The cardiorenal syndrome (CRS) involves both kidney failure and heart failure, with the failing organ initially being either the heart or the kidney; usually one failing organ leads to the failure of the other. While use of biomarkers and imaging techniques can assess cardiovascular function, the assessment of renal injury and function is complex in CRS patients. This article discusses some of the novel markers for the assessment of renal function and injury.
Renal dysfunction is an independent and significant contributor to poor heart failure outcomes. Serum creatinine (SCr), the most frequently used marker for clinical assessment of renal dysfunction and injury, is at best a retrospective window. Glomerular filtration rate, GFR, considered the best overall measure of renal function (RF), is similarly affected by multiple variables, but novel biomarkers of renal injury and function are now available. These biomarkers also have limitations but they address gaps in the information provided from use of conventional biomarkers
A marker of renal function: cystatin-C
Cystatin C, an endogenous proteinase inhibitor of low molecular weight (13-kDa), possesses many features that make it attractive as a surrogate marker of RF and GFR. It is synthesised and released into plasma by all nucleated cells at a constant rate, is freely filtered by the glomerulus and completely reabsorbed by the proximal tubules. It can be easily measured in the serum and plasma without the need for a urine sample or complex equations. It is not affected by changes in body mass, nutrition, age or gender, making it potentially more beneficial in critically ill patients, elderly and children. It has been validated as a marker of GFR in patients with pre-existing renal dysfunction and acute kidney injury (AKI) as levels increase before SCr. In congestive heart failure (CHF) Cys-C is superior to SCr based estimates, which underestimate GFR. This appears to extend to acure decompensated heart failure (ADHF) admissions without advanced RF. Cys-C also reflects myocardial stress and damage, reflects more advanced left ventricular diastolic and right ventricular systolic dysfunction and is an independent predictor of long-term prognosis after adjusting for myocardial factors. The advantage of Cys-C over SCr appears greater and more conclusive for ruling in renal injury in affected patients. Other benefits include predicting future CV events in intermediate-risk individuals, mainly through the identification of those unlikely to develop events; it is a stronger predictor of adverse events than conventional measurement of RF and, in combination with cardiac troponin T and N-terminal–pro-brain natriuretic peptide, it improves risk stratification for CV mortality (inclusive of HF) beyond models of established risk factors.
Novel assessments of renal injury
The potential to attenuate or reverse renal injury is far less likely when renal dysfunction is already evident, at least based on current assessment methods. Several promising AKI biomarkers arenow available.
Neutrophil gelatinase-associated lipocalin
Human neutrophil gelatinase-associated lipocalin (NGAL) is a 25-kDa protein initially described to be bound to gelatinase in specific granules of the neutrophil, with recent evidence suggesting physiological activity in the kidney. It is expressed and secreted by immune cells, hepatocytes and renal tubular cells in various pathologic states. NGAL exerts bacteriostatic effects, which are explained by its ability to capture and deplete siderophores, small iron-binding molecules that are synthesised by certain bacteria as a means of iron acquisition and role in cell survival, inflammation and matrix degradation. NGAL is up regulated more than 10-fold in post ischaemic renal injury in a mouse model and secreted relatively early into the urine. Several recent studies in homogenous (adult and paediatric cardiac surgery), heterogeneous (intensive care and emergency department) and chronic kidney disease (CKD) populations have supported the use of NGAL as an important biomarker in early diagnosis and prediction of duration and severity of AKI. NGAL differentiates AKI from changes in GFR due to chronic disease progression, predicts duration of ICU stay and provides prognostic value. Specifically, a single urine level of NGAL in the emergency department differentiates AKI from normal function and from pre-renal azotaemia, and predicts poor in-patient outcome.
A recent multicentre pooled analysis of published data on 2322 critically ill children and adults with the cardiorenal syndrome revealed the surprising finding that approximately 20% of patients display early elevations in NGAL concentrations but never develop increases in serum creatinine. Importantly, this sub-group of ‘NGAL-positive creatinine-negative’ subjects encountered a substantial increase in adverse clinical outcomes, including mortality, dialysis requirement, ICU stay and overall hospital stay. Thus, early NGAL measurements can identify patients with sub-clinical AKI who have an increased risk of adverse outcomes, even in the absence of diagnostic increases in serum creatinine.
