The findings in this study describe the unbiased identification, initial testing, and systematic validation of three urinary biomarkers of AKI after cardiopulmonary bypass, elucidated through recently established phases of the diagnostic test development process (16
). The three biomarkers demonstrated excellent separation at all post-operative time points between patients with and without AKI. When accuracy was assessed by receiver-operating characteristic curves, these biomarkers performed in the very good (for albumin) to excellent (for α1
-microglobulin and α1
-acid glycoprotein) range. The biomarkers also demonstrated excellent prognostic potential such that increasing quartiles of biomarker concentrations were associated with duration of AKI and length of hospital stay.
We employed a temporally predictable human model of AKI to uncover and validate these biomarkers, namely subjects undergoing cardiopulmonary bypass, the most frequent major surgical procedure performed in hospitals worldwide. AKI requiring dialysis represents the strongest independent risk factor for death in these patients (19
). Even a minor degree of post-operative AKI as manifest by only a 0.2–0.3 mg/dl rise in serum creatinine from baseline is associated with a significant increase in mortality after cardiac surgery in adult cohorts (20
). Additionally, AKI after adult cardiac surgery is associated with adverse outcomes such as prolonged intensive care and hospital stay, dialysis dependency, and increased long-term mortality (21
). Children with congenital heart diseases may be especially vulnerable to developing AKI, since many require multiple surgeries for step-by-step repair of complex congenital anomalies. Indeed, the incidence of AKI in the validation set was 37%, similar to that previously reported in pediatric cardiac surgery cohorts (11
-microglobulin (A1M) is a 27 kDa glycoprotein produced in the liver. It’s biological functions remain unclear (24
). It is freely filtered by the glomerulus and efficiently reabsorbed by the proximal tubule. A previous study (25
) has shown that in adult patients with established acute kidney injury, an increase in urinary A1M predicted subsequent requirement of renal replacement therapy, with an AUC of 0.86 (CI 78–0.92). In a study of adults presenting to the emergency department (26
), a single measurement of urinary A1M predicted the subsequent development of acute kidney injury, with an AUC of 0.89 (CI 0.84–0.93). A recent study of adults undergoing coronary artery bypass grafting revealed urinary A1M to be significantly elevated within 1 hour of initiating cardiopulmonary bypass in subjects who subsequently developed AKI (10
), but biomarker characteristics were not reported. The findings reported herein validate A1M as a promising urinary biomarker for the prediction of AKI and its outcomes after cardiopulmonary bypass. However, the reported presence of urinary A1M in subjects with chronic glomerular disorders (8
) as well in patients with sepsis (27
) may limit the utility of this biomarker.
-acid glycoprotein is a 43 kDa acute phase reactant glycoprotein synthesized primarily in the liver. It has two major biological functions – transport of endogenous substances and an immunomodulatory role (28
). Measurement of urinary α1
-acid glycoprotein in adults presenting to the emergency department (26
) predicted the subsequent development of acute kidney injury, with an AUC of 0.83 (CI 0.77–0.89). Our findings reported herein validate α1
-acid glycoprotein as an excellent urinary biomarker for the prediction of AKI and its clinical outcomes in the pediatric cardiac surgical population. However, increases in urinary α1
-acid glycoprotein concentrations have also been reported in nephrotic syndrome (8
), acute inflammation (28
), and type 2 diabetes with normal glomerular filtration rates (29
Microalbuminuria is a well known consequence of many acute conditions, even in the absence of kidney injury. Postulated mechanisms include acute changes in capillary permeability (30
) and an inflammatory insult (31
). Studies have shown the predictive role of microalbuminuria for the development of multi-organ failure and mortality in critically ill subjects (32
). The results reported here identify and validate the potential utility of microalbuminuria as a simple and widely available test for the early diagnosis and prognosis of AKI after cardiopulmonary bypass.
This study has several strengths. First, we employed a prospective cohort design of children undergoing cardiopulmonary bypass, with distinct testing and validation datasets. Our homogeneous cohort of pediatric subjects with normal baseline kidney function in whom the only obvious etiology for AKI would be the result of cardiopulmonary bypass comprises an ideal population for the initial discovery and validation of AKI biomarkers (34
). Second, the biomarkers themselves were uncovered using an unbiased, proteomic approach. Fortunately, all three identified biomarkers could be measured non-invasively by widely available clinical platforms, which accelerated the progress from discovery to validation.
This study also has important limitations. First, the SELDI-TOF MS proteomic technique has several shortcomings. It has limited sensitivity for the detection of proteins above the 10–20 kDa range, and other proteomic platforms will be necessary to explore the entire urine proteome in greater detail. SELDI-TOF MS is also only semi-quantitative, due to ion suppression effects that can confound mass-charge ratios. Second, our results will need to be validated in a larger randomized prospective trial, including adults undergoing cardiopulmonary bypass, in whom the biomarker performance may differ (23
). Third, ours was a cohort with normal kidney function at recruitment, and it will be important to confirm our findings in documented high-risk settings such as pre-existing kidney dysfunction, diabetes mellitus, and concomitant nephrotoxic drug use. Fourth, the definition of AKI was based on elevations in serum creatinine, which raises the conundrum of using a flawed outcome variable to analyze the performance of novel biomarkers. This study may have yielded different results had there been a true “gold standard” for AKI. Instead, using a change in serum creatinine potentially sets up the biomarkers for lack of accuracy due to either false positives (true tubular injury but no significant change in serum creatinine) or false negatives (absence of true tubular injury, but elevations in serum creatinine due to pre-renal or other causes).
Finally, simultaneous examination of other urinary biomarkers as potential predictors of AKI may provide additional information (5
). Recent studies have uncovered other AKI biomarkers such as neutrophil gelatinase-associated lipocalin (NGAL) (23
), interleukin 18 (IL-18) (38
) and kidney injury molecule 1 (KIM-1) (39
) in clinical cohorts similar to that employed in this study. A collection of strategically selected candidates, perhaps including the three biomarkers reported herein, may prove of value for early and rapid diagnosis of AKI. Experimental studies continue to reveal therapies such as growth factors, stem cells, anti-apoptotic, anti-inflammatory, and anti-oxidant approaches that are effective in early AKI (6
). It is anticipated that emerging biomarker panels will enable these promising agents to be investigated in humans with AKI, especially in temporally defined clinical situations such as subjects undergoing cardiopulmonary bypass.