Hyeonji Sim, Yoojin An, Sung-Soo Kim, Danbee Kwon, Jeongmin Lee, Kichang Lee, Hakyoung Yoon
Background
Kidney disease in cats, including acute kidney injury (AKI) and chronic kidney disease (CKD), leads to changes in renal morphology. Traditional ultrasonographic parameters, such as kidney length, are influenced by body size and may not accurately reflect disease severity. The renal cortical thickness (RCT), when normalized to aortic diameter (RCT:Ao ratio), has shown promise as a stable indicator of renal pathology in dogs, independent of body conformation. This study sought to establish normal reference ranges for the feline RCT:Ao ratio and assess its diagnostic value in distinguishing healthy, AKI, CKD, and acute-on-chronic kidney disease (ACKD) cases.
Methods
This retrospective multicenter study analyzed ultrasonographic data from 357 cats (152 normal, 171 CKD, 19 AKI, 15 ACKD) collected between 2021 and 2024 across five veterinary centers. Bilateral kidneys were imaged in the midsagittal plane to measure RCT and aortic diameter using standardized 12-MHz linear transducers. Cats were classified based on International Renal Interest Society (IRIS) criteria. Statistical analyses assessed relationships between RCT, body weight (BW), and body condition score (BCS), as well as intergroup differences in RCT:Ao ratios. Receiver operating characteristic (ROC) curves determined diagnostic cutoffs for disease detection.
Results
Normal feline RCT averaged 3.71 ± 0.53 mm, with a mean RCT:Ao ratio of 1.27 ± 0.15. RCT positively correlated with BW (p < 0.001) but not with BCS (p = 0.343), while the RCT:Ao ratio was unaffected by either factor. Significant differences in RCT:Ao ratio were found between all disease groups (p < 0.001), except between AKI and ACKD. CKD cats showed lower ratios (mean 1.05), whereas AKI cats showed higher ratios (mean 1.70). Optimal diagnostic cutoffs were 1.15 for CKD (75% sensitivity, 80% specificity) and 1.45 for AKI (90% sensitivity, 89% specificity). The ratio decreased progressively across CKD stages but plateaued between stages 3 and 4.
Limitations
The study was retrospective and unevenly distributed across BCS and CKD stages, particularly at advanced stages. The limited sample of ACKD cats reduced statistical power. Additionally, the high cortical echogenicity typical in feline kidneys might affect cortical boundary visualization, potentially influencing measurement accuracy. Lack of histopathologic correlation also restricts definitive validation.
Conclusions
The RCT:Ao ratio is a reliable, body size–independent ultrasonographic parameter for evaluating feline kidney disease. A reduced ratio suggests CKD, whereas an elevated ratio indicates AKI. Cutoff values of ≤1.15 and ≥1.45 enable effective differentiation between normal, CKD, and AKI kidneys. This metric offers a practical and noninvasive tool for early disease detection and staging, potentially improving diagnostic precision in feline nephrology.
(A) Measurements of the RCT in clinically normal kidneys without MRS and (B) with MRS; (C) the abdominal Ao diameter. Midsagittal plane (A) of the kidney showing two parallel hyperechoic cross-sectional pelvic diverticular lines (asterisks) was used. The three different shortest perpendicular distances (bidirectional arrows) from the trailing edge of hyperechoic renal capsule to the leading edge of the base of the renal pyramid were measured. (B) Thin hyperechoic lines (arrow) at the medulla were defined as MRS. The same measurement sites and methods were used for the kidneys without MRS. (C) Sagittal plane of the abdominal Ao. The aortic luminal diameter was measured from the trailing edge to the leading edge of the distance perpendicular to the vessel wall (bidirectional arrows), just posterior to the renal artery bifurcation (arrowhead), in the systolic phase. Ao, aorta; MRS, medullary rim sign; RCT, renal cortical thickness.
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