Sensitivity and Specificity of a Single Emergency Department Measurement of Urinary Neutrophil GelatinaseAssociated Lipocalin for Diagnosing Acute Kidney Injury

Context Studies in surgical patients suggest that urinary neutrophil gelatinaseassociated lipocalin (NGAL) may be an effective test for diagnosing acute kidney injury. Contribution The authors measured markers of renal function in 635 consecutive adults who were admitted to the emergency department of 1 urban hospital; of these patients, 30 had a final clinical diagnosis of acute kidney injury. When urinary NGAL was at least 130 g/g creatinine, the test was highly discriminatory for acute kidney injury (sensitivity, 0.90; specificity, 0.995; positive likelihood ratio, 181.5; and negative likelihood ratio, 0.10). Caution Patients were from 1 setting in 1 hospital. Kidney diagnoses were clinical, not biopsy-proven. Implication Urinary NGAL is a promising test because of its large effect on the probability of acute kidney injury. The Editors Acute kidney injury is a common complication among ambulatory and hospitalized patients, and its incidence has increased by 11% in recent years (1). It is a rapidly progressive illness that independently predicts excess morbidity and mortality (24). Twenty percent to 60% of patients with acute kidney injury require dialysis (5), and mortality rates range from 15% in the community setting (2, 3) to 50% to 80% in the setting of multiorgan failure (6, 7) and more than 80% in the postoperative setting (8, 9). Less severe forms of acute kidney injury may also result in prolonged hospitalization (10). These characteristics contrast with those of other kidney diseases, such as chronic kidney disease, which is typified by an insidious decline in renal function and is usually nonprogressive during hospitalization. Acute kidney injury is also distinct from prerenal azotemia, a physiologic response of the kidney to various predisposing factors (volume depletion, diuretic use, reninangiotensin blockade) that promptly resolve on fluid administration, regimen modification, or amelioration of the extrakidney organ malfunction (11). It is critical to distinguish acute kidney injury from prerenal azotemia and chronic kidney disease at the time of patient presentation to rapidly manage associated illness. However, the initial measurement of serum creatinine, the standard marker of kidney function, does not distinguish acute kidney injury from prerenal azotemia (12) or chronic kidney disease. In addition, the initial measurement of serum creatinine cannot reflect the extent of injury because its accumulation always lags behind the insult (13). Even a large decline in glomerular filtration rate (GFR) may manifest as a small change in serum creatinine level, particularly in the initial 48 hours after acute kidney injury before steady-state equilibrium is reached (13, 14). Serum creatinine may also vary by age, race, sex, muscle mass, metabolism, nutritional status, comorbid conditions, hydration status, and medication use and consequently may not increase in proportion to the severity of the injury. As a result, the diagnosis of acute kidney injury currently requires measuring serum creatinine repeatedly and delaying maneuvers to prevent ongoing kidney damage, such as stopping use of nonsteroidal anti-inflammatory drugs, adjusting medication dosages, or correcting hemodynamic status. Even elevations of serum creatinine level that do not meet established criteria for acute kidney injury (15) are associated with excess mortality (16), prolonged hospitalization (17), functional decline (18), and greater financial costs (19), and this highlights the insensitivity of serum creatinine measurement as a diagnostic test. These limitations in the use of serum creatinine provide the rationale for the discovery of kidney proteins that are expressed at the onset of injury and are more sensitive and specific for the diagnosis of acute injury than current diagnostic tests. Neutrophil gelatinaseassociated lipocalin (NGAL) is secreted into the urine by the thick ascending limb of Henle and collecting ducts of the kidney (20, 21). At these sites, NGAL is likely to play a critical role in host defense by chelating ironsiderophore complexes that enhance microbial growth (2224) or mediate oxidative damage. In some segments of the nephron, NGAL may also recycle the iron complexes by endocytosis (25, 26). Urinary NGAL is expressed in proportion to the degree of acute injury (27), whereas in chronic kidney disease, urinary NGAL is expressed in patients with progressive but not stable kidney failure (22). Volume depletion or diuretics do not increase urinary NGAL levels in mice (28), again reflecting the specificity of NGAL for ongoing tubular damage. These observations suggest not only that urinary NGAL detects acute kidney injury but also that its degree of expression might distinguish among acute kidney injury, prerenal azotemia, and chronic kidney disease. In addition, because NGAL is detectable before the accumulation of serum creatinine (20, 27, 29), NGAL might be used to diagnose acute kidney injury at patient presentation even when changes in serum creatinine level are incipient. Because of the diagnostic ambiguities that occur with current tests at patient presentation, we conducted a prospective cohort study in an inner-city emergency department to determine the accuracy of urinary NGAL to identify acute kidney injury. We subsequently followed each participant's hospital course to determine the relationship among the presenting level of NGAL and other urinary proteins, serum creatinine level, and patient outcome. Our hypothesis was that a single measurement of urinary NGAL is superior to conventional and novel biomarkers in predicting acute kidney injury and its comorbid conditions. Methods This study was approved by the Columbia University Medical Center Institutional Review Board, and informed consent was obtained before enrollment. We recruited consecutive patients 18 years and older who visited the Columbia University Medical Center emergency department between 6 a.m. and 12 a.m. from March to August 2007. We obtained the first sample of donated urine and blood. We excluded 17 patients who were receiving hemodialysis and 230 patients without subsequent creatinine measurements from further analysis (Figure 1). Figure 1. Study flow diagram. Diagnosis of Kidney Disease We defined altered kidney function by age- and sex-based criteria: men and women between 18 and 50 years of age with a serum creatinine level greater than 106 mol/L (>1.2 mg/dL), men older than 50 years with a level greater than 88.4 mol/L (>1.0 mg/dL), and women with a level greater than 70 mol/L (>0.8 mg/dL). We estimated GFR by using the Modification of Diet and Renal Disease formula (30). We determined baseline kidney function for 509 patients from a retrospective analysis of serum creatinine, medical history, and demographic characteristics recorded in the Columbia University Medical Center electronic records for the 1 to 12 months before admission. A prospective analysis of kidney function after hospital admission consisted of daily serum chemistry studies and renal ultrasonography. On the basis of the retrospective and prospective studies, a coordinator and an internist who were blinded to the experimental measurements independently assigned patients to 1 of 4 diagnostic categories (normal kidney function, nonprogressive chronic kidney disease, prerenal azotemia, or acute kidney injury); a nephrologist adjudicated the 24 cases of disagreement. We defined normal kidney function as a baseline estimated GFR greater than 60 mL/min per 1.73 m2 and no transient or sustained increases in serum creatinine level or decreases in estimated GFR during the patient's stay in the hospital. We defined nonprogressive chronic kidney disease as a sustained and unchanging (<25% change from baseline) increase in serum creatinine level that met our criteria for altered kidney function and persisted for more than 3 months before hospitalization, reflecting a stably reduced estimated GFR of less than 60 mL/min per 1.73 m2 that is consistent with the National Kidney Foundation's Kidney Disease Outcomes Quality Initiative definition of chronic kidney disease (31). In the absence of retrospective data, presumptive nonprogressive chronic kidney disease was diagnosed if patients had a sustained elevated serum creatinine level that did not change during hospitalization and reflected a stable estimated GFR of less than 60 mL/min per 1.73 m2 despite volume resuscitation. We defined prerenal azotemia as a new-onset increase in serum creatinine level that satisfied our criteria for altered kidney function and either resolved within 3 days with treatment aimed at restoring perfusion, such as intravenous volume repletion or discontinuation of diuretics (in the setting of historical and laboratory data suggesting decreased renal perfusion), or was accompanied by fractional excretion of sodium less than 1% at presentation. We defined acute kidney injury as a new-onset 1.5-fold increase in serum creatinine level or a 25% decrease in estimated GFR from baseline values that satisfied minimal RIFLE (risk for kidney dysfunction, injury to the kidney, failure of kidney function, loss of kidney function, and end-stage kidney disease) criteria for serum creatinine and GFR that was sustained for at least 3 days despite volume resuscitation. The RIFLE criteria provide a meaningful way to stratify patients at different stages of kidney failure on the basis of severity (risk, injury, failure) and outcome (loss and end-stage disease) (Appendix Table) (15). Appendix Table. The RIFLE Criteria We prospectively identified patient outcomes (nephrology consultation, intensive care admission, dialysis initiation, mortality) from electronic medical records. Laboratory Measurements We centrifuged the urine samples at 12000 rpm for 10 minutes and stored the supernatants at 80C. Urinary NGAL (10 L) was quantified by immunoblots with nonreducing 4% to 20% gradient polyacrylamide gels (Bio-Rad Labora