Association of Renal Structural Parameters on Ultrasound with Estimated Glomerular Filtration Rate in Canine Chronic Kidney Disease

Abstract

Many human studies have investigated the relationship between estimated glomerular filtration rate (eGFR) and renal structural parameters, including renal length (RL), renal cortical thickness (RCT), or renal cortical echogenicity (RCE), using ultrasonography (US). Currently, no studies have quantitatively evaluated canine renal structural parameters using US compared with eGFR for application in animals. Because the distinction between the renal cortex and outer medulla (OM) is clear in normal canine kidneys using US, the author hypothesized that the degree of differentiation between the cortex and OM would be related to renal function. It was also hypothesized that renal structural parameters estimated using US in canine chronic kidney disease (CKD) would have a linear relationship with eGFR

similar to results of human studies. The present study proposed methods for evaluating RCE using canine kidney characteristics via US (rather than through comparisons with other organs) and discussed the clinical significance of US parameters of the renal structure in canine CKD patients. Symmetric dimethylarginine (SDMA) and serum creatinine concentrations were measured in all dogs

kidneys were evaluated using US within 1 month of the SDMA and creatinine concentration measurements. The US digital imaging and communications in medicine (DICOM) images were evaluated and RL, RCT, OM thickness (OMT), parenchymal thickness (PT

RCT + OMT), and RCE grade were measured. RCE was classified according to the degree of distinction of the renal cortex and OM. The measurements of RL, RCT, OMT, and PT were divided by body surface area for standardization (RLS, RCTS, OMTS, and PTS, respectively). All patients were classified into the normal kidney function (N) group or the CKD (C) group using the medical recording charts. Group C was subdivided according to whether the cortex and OM were distinct or not (C-1 and C-2, respectively). The differences in eGFR between group N and C-1 and C-2 and the tendency of the structural parameters (RLS, RCTS, OMTS, and PTS) in group C were analyzed based on eGFR using SDMA and creatinine measurements. A significant difference was observed in eGFR between groups N and C (p < 0.0001). The eGFR tended to decline from group N to C-1 and C-2 (p < 0.0001, St. J-T Statistic: −4.659). As the mean RCE grade (the mean RCE grades of the left and right kidneys) increased, the proportion of cases in group C tended to increase significantly (p < 0.0001). The mean RCE grade was negatively correlated with eGFR (p < 0.001). Positive correlations were also observed between RLS and eGFR and between PTS and eGFR in group C (p < 0.05 and < 0.02, respectively). The RCE grade classified by the degree of distinction of the renal cortex and OM can be used as an objective method for evaluating RCE. Moreover, similar to human studies, renal structural parameters measured using US, including PTS and RLS, have a linear relationship with eGFR in canine CKD patients.