Plasma neutrophil gelatinase-associated lipocalin for the prediction of acute kidney injury in acute heart failure

Acute kidney injury (AKI) is an increasingly common complication in hospitalized patients. Over the last decade the incidence of AKI has steadily increased [1]. Current estimates assume that annually about 17 million hospital admissions in the United States are complicated by AKI, resulting in additional health care costs of 10 billion dollars [2].

AKI occurring during acute heart failure (AHF) has been termed the cardiorenal syndrome Type I [3]. AHF patients developing AKI have been shown to have increased morbidity, mortality and treatment costs [4, 5]. Unfortunately, the accurate prediction of AKI is an unmet clinical need [6].

Neutrophil gelatinase-associated lipocalin (NGAL; also known as human neutrophil lipocalin, lipocalin-2, siderocalin, 24p3, or LCN2) is a small molecule that belongs to the well-defined superfamily of proteins called lipocalins [7]. Human NGAL was originally identified as a 25-kDa protein associated with purified gelatinase obtained from the supernatant of activated neutrophils, which normally are the main cellular source of circulating NGAL [7]. Subsequent studies have identified tubularly secreted NGAL as a novel and specific biomarker for the early detection of AKI in several settings including cardiac surgery [8, 9], contrast agent administration [10, 11], unselected patients in the emergency department [12] and in critically ill patients [13‐15].

As the main pathophysiological mechanisms of AKI in AHF as well as the timing of insults may differ from the settings mentioned above, it is unknown whether these promising initial findings can be extrapolated to AHF. Therefore, we aimed to examine NGAL levels in the prediction of AKI in patients with AHF.

In this investigation we examined the potential of plasma NGAL levels at admission and serially thereafter to predict the occurrence of AKI and WRF in 207 patients with AHF. There are several key findings in this study. First, in AHF patients, plasma NGAL levels are significantly correlated to renal indices at presentation but also to age and baseline renal function. However, plasma NGAL levels are not associated with the extent of hemodynamic cardiac stress as quantified by BNP, the extent of venous congestion or inflammation. Second and most importantly, we found that spot measurements of plasma NGAL at presentation failed to improve the prediction of AKI over creatinine measurement. While both markers were significantly higher in patients experiencing AKI, their accuracy for the prediction of AKI was only modest. Third, the combination of plasma NGAL levels and creatinine did not result in a higher accuracy than creatinine alone. Fourth, the accuracy of plasma NGAL was even lower in patients presenting with creatinine values within the normal range. Fifth, plasma NGAL levels did not show a clear trend preceding the occurrence of AKI. Consequently, serial measurements did not add to the diagnostic potential of plasma NGAL at presentation.

Our results represent the first larger scale study investigating the potential of plasma NGAL in the setting of AHF. The results of an earlier study enrolling 91 patients with AHF support our observations. Aghel and colleagues also found spot measurements of serum NGAL to be significantly higher in patients experiencing early AKI [18]. Furthermore, their study described comparable AUCs for serum NGAL and estimated glomerular filtration rate (eGFR) for the prediction of early AKI (0.7 and 0.61, respectively). Unfortunately, due to the small number of patients in their study no extensive multivariate analysis could be performed. Contrastingly, a very recent study by Alvelos and co-workers found spot measurements of NGAL to powerfully predict AKI in 114 hospitalized AHF patients (AUC 0.93)[19]. However, their study differs significantly from the present study and also the work presented by Aghel, since it selectively assessed the predictive potential of NGAL levels drawn on the first in-hospital day and only considered AKI occurring between day two and day three. Applying these criteria to our population led to the exclusion of 24 (40%) early AKI cases, explaining the lower incidence of AKI occurring in the Alvelos study (12% versus 29%). Additionally, using the Alvelos definition of early AKI in a secondary analysis, we found the predictive accuracy of NGAL levels at presentation to be comparable to our primary analysis (AUC 0.61) and were therefore not able to confirm their findings. It can only be speculated that the narrower and sub-acute observational period in the Alvelos study reflects a more specific, NGAL-detectable AKI pathology, while the more inclusive observational period in this study might cover a wider range of AKI triggers.

