Our results confirm that urine NGAL, IL-18, L-FABP and KIM-1 are predictors of post-cardiac surgery AKI. To our knowledge, the current study is the first to 1) establish a temporal pattern of biomarker elevation after CPB and 2) demonstrate the utility of biomarker combinations for the improved prediction of AKI beyond clinical models. These findings could enhance the potential for appropriately timed application of therapeutic interventions. Additionally, these biomarkers offer severity and prognostic information at early time-points.
The etiology of AKI following CPB is multi-factorial and incompletely understood. Since many pathways are involved, it is not surprising that combinations of biomarkers with different properties may prove most predictive. In this study, we found that a clinical model of age and CPB time could predict AKI with reasonable certainty. The addition of biomarkers, however, particularly in combination, significantly increased the predictive value of the model. Thus, a panel of strategically selected and rapidly testable biomarkers may prove optimal for early diagnosis and institution of treatment.
We studied four biomarkers as candidates for a sequential AKI panel. Urine IL-18, NGAL, L-FABP and KIM-1 are all up-regulated and released by the kidney tubules during injury and are biological intermediates in the causal mechanisms of ischemia-reperfusion injury to the kidney. These biomarkers are present at low concentrations pre-operatively, and their levels increase by several fold in patients who develop kidney injury. IL-18, a mediator of inflammation, is produced by proximal tubules, and is activated by caspase-1
following AKI (20
). It is more specific to ischemic AKI and is not significantly affected by nephrotoxins or chronic kidney disease. The gene for NGAL is significantly up-regulated in the kidney after ischemic and nephrotoxic injury, and the protein is over-expressed in distal tubule cells (21
). NGAL may play a primary role in renal tubular survival and recovery, and has been used therapeutically in ischemia-reperfusion injury in animal models (22
). Recent studies using an NGAL reporter mouse have established that the kidney is the primary source of urinary NGAL in AKI (23
). L-FABP is an inflammatory proximal tubular protein that is upregulated following a variety of acute kidney injuries (9
). KIM-1 is a trans-membrane protein that is over-expressed in de-differentiated proximal tubule cells after ischemic or toxic injury (10
We found urine NGAL concentrations increased at the earliest time-point after CPB and were associated with the highest predictive value. We do note that, in some studies, NGAL has not performed as well (26
). We hypothesize that this may be related to the confounding disease states (such as diabetes, lung disease, inflammation) that may be more prevalent in adults and may affect NGAL concentrations. This further highlights the need for a comprehensive “panel” of biomarkers, optimizing both sensitivity and specificity. Notably, our results demonstrate that combinations of biomarkers provide additional predictive value. Interestingly, we found small (but significantly) higher pre-operative NGAL levels in patients who developed post-CPB AKI. It is possible that some of our patients had subclinical pre-operative AKI, which may have predisposed them to development of AKI.
With the exception of IL-18 in the RIFLE “F” patients, all biomarker concentrations increased in a step-wise fashion with worsening kidney injury. It is unclear why IL-18 did not increase in the same pattern in RIFLE “F” patients, particularly since it has performed well in other studies (8
). It is possible that this is due to the small number of “F” patients analyzed but further evaluation in larger studies is warranted.
The importance of determining the temporal sequence of the biomarkers is underscored by the fact that the course of experimental AKI proceeds in four phases: initiation, extension, maintenance, and recovery (27
). The initiation phase is the period during which initial exposure to the ischemic insult occurs, intracellular ATP depletion is profound, and generation of reactive oxygen molecules and labile iron is initiated. Vasodilator, ATP-donor, anti-oxidant, and iron chelation therapies may be especially effective during this phase, and the appearance of the earliest non-invasive biomarkers such as NGAL may be used to trigger such therapies. Prolongation of ischemia followed by reperfusion ushers in the extension phase. Tubules undergo reperfusion-mediated cell death, and the injured endothelial and epithelial cells amplify the inflammatory cascades. This phase probably represents a window of opportunity for early diagnosis with intermediate biomarkers such as L-FABP and IL-18, and active therapeutic intervention with anti-apoptotic and anti-inflammatory strategies. During the maintenance phase, both cell injury and regeneration occur simultaneously. Measures such as growth factors and stem cells that accelerate the endogenous regeneration processes, initiated by later biomarkers with high specificity such as KIM-1, may be most effective during this phase.
Our study has several strengths. First, we utilized a prospective cohort design and employed a rigorous protocol to collect specimens, followed by blinded measurements of biomarkers. Second, we enrolled a large, relatively homogenous cohort of subjects in whom the most proximate etiology for AKI would be CPB. The study design also allowed for the precise determination of the temporal rise in biomarkers following CPB.
This study does have important limitations. First, this remains a single center study which needs validation at the multi-center level. Second, our results may not be generalizable to adults undergoing CPB, or to the myriad other clinical scenarios that commonly lead to AKI in hospitalized patients. Third, SCr
levels were not measured at the same frequency as the urinary biomarkers examined, and the exact timing of SCr
rise in our study population is uncertain. Fourth, the definition of AKI was based on elevations in SCr
, making it very likely that we captured only those with greater than mild injury. Additionally, since patients with congenital heart disease often have decreased muscle mass, elevations in SCr
may reflect more injury than in a healthy population. Indeed, in a recent multicenter pooled analysis of 2322 critically ill children and adults with cardio-renal syndrome, 20% of patients had early elevations in NGAL but never developed increases in SCr
). Importantly, this sub-group of “NGAL-positive, SCr
-negative” subjects encountered a substantial increase in adverse outcomes, including mortality, dialysis requirement, ICU stay, and prolonged hospital stay. Thus, early biomarker measurements may identify patients with sub-clinical AKI who have an increased risk of adverse outcomes, even in the absence of SCr
This study is the first to demonstrate the temporal elevation and progression of clinically relevant and predictive biomarkers following AKI, and the first to demonstrate the enhanced prediction of AKI using biomarker combinations. The application of biomarker technology to create a bedside AKI “panel” could allow clinicians the ability to pinpoint timing of insult to the kidney and perhaps direct therapeutic interventions. Results of this study support the inclusion of urine NGAL, IL-18, L-FABP and KIM-1 in such a panel.