Biomarkers of kidney disease: potential utilities

(2018)

Gilad Segev, Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Israel

Despite advances in the management of kidney diseases, including the introduction of renal replacement therapies, the mortality rate among human and animal patients remains high. One of the speculated reasons for the high mortality is the late recognition of the disease and consequently the narrow window of opportunity for therapy. Therefore, early recognition of the disease, before overt renal failure is evident, is crucial to allow timely and thus potentially more effective therapy. The need for early diagnosis is further emphasized in veterinary medicine, because renal replacement therapies are not readily available.

Limitations of blood creatinine concentration

Despite the diagnostic advancements made in other medical fields (e.g., the use of biomarkers in cardiology), blood creatinine (Cr) is still being used as the marker for kidney function despite its multiple shortcomings. These include: 1) High variability among dog breeds, resulting in a wide reference range for the species. Consequently, Cr is not expected to rise above the reference range in most dog breeds until ~75% of nephrons become non-functional. 2) Cr is affected by extra-renal factors, particularly muscle mass, therefore lacks specificity. 3) Cr is a functional marker thus does not increase in kidney injury that is not accompanied by decreased glomerular filtration rate (GFR). 4) Cr does not represent the severity of the dysfunction until a steady state has been reached. Consequently, substantial changes in GFR at the early stages of kidney injury are associated with relatively small changes in Cr.

The above limitations of Cr are reflected by the findings of several studies indicating that small, and even transient, increases in Cr in human patients are detrimental. In one study, as little as 0.5 mg/dL (45 µmol/l) increase was associated with greater in-hospital mortality (Chertow et al., 2005). In another study, a transient increase in Cr for 1–3 days was also associated with increased odds ratio for in-hospital mortality (Uchino et al., 2010). Furthermore, even a small and transient increase in Cr in patients that were discharged from the hospital was associated with the need for chronic dialysis over the ensuing three years (Wald et al., 2009). These studies imply that relying on Cr as the only marker of kidney function does not provide all the information needed to accurately assess kidney function, to promote timely intervention, and to determine the prognosis.

Renal biomarkers – characteristics

In recent years, increasing research efforts have been undertaken to identify sensitive and specific renal biomarkers. A biomarker should meet multiple requirements to be considered an ideal marker, it should: 1) be detectable in urine and/or blood, 2) be detectable using methods that are readily available and cost effective, 3) be highly predictive of kidney injury (with high sensitivity and specificity), 4) provide information regarding the etiology, 5) provide information regarding the location of the injury (i.e., glomeruli and/or tubules), 6) reflect the severity of the injury, 7) indicate kidney injury and/or repair processes, 8) predict the likelihood of recovery.

It is yet to be determined when and if biomarkers can meet all of these requirements but it is unlikely that a single one will provide all this information. It is more likely that an array of biomarkers will be needed, each providing one piece of the puzzle. Because a full panel of biomarkers might be cost-prohibitive; a subset of biomarkers may have to be selected, each one providing unique and specific information that complements the others.

Potential utilities of biomarkers

Early diagnosis of kidney injury

Renal biomarkers have several potential advantages, early diagnosis being a major one (Yerramilli et al., 2016). It has been shown that kidney injury can be identified using sensitive biomarkers days before any increase in Cr is documented, thus measurement of biomarkers may direct the clinician's attention to ongoing kidney damage before there is measurable reduction in GFR (Palm et al., 2016). For example if a nephrotoxic drug is being administered, it would be more rational to monitor the patient using biomarkers capable of indicating kidney injury, rather than markers that can only indicate decreased kidney function. Once kidney injury is identified, the drug can be discontinued before kidney failure occurs. This might be expected to lead to a better outcome, since once kidney function decreases, recovery is expected to be slow and the patient might die before recovery has occurred.

Screening patients at risk for acute kidney injury

Biomarkers indicating active kidney injury are likely more sensitive than markers indicating decreased function (i.e. surrogates for GFR); therefore the former should be used to screen patients at high risk for kidney injury. It has been shown that the prevalence of acute kidney injury (AKI) in hospitalized dogs is relatively high when using human criteria for AKI (Thoen and Kerl, 2011). The human criteria and the International Renal Interest Society (IRIS) guidelines rely on changes in Cr concentration (or urine production) to define AKI, and therefore probably underestimate the true prevalence of AKI in hospitalized dogs and cats. Early recognition of kidney injury in hospitalized patients might allow identification, and potentially elimination, of the cause and might allow treatment to begin before the injury progresses to failure, improving the chance of a favorable outcome. A current limitation is the poor availability of commercial assays for assessment of candidate biomarkers, but the availability of useful assays is expected to increase in future years.

Differentiating upper and lower urinary tract infection

Neutrophil gelatinase-associated lipocalin (NGAL) concentration was shown to increase in a subset of dogs with apparently lower urinary tract disease, potentially indicating concurrent upper urinary tract involvement (Segev et al., 2013). Yet, an increase in urinary NGAL concentration might result from local inflammation within the lower urinary system, and not necessarily kidney damage, since NGAL originates also from neutrophils recruited as part of the local inflammatory process (Decavele et al., 2011). Recent unpublished data suggest that other, kidney-specific biomarkers are also increased in dogs with apparently lower urinary tract infection (UTI), indicating that some of these dogs have subclinical pyelonephritis and ongoing kidney injury that would go unnoticed based on current concepts. It is thus possible that in the absence of sensitive and specific tools to aid differentiating upper and lower UTI, some patients with apparent cystitis in fact sustain pyelonephritis, and are consequently being undertreated. With the growing availability of kidney-specific biomarkers, differentiation between upper and lower urinary tract infection may become easier when UTI is documented, and sequential changes in the biomarker concentrations could guide treatment.