The results obtained for AHF patients differ from the results obtained in other clinical settings, such as after cardiac surgery [8, 9] and contrast agent administration [10, 11], in which NGAL was established as a promising early marker of AKI. However, contrast-induced nephropathy and post-surgical AKI are marked by a clear time of injury. In these settings NGAL has repeatedly been proven to increase significantly three to four hours after the insult, while creatinine only increases 48 to 72 hours later. In contrast, the pathophysiological mechanisms underlying renal dysfunction in patients with AHF appear to be multifactorial and not well defined. Overall, an imbalance between the failing heart, the neurohumoral system and inflammatory responses has been associated with the occurrence of renal dysfunction [20]. Additionally, hemodynamic factors including the adequacy of renal perfusion [21] and the degree of venous congestion [22, 23] as well as drug nephrotoxicity appear to contribute to the development of AKI in AHF. Hence, AKI in AHF might well be caused by serial insults making its prediction especially difficult. Similarly, two studies directly comparing the predictive potential of admission NGAL to admission creatinine [13] or clinical prediction models [15] in critically ill patients, found classical predictors of AKI to at least equal if not surpass the potential of NGAL. Hence, even in the setting of critically ill patients, NGAL levels might not be able to act as a stand-alone test [24, 25].

The significant correlation of admission NGAL levels with baseline creatinine levels observed in this study and the correlation of urinary NGAL levels with renal indices in stable chronic heart failure [26, 27] suggest that NGAL levels in AHF may reflect chronic structural tubular damage. This thesis is further supported by the stable NGAL levels over time in patients experiencing AKI in this study. A chronic structural tubular damage and persistently elevated NGAL levels appear to be widespread in the setting of heart failure [26] and elevated NGAL levels have been associated with increased short-term mortality [28, 29]. Consequently, the dampened kinetics of NGAL levels before the occurrence of AKI might contribute to the limited predictive potential of plasma NGAL in AHF.

Additionally, recent studies found circulating NGAL levels to be influenced by various extra-renal factors [30]. Systemically elevated NGAL levels have been observed in infectious diseases [7], primary and secondary anemia [31], various human cancers [30] and animal models of obesity [32]. In the setting of cardiovascular diseases, elevated NGAL levels were found in coronary plaques, the failing myocardium and in the circulation of heart failure patients [33]. The combined influence of this systemic NGAL pool might contribute to the limited predictive potential of plasma NGAL in AHF. The predictive potential of urinary NGAL, which appears to be less impacted by extra-renal factors, has not been selectively examined in AHF patients yet. However, a recent meta-analysis including patients from 19 studies showed comparable performances for plasma and urinary NGAL [34].

Potential limitations of the current study merit consideration. First, only Swiss centers participated in this study. However as baseline characteristics, treatment and the frequency of AKI rates were similar to those observed in large registry studies, we consider our results representative. Second, we did not include the urine output criteria of the AKIN classification into the definition and grading of AKI, as loop diuretics impacting urine were used in the majority of our patients and adequate urine output collection is especially difficult during the early in-hospital period marked by repeated changes of the care-team. However, these difficulties are shared by previously published studies assessing the predictive potential of NGAL in the non-ICU setting, which also opted to exclude the urine output criteria for AKI diagnosis [12, 18, 19]. Third, we cannot differentiate between 'pre-renal' and 'structural" disturbances of kidney function as the reason for AKI. However, independent of the underlying pathology, minor changes of renal function have repeatedly been associated with a worse patient outcome [35, 36]. We therefore believe that adequate prediction of AKI in AHF should be able to foresee these prognostically important creatinine changes. Fourth, the definition of AKI is based on serial measurements of serum creatinine, hence an incorporation bias [37, 38] might have favored presentation values of serum creatinine in predicting AKI in ROC curve and regression analyses. However, in the absence of a non-creatinine based definition of AKI, we feel that to provide tangible clinical benefit plasma NGAL would have to improve the early detection of AKI over the current gold standard. Finally, clinical parameters (most importantly chronic kidney disease) previously shown to predict in-hospital AKI (observational period presentation to discharge) [6, 39, 40] failed to independently predict early AKI in this study. These differences most likely represent the increased importance of acute parameters (serum creatinine incorporating community acquired AKI and chronic kidney disease) versus chronic parameters (chronic kidney disease, outpatient diuretic therapy) in the prediction of early versus late AKI. Nevertheless, these clinical parameters remained non-significantly associated with an increased occurrence of early AKI.

Plasma NGAL levels do not adequately predict AKI and WRF in patients with AHF. This lack of predictive power is probably caused by the multifactorial pathomechanism underlying the cardiorenal syndrome and chronically elevated NGAL levels caused by chronic tubular injury and extrarenal NGAL production in heart failure patients.