Study of renal resistive index and renal pulsatility index in non-azotaemic healthy dogs segmented by age group

Renal resistive index (RRI) and pulsatility index (RPI) are valuable tools for the early diagnosis and monitoring of acute and chronic kidney injury, yet age-specific reference values in non-azotemic healthy dogs have not been clearly defined. This study aimed to establish these reference values by evaluating 85 healthy, client-owned dogs, which were divided into five age groups (juvenile, young adult, adult, senior, and geriatric), with 17 dogs per group. Each dog underwent blood tests and physical examinations without abnormalities. Using a standardized protocol, a single operator measured RRI and RPI, revealing a moderate significant correlation between age and both indices (RRI ρ =.41, p < .001; RPI ρ =.40, p < .001). In the total sample, the mean RRI and RPI values were 0.613 ± 0.062 (95% confidence interval (95% CI): 0.600 - 0.627) and 1.144 ± 0.236 (95% CI: 1.093 - 1.195), respectively. In geriatric dogs, these values increased to 0.663 ± 0.064 (95% CI: 0.630 - 0.695) for RRI and 1.305 ± 0.263 (95% CI: 1.170 - 1.440) for RPI. Notably, significant differences in RRI and RPI were found between all groups of dogs younger than 11 years and the geriatric dog group (p = .0004 and p = .0007, respectively). The proposed reference values were 0.71 for RRI and 1.51 for RPI in dogs younger than 11 years old, compared to 0.79 and 1.83, respectively, in geriatric dogs. These findings demonstrate that RRI and RPI vary with age, with younger dogs showing lower values than older dogs, emphasizing the importance of age-specific thresholds to improve the accuracy of renal assessment in dogs.