Markers of chronic kidney disease progression

The traditional concept is that AKI and chronic kidney disease (CKD) represent two distinct processes of kidney damage: AKI represents rapidly progressing active damage due to various causes, whereas CKD represents slowly progressive damage. Lately it has been suggested that AKI and CKD might not be separate entities, as they are influenced by various similar conditions, share common risk factors and ultimately impact each other (AKI is a risk factor for CKD and vice versa) (Cowgill et al., 2016). Results of recent studies indicate that active kidney damage is present in a subset of dogs with apparently stable (based on Cr) CKD. This finding might account, at least in part, for the wide variation in progression rate of CKD among dogs and cats (Jacob et al., 2002; Ross et al., 2006). The presence of active kidney injury, demonstrated by increased concentrations of biomarkers, likely predicts risk for rapid progression of the disease, whereas absence of active kidney injury, probably predicts a more slowly progressive kidney disease. The documentation of ongoing active kidney damage in patients with CKD might suggest that the pathophysiology of AKI and CKD share more characteristics than currently recognized, and the main difference between them being the rate of disease progression (Cowgill et al., 2016).

Identification of markers of kidney damage in dogs and cats with CKD might also facilitate the diagnosis of IRIS CKD Stage 1, which is currently challenging with available tools. Early identification of the disease could allow early intervention aimed to preserve kidney function. Moreover, assessment of biomarkers indicating active kidney damage in animals with CKD would facilitate the investigation of novel therapeutic interventions. Active injury biomarkers could be monitored sequentially after the application of these therapies, in the same way alanine aminotransferase is used to assess efficacy of interventions during management of liver diseases. The currently available markers of kidney damage preclude such assessment, as short-term interventions are unlikely to alter kidney function, despite either benefit or harmful effects.

Candidate biomarkers

Symmetric dimethylated arginine (see separate article on SDMA within Emerging themes http://www.iris-kidney.com/emerging-theme/emerging_theme_index.html)

Symmetric dimethylated arginine (SDMA) is a methylated form of the amino acid arginine. Like creatinine, SDMA is a filtration marker, but it is not influenced by muscle mass and therefore its reference range is more uniform (Hall et al., 2014a; Hall et al., 2014b; Hall et al., 2015). Studies in dogs and cats reported earlier detection of CKD using SDMA than with Cr (Nabity et al., 2015; Yerramilli et al., 2016).

Retinol Binding Protein

Retinol binding protein (RBP) is a low molecular weight protein (21 kDa), synthesized by the liver and functions as a carrier protein for retinol (Vitamin A1). When unbound, it is freely filtered and then metabolized and reabsorbed by the proximal renal tubules, thus has been suggested as a marker of proximal tubular dysfunction. Several veterinary studies have evaluated the utility of RBP as an early marker of AKI. In a study of X-linked hereditary nephropathy, increased urinary RBP to creatinine ratio correlated with Cr and GFR. Moreover, RBP continued to increase as the disease progressed, while other biomarkers peaked and reached a plateau early in the course of disease (Nabity et al., 2012). Urinary RBP concentrations were assessed in dogs with pyometra as markers of proximal tubular function and were found to be were significantly increased compared with healthy controls (Maddens et al., 2011). In a study characterizing kidney damage during naturally occurring canine heatstroke, urinary RBP was also increased, often before any increase in Cr was documented (Segev et al., 2015).

Neutrophil gelatinase-associated lipocalin

Neutrophil gelatinase-associated lipocalin (NGAL) is one of the most studied biomarkers thus far in veterinary medicine (Cianciolo et al., 2016; Hokamp et al., 2016). It is a 25 kDa protein, originally discovered in the granules of neutrophils, and is released in response to bacterial infection (Mårtensson et al., 2012). In a prospective study evaluating NGAL as a marker of kidney injury in dogs, urinary NGAL to creatinine ratio (UNCR) was higher in dogs with AKI compared with other urinary tract diseases (CKD, UTI). Receiver operator characteristics analysis, performed to assess UNCR as a predictor of AKI had an area under the curve of 0.94 (Segev et al., 2013). The median UNCR of non-azotemic IRIS AKI Grade I dogs was significantly higher than in the other groups, but no different from azotemic AKI dogs (IRIS AKI Grades II and above), indicating that UNCR increases before Cr, and thus may be used as an early marker of the disease. In experimentally induced AKI, UNCR preceded the increase in Cr by approximately 7 days (Palm et al., 2016).

Despite being a very sensitive and early marker of kidney injury, NGAL's specificity is questionable, as it originates from multiple tissues as well as from neutrophils and therefore may increase during inflammation and other disease processes accompanied by recruitment of neutrophil.

Other possible candidate biomarkers

Serum inosine, urinary cystatin B and urinary clusterin, are some of the promising biomarkers currently under investigation (Yerramilli et al., 2016). These markers are alluring, as kidney specific assays are being developed to assess their concentration in dogs and cats. Therefore, they are expected to be more specific and encompass the potential to predict accurately active and ongoing injury processes. In addition to early detection of AKI, preliminary studies suggest their usefulness in documenting subclinical active kidney injury associated with concurrent urinary diseases or other systemic processes that may influence the kidney.

Conclusion

The development and availability of sensitive and specific kidney biomarkers in coming years will likely change some of the current approaches to diagnosis and therapy in veterinary nephrology. However, data regarding use of these agents in veterinary medicine is scarce at present and further research is warranted. Each of the biomarkers investigated so far has advantages and weaknesses and it is most likely that an array of biomarkers will be required to provide all the information required.

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