Mouse kidney microarray analysis revealed NGAL as one of the most upregulated genes at early time points after AKI 7
. Downstream proteomic studies showed NGAL to be one of the most rapidly and robustly induced proteins in the kidney after ischemic or nephrotoxic AKI in animal models, and NGAL protein was easily detected in the blood and urine very early in the course of AKI 8, 9
. These findings generated several translational studies to evaluate NGAL as a novel biomarker in human AKI. In a prospective study of children undergoing cardiopulmonary bypass (CPB), the diagnosis of AKI (defined as a 50% increase in serum creatinine) was only possible 1–3 days after surgery 10
. In contrast, NGAL measurements by ELISA revealed a marked increase in the urine and plasma within 2–6 hours of the surgery in patients who subsequently developed AKI. Using a cut-off of 50 ng/ml, both urine and plasma NGAL were independent predictors of AKI, with an outstanding AUC of 0.99 for the 2 hour urine NGAL and 0.91 for the 2 hour plasma NGAL measurement 10
. The 2 hour urine NGAL level also represented an independent predictor of duration of AKI among cases 11
. In a subsequent study, urine NGAL measured at 4 hours after initiation of CPB in children showed an AUC of 1.000 12
. For an NGAL cut-point concentration of 100 ng/mg creatinine, both sensitivity (1.000) and specificity (1.000) were perfect for the prediction of AKI. Thus, NGAL has emerged as a sensitive, specific, and highly predictive early biomarker of AKI in the urine and plasma, after CPB in children.
NGAL has also been evaluated in pediatric kidney transplantation. Biopsies of kidneys obtained 1 hour after vascular anastomosis revealed a significant correlation between NGAL staining intensity and the subsequent development of delayed graft function 13
. In a prospective multicenter study of children and adults, urine NGAL levels in samples collected on the day of transplant predicted subsequent delayed graft function and dialysis requirement (which typically occurred 2–4 days later) with an AUC of 0.9 14
. In a retrospective study of kidney transplant patients undergoing either protocol biopsies or clinically indicated biopsies, urine NGAL measurements were found to be significantly increased in subjects with tubulitis or other tubular pathologies, suggesting NGAL as a non-invasive screening tool 15
Several investigators have examined the role of NGAL as a predictive biomarker of nephrotoxicity following contrast administration, with promising results. In a prospective study of children undergoing elective cardiac catheterization with contrast administration, both urine and plasma NGAL predicted contrast-induced nephropathy (defined as a 50% increase in serum creatinine from baseline) within 2 hours after contrast administration 16
. Using a cut-off value of 100 ng/ml, the AUC for prediction of contrast nephropathy was excellent for the 2 hour urine NGAL (0.92) as well as the 2 hour plasma NGAL (0.91). By multivariate analysis, the 2 hour NGAL concentrations in the urine and plasma were found to be powerful independent predictors of contrast nephropathy 16
Urine NGAL has also been shown to predict the severity of AKI and dialysis requirement in a multicenter study of children with diarrhea-associated hemolytic uremic syndrome, with high sensitivity but low specificity 17
. Using a cut-off of 200 ng/ml, NGAL in urine obtained soon after hospitalization was significantly increased in those children who subsequently developed severe AKI requiring dialysis. Urine NGAL measurements also represent early biomarkers of AKI in the pediatric intensive care setting, being able to predict this complication about 2 days prior to the rise in serum creatinine 18
. Early urine NGAL measurements were also predictive of duration of AKI as well as worsening of AKI in critically ill subjects. Thus, NGAL may represent an early diagnostic and prognostic AKI marker even in a heterogeneous group of children with unknown timing of kidney injury.
All results described thus far have been obtained using research-based assays. The availability of validated clinical tools for NGAL measurements could revolutionize renal diagnostics. In this regard, a standardized point-of-care kit is under development for the measurement of plasma NGAL (Triage® NGAL Device, Biosite Incorporated). The assay is easy to perform, with quantitative results available in15 minutes, and requires only microliter quantities of whole blood. In a pilot study with 120 children undergoing CPB, plasma NGAL levels measured by Triage® NGAL Device increased three-fold within 2 hours of CPB in those who subsequently developed AKI 19
. By multivariate analysis, plasma NGAL at 2 hours post-CPB was the most powerful independent predictor of AKI. For the 2 hour plasma NGAL measurement, the AUC was 0.96, sensitivity was 0.84, and the specificity was 0.94 for prediction of AKI using a cutoff value of 150 ng/ml. The early plasma NGAL levels strongly correlated with change in creatinine, duration of AKI, length of hospital stay, and mortality.
In addition, a urine NGAL immunoassay is being developed for a standardized clinical platform (ARCHITECT® analyzer, Abbott Diagnostics). This assay is also easy to perform, with a first result available within 35 minutes, and it requires only 150 µl of urine. In a pilot study of 196 children undergoing CPB, urine NGAL measured by ARCHITECT® analyzer increased 15-fold within 2 hours, in those who subsequently developed AKI 20
. For the 2 hour urine NGAL measurement, the AUC was 0.95, sensitivity was 0.82, and the specificity was 0.90 for prediction of AKI using a cutoff value of 100 mg/ml. The 2 hour urine NGAL levels highly correlated with severity of AKI, duration of AKI, length of hospital stay, dialysis requirement, and death.
In summary, NGAL is emerging as a center-stage player in the AKI field, as a novel predictive biomarker, for prominent inclusion in the urinary “AKI Biomarker Panel”. However, NGAL measurements may be influenced by a number of coexisting variables such as systemic infections, inflammatory conditions, and malignancies 21
. There is also an emerging literature suggesting that NGAL is a marker of chronic kidney disease severity 22