Among acute decompensated heart failure patients, high admission serum NGAL levels were associated with increased risk of worsening RF. In particular, patients with an NGAL of >140 ng/mL on admission had a 7.4-fold increased risk, with a sensitivity and specificity of 86% and 54%, respectively. NGAL values are also significantly increased and parallel the clinical severity of CHF. After a 2-year follow up, patients with baseline NGAL > 783 ng/mL had a significantly higher mortality. These findings may suggest that NGAL plays a pivotal role in the systemic adaptation to CHF. Elevated baseline serum levels in acute post-myocardial infarction and CHF correlated with clinical and neurohormonal deterioration and adverse outcomes. In a rat model of post-MI HF, NGAL/lipocalin-2 gene expression was increased in the non-ischaemic left ventricle segments, primarily located to cardiomyocytes. Strong NGAL immunostaining was found within cardiomyoctes in experimental and clinical HF. Furthermore interleukin-1β and agonists for toll-like receptors 2 and 4 were potent inducers of NGAL/lipocalin-2 in isolated neonatal cardiomyocytes supporting a role for the innate immune system in HF pathogensesis. Urinary NGAL also increases in parallel with the NYHA classes for HF and is also closely correlated with serum NGAL, Cys-C, SCr and eGFR. This suggests tubular damage may accompany renal dysfunction in CHF, which has prognostic consequences.
Evidence is continuing to accumulate and NGAL measurement appears to be of diagnostic and prognostic value. In a recent meta-analysis, NGAL levels predicted renal replacement therapy initiation and in-hospital mortality. Several recent studies showing the response of urinary levels to therapy suggest a future role for NGAL for follow up and monitoring the status and treatment of diverse renal diseases reflecting defects in the glomerular filtration barrier, proximal tubule reabsorption and distal nephrons. Thus the prospects of NGAL being used as a diagnostic tool, even beyond the realms of nephrology, are exciting but require further clinical research. The commercial availability of standardised clinical platforms for the accurate and rapid measurement of NGAL in the urine and plasma will facilitate future investigations as well as direct clinical applications.
Interleukin (IL)-18 is a proinflammatory cytokine which induces interferon-g production in T cells and natural killer cells. It is synthesised as a biologically inactive precursor, which requires cleavage into an active molecule by an intracellular cysteine protease similar to IL-1b. IL-18 is both a mediator and biomarker of ischaemic AKI. Several early studies demonstrate increases in patients with acute tubular necrosis, prerenal azotaemia, nephrotic syndrome, delayed graft function after renal transplantation, chronic renal insufficiency and urinary tract infections. In contrast nephropathy, cardiopulmonary bypass, critically ill children and kidney transplantation, urinary IL-18 rises two days earlier than SCr. Urine IL-18 increases four to six hours after cardiopulmonary bypass, peaks at over 25-fold at 12 hours, and remains elevated up to 48 hours later. IL-18 levels also predict graft recovery and need for dialysis up to three months later. There is also significant evidence that IL-18 contributes to clinical HF and other acute and chronic cardiovascular presentations. Presently there are no studies with patients with CRS.
Major concerns over IL-18 surround its discriminatory capacity and appropriate use. One concern is a spill over into the urine and its effects as a confounder, differentiating elevated cardiac as opposed to renal injury. Additionally, serum IL-18 may be increased in other disease states e.g. autoimmune disorders such as SLE, certain leukaemias, postoperative sepsis, chronic liver disease and acute coronary syndromes. On a positive note serum IL-18 levels were not different between those with and without AKI post paediatric cardiac surgery, and data suggesting its pathophysiological contribution to the renal damage observed during ischaemia/reperfusion are positive signs for its discriminatory values and causative effects in renal injury. Thus IL-18 appears to be a worthwhile addition to a biomarker panel in the assessment of AKI.
Kidney injury molecule-1
Kidney injury molecule-1 (KIM-1) is a transmembrane protein that is highly over expressed in proximal tubule cells after ischaemic or nephrotoxic AKI. Several studies have shown KIM-1 in urine and renal biopsy to be elevated from predominately ischaemic AKI and not from prerenal azotaemia, chronic renal disease, and contrast nephropathy. KIM-1 appears to play a role in the pathogenesis of tubular cell damage and repair in experimental and human kidney disease. KIM-1 is a sensitive marker for the presence of tubular damage. It is virtually undetectable in healthy kidney tissue, but tubular KIM-1 expression is strongly induced in acute and chronic kidney disease as well as transplant dysfunction, where it is significantly associated with tubulointerstitial damage and inflammation. Elevated urinary KIM-1 levels are strongly related to tubular KIM-1 expression in experimental and human renal disease, indicating that urinary KIM-1 is a very promising biomarker for the presence of tubulo-interstitial pathology and damage. Furthermore, urinary excretion of KIM-1 is an independent predictor of graft loss in renal transplant recipients, demonstrating its prognostic impact. Studies after cardiopulmonary bypass surgery have noted similar findings. KIM-1 also predicts adverse clinical outcomes in various forms of AKI. Data from non-diabetic proteinuric patients suggest that urinary excretion of KIM-1 may have the potential to guide renoprotective intervention therapy. KIM-1 could potentially provide additional prognostic data for tubular damage in CHF. Future studies will reveal whether the sensitive biomarker KIM-1 will become a therapeutic target itself. Kim-1/KIM-1 dipsticks can provide sensitive and accurate detection of Kim-1/KIM-1, thereby providing a rapid diagnostic assay for kidney damage and facilitating the rapid and early detection of kidney injury in preclinical and clinical studies. The primary limitation of KIM-1 is that time to peak is 12 to 24 hours after insult. While it may be of limited use as an early AKI biomarker, the ability to detect KIM-1 in urine makes it an attractive option, possibly in a biomarker panel togther with NGAL and IL-18.
The fatty acid-binding proteins (FABPs) are 15kDa cytoplasmic proteins. There are two types: heart type (H-FABP) located in the distal tubular cells and liver type (L-FABP) which is expressed in the proximal tubular cells. Both markers have been suggested as useful for the rapid detection and monitoring of renal injury. H-FABP has been tested as a marker for ischaemic injury in donor kidneys. L-FABP has been tested in progressive ESRD as well as renal injury post renal transplantation and cardiopulmonary bypass and more recently acute coronary syndromes. In patients undergoing PCI for unstable angina, urine L-FABP levels were significantly elevated after two and four hours and remained elevated for 48 hours. SCr did not change significantly during the study period. Among nondiabetic CKD patients, urine L-FABP levels correlated with urine protein and SCr levels. Notably, L-FABP levels were significantly higher in patients with mild CKD who progressed to more severe disease. Neither SCr nor urine protein differed between those same groups. H-FABP however is produced by myocardial damage and its clearance is determined by RF. The ratio of H-FABP to myoglobin after haemodialysis may be a useful marker for estimating cardiac damage and volume overload in haemodialysis. This may be an advantage for FABP in a panel of biomarkers to discrimnate background noise and cases where troponin levels can’t be interpreted. The main drawbacks are lack of evidence in the HF setting, small sample sizes in existing studies, and non-availability of a commercially usable assay. Additional longitudinal studies are needed to demonstrate the ability of L-FABP to predict AKI as well as CKD and its progression in cohorts with CKD of multiple aetiologies.
Each of these biomarkers has advantages and limitations. It will be a while yet before any of these biomarkers match serum troponin and act as a ‘standalone’ marker. However they do provide a safety mechanism initially to highlight anticipated risk, as well as additional information on likely renal pathophysiology. It may ultimately be that a panel of biomarkers is required. Candidates for inclusion are NGAL, IL-18, KIM-1, Cys C and L-FABP. The alternative may be selective use of markers when renal injury is anticipated, a strategy synonymous with acute coronary syndromes with serial cardiac enzymes, i.e.’serial renal enzymes’. Ultimately a point of care device would be ideal, and some kits are already in place. It is however clear that the learning paradigm is still ongoing. Future studies will need to validate these biomarker panels in a large heterogenous cohort.
The future of renal assessments in cardiovascular patients
Renal dysfunction is an independent and significant contributor to poor heart failure outcomes. Idiosyncracies in cardiorenal physiology and limitations of conventional diagnostic tools are factors in the poor prescribing of proven heart failure therapies in these patients. Novel biomarkers of renal injury and function are currently available. These biomarkers do have limitations but they address gaps in the information gained from conventional biomarkers i.e. improvements in injury chronology and functional accuracy. Significant limitations in how these markers are used as well as issues of availability and cost can only be addressed by further work. Future research studies should consider addressing these questions.
Abstracted with permission from Iyngkaran P et al. Cardiorenal syndrome – definition, classification and new perspective in diagnostics. Seminars in Nephrology 2012; 32: 3-17.