oliguria. Measurements of markers for AKI in serum may be especially useful in patients with severe oliguria and potential changes in urinary biomarker concentraD ow nl oa de d by : 54 .7 0. 40 .1 1 10 /6 /2 01 7 4: 09 :4 5 A M Serum and Urinary Biomarkers of Acute Kidney Injury Blood Purif 2010;29:357–365 359 tions induced by hydration status and diuretic therapy [1, 2, 17] . However, increasing evidence from published studies indicates that the creatinine concentration is not a decisive marker in diagnosing AKI [1, 7, 10, 18, 19] : (1) elevated serum creatinine concentrations are not specific for AKI and require differentiation from other prerenal or extrarenal causes of azotemia; (2) serum creatinine concentrations are not specific for renal tubular lesions, pathogenetically related to AKI development, but seem to reflect the loss of glomerular filtration function, accompanying the development of AKI; (3) increases in serum creatinine are detected later than the actual GFR changes as creatinine accumulates over time; (4) serum creatinine is a poor marker of kidney dysfunction as changes in its concentrations are neither sensitive nor specific in response to slight GFR alterations and become apparent only when the kidneys have lost 50% of their functional capacity, and (5) changes in serum creatinine may be influenced by other factors, which are not directly related to kidney damage, such as age, sex, body mass, and hydration and nutritional status. Neutrophil gelatinase-associated lipocalin (NGAL), a 25-kDa protein covalently bound to gelatinase from human neutrophils and determined in serum, has emerged as a sensitive, specific and highly predictive early biomarker for AKI. Serum NGAL measurements predict AKI following cardiopulmonary bypass and radiocontrast agent administration and are a highly sensitive but nonspecific predictor of AKI in critically ill children with septic shock. In the early phases of AKI from diverse etiologies, NGAL accumulates within 2 distinct pools, namely a systemic and a renal pool. Additional contributions of this parameter to the systemic pool in AKI may derive from neutrophils, macrophages and other immune cells. Besides, a decrease in the GFR resulting from AKI would be expected to decrease the clearance of NGAL with further accumulation in the systemic pool. Serum NGAL concentrations greater than 25 g/l reliably predicted AKI in children [17, 20, 21] . Cystatin C, determined in serum, is an endogenous marker of kidney function, detected earlier than creatinine. It is both to help diagnose renal dysfunction and to identify progress of AKI. Cystatin C is a nonglycosylated protein, an endogenous inhibitor of cysteine proteinases, produced by all nucleated cells of the body and released into the bloodstream at a constant rate. Serum cystatin C concentrations are independent of age, sex, race, body mass and hydration level, and may be measured by a simple nephelometric method. A relatively high concentration in body fluids, low molecular weight (13.3 kDa), and positive charge of the protein molecule facilitate its free glomerular filtration into primary urine. It is completely reabsorbed and catabolized in proximal tubular cells. Due to the constant rate of its production, the assessment of serum cystatin C concentrations may be a marker of glomerular filtration, which is not affected by infection, liver disease or inflammation [2, 4, 22] . Serum concentrations of cystatin C may be used to detect renal dysfunction in critically ill patients with AKI 24–48 h earlier than creatinine measurements. However, cystatin C is not diagnostically specific for AKI, because it is an early marker of impaired glomerular filtration rather than of tubular lesion [4, 18] . Recently, many authors have proposed serum uric acid measurements for early detection and assessment of AKI development. Much is known about the association between hyperuricemia and acute urate nephropathy, but increased serum concentrations of uric acid have also been demonstrated in many other types of AKI. Since elevated serum uric acid concentrations result from its increased production (the effect caused by ischemia, for example) as well as decreased glomerular filtration which reduces its excretion with urine, it is believed that uric acid detected in serum may be a simple marker of imminent onset of AKI. The increased glomerular filtration of uric acid into primary urine results in supersaturation of intratubular fluid with uric acid, its crystallization with subsequent occlusion of the tubular lumen, and development of local kidney inflammatory response, with infiltration by inflammatory cells. Uric acid is not only a diagnostic marker, but also a biologically active indicator of intrarenal injury to the microvasculature, a potent regulator of endothelial NO levels, and inhibitor of proliferation and migration of epithelial cells, causing endothelial cell apoptosis and stimulation of the proinflammatory agents: monocyte chemoattractant protein 1 and C-reactive protein [23, 24] . Urine as Clinical Material for the Detection of