Routine Diagnosis of Acute Kidney Injury

In textbooks, acute kidney injury (AKI) is defined as an increase in serum creatinine above the upper reference limit in the absence of chronicity. Classically, AKI is diagnosed based on acute (i.e. less than 1 week) clinical signs (e.g. anorexia, lethargy, vomiting, diarrhea, polyuria/polydipsia) and laboratory abnormalities such as azotemia, isosthenuria or poorly concentrated urine (urine specific gravity <1.030 in dogs or <1.035 in cats), proteinuria and/or glucosuria. Abdominal palpation and medical imaging usually reveal normal to enlarged kidneys in contrast to the reduced kidney size which is typically found in chronic kidney disease (CKD). A grading scheme with grades I to V has been proposed by the International Renal Interest Society (IRIS) to classify dogs and cats with AKI based on serum creatinine and clinical parameters (e.g. volume responsiveness, urine output). Further subgrading can be done based on the presence of non-oliguria or oliguria-anuria and the need for renal replacement therapy.

Early Diagnosis of AKI

Acute kidney injury begins without clinical signs as an ischemic or nephrotoxic insult. This means that grade I AKI or nonazotemic AKI is challenging to diagnose using routine diagnostic tests. However, timely diagnosis shortly after the initial intrinsic injury and before the occurrence of more severe structural damage and functional alterations would facilitate more effective management and probably improve the prognosis. This is mainly essential to detect hospital-acquired AKI in critically ill dogs and cats (e.g., sepsis, pancreatitis, postoperative) that often have multiple risk factors for AKI. Using routine diagnostic tests, an increase in serum creatinine of ≥26.4 µmol/l (≥0.3 mg/dl) from baseline can indicate AKI even when serum creatinine remains within the reference interval. Because of the curvilinear relationship between serum creatinine and glomerular filtration rate (GFR), such small change in creatinine may represent an important decline in renal function. However, this approach is limited by the delay in serum creatinine elevation to 24–72 h after the initial insult or injury. Detection of glucosuria or cylindruria in routine urinalysis is reported to be neither sensitive nor specific for AKI. Estimation of the GFR might be helpful to identify mild functional decline but is rarely done in cats and dogs suspected of AKI, mainly because of practical and financial reasons. So, early diagnosis of AKI—in particular hospital-acquired AKI—continues to challenge clinicians and there is an urgent need for more sensitive biomarkers than serum creatinine to detect early AKI, ideally biomarkers that detect renal damage before GFR decreases.

Biomarkers to Diagnose AKI

An ideal biomarker for AKI should be able to detect kidney damage or dysfunction in an early stage, localize kidney injury, differentiate renal injury from pre-, post- and nonrenal injury and acute from chronic kidney disease, give indication on duration and severity of renal injury, provide a prognosis and monitor the effects of intervention. Additionally, biomarkers should be accurate, easy and inexpensive to measure and noninvasive. Because of the heterogenous and often multicausal nature of AKI, these goals will only be achieved by using a panel of biomarkers, monitored over time. Biomarkers to diagnose AKI can be categorized in functional biomarkers, damage biomarkers and pre-injury phase biomarkers.

Whether indirect markers for GFR such as serum symmetric dimethylarginine (SDMA) or serum cystatin C have a role in early AKI diagnosis in dogs and cats is still under investigation. SDMA has been shown to be increased in dogs with azotemic AKI (mostly grade III–V), but SDMA could not differentiate AKI from CKD and was not of prognostic value in a small number of critically ill dogs. In a recent study, SDMA appeared a moderate predictor for the development of AKI in hospitalized dogs with a sensitivity of 72% and specificity of 63% for a cutoff of 14 µg/dL.

Damage biomarkers have been the focus of small animal research regarding AKI diagnosis in the last decades. Because the proximal tubule and loop of Henle are predisposed to toxic and ischemic damage, low molecular weight and tubular proteins are the most promising urinary AKI biomarkers in dogs and cats. Inflammatory proteins that can be produced locally or systemically are also diagnostically valuable. To date, studies have mostly followed a targeted approach in which one or more specific biomarkers were investigated in contrast to the non-targeted approach such as metabolomics and proteomics that recently gained interest in other research domains. Also, the majority of research is on dogs or cats with azotemic AKI instead of nonazotemic, grade I AKI. The most studied damage biomarker in human and veterinary medicine is neutrophil gelatinase-associated lipocalin (NGAL). Similar to human medicine, urinary NGAL has been proven to be increased in dogs with AKI, also in cases with nonazotemic (IRIS grade I) AKI. The improved sensitivity compared to serum creatinine is an important advantage, but the specificity of uNGAL remains questionable and warrants further study. Other proximal tubular or inflammatory biomarkers that show potential to diagnose naturally occurring nonazotemic canine AKI are kidney injury molecule-1 (KIM-1), gamma-glutamyl transferase (GGT), retinol-binding protein (RBP), heat shock protein-70 (HSP70), N-acetyl-β-D-glucosaminidase (NAG) and interleukin-6 (IL-6), but many others were not yet evaluated in this target population. Research in cats is currently scarce, but urinary NGAL, KIM-1, heat shock protein-72 (HSP-72) and hemojuvelin are increased in cats with naturally occurring AKI, mostly azotemic AKI. In the last years, there is a paradigm shift in human research towards biomarkers that can detect “acute kidney stress”, which is the pre-injury phase occurring prior to renal damage or functional decline. Important examples are the cell cycle arrest biomarkers tissue inhibitor of metalloproteinase-2 (TIMP-2) and insulin-like growth factor-binding protein 7 (IGFBP7). Usually, these markers are evaluated together and in human medicine, this combination appears superior to previously described markers of AKI (e.g. NGAL). Other strategies that are under investigation in people are development-related molecules such as wingless-related integration site (Wnt) and the Dickkopf family of proteins (DKK).

Implementation of Biomarkers in Clinical Practice

At this moment, most biomarkers for AKI diagnosis in human and small animal medicine have not been implemented in clinical practice. Possible reasons are incomplete assay validation (i.e. analytical, biological, clinical validation) or assays that are not appropriate for clinical settings (e.g. expensive or time-consuming ELISA). Also, several biomarkers are not entirely specific for AKI or the specificity has not been thoroughly evaluated. Thirdly, many studies evaluated the diagnostic performance of biomarkers for nonazotemic AKI against healthy controls which might not be the most appropriate control group to detect hospital-acquired AKI. Fourthly, several studies are limited by the use of serum creatinine as a reference standard because GFR estimation or kidney biopsies are often not feasible in dogs or cats at risk for or with AKI. Before new biomarkers will become available in practice, large prospective studies in dogs and cats with various stages (including the nonazotemic stage) of AKI and with various control groups (e.g. critically ill dogs or cats without evidence of AKI) will need to prove the patient benefit of those biomarkers.

References

References available upon request.