Early Diagnosis of Chronic Kidney Disease in Asymptomatic Cats - Association of Renal Doppler Resistive Index with Elevated Symmetric Dimethylarginine

Background: Chronic kidney disease (CKD) in cats often progresses silently, making early diagnosis challenging. Symmetric dimethylarginine (sSDMA) is a sensitive biomarker for early renal dysfunction, while the resistive index (RI), obtained via Doppler ultrasonography, reflects renal vascular resistance. Combining these tools may improve early CKD detection. This study aimed to evaluate the correlation of RI with sSDMA in asymptomatic cats. Materials, Methods & Results: A prospective, blinded cohort study assessed the correlation between renal RI and sSDMA in 101 clinically asymptomatic cats. Examinations were performed without sedation, with manual restraint, and approved by the Federal University of Paraná's Animal Ethics Committee. Triplex Doppler ultrasonography used a GE Logic 5 machine with a 7.5-12 MHz linear transducer. B-mode assessed renal morphology, followed by color and pulsed Doppler for vasculature and interlobar arteries. RI was automatically calculated from three waveforms per kidney. An RI ≥0.59 (left) or ≥0.54 (right) was considered elevated. Blood samples were collected from the jugular vein. Creatinine and sSDMA were measured at IDEXX Laboratories (São Paulo, Brazil) using liquid chromatography-mass spectrometry. Urine was collected via cystocentesis. sSDMA values ≥ 14 μg/dL were considered elevated. Cats were grouped by sSDMA levels. Statistical analysis used SPSS 21.0, with P < 0.05 set as significant. Reliable RI measurements were obtained in 91 cats; 10 were excluded due to poor waveform acquisition (e.g., patient movement, panting, poor corticomedullary definition). Mean RI was 0.61 ± 0.07 (left) and 0.62 ± 0.07 (right), with no significant difference. Increased RI was seen in 55.9% (47/84) of left kidneys and 86% (74/86) of right kidneys. Elevated sSDMA was detected in 14.9% (15/101) of cats. A statistically significant correlation was found between elevated RI in the right kidney and increased sSDMA (P = 0.001), but not the left (P = 0.246). Creatinine also positively correlated with elevated sSDMA (P = 0.022), supporting the link between renal hemodynamic changes and early biochemical markers. These results suggest RI, particularly in the right kidney, may be a useful non-invasive parameter for early CKD detection in cats, especially with sSDMA. Discussion: While renal ultrasonography is common in cats, few studies explore RI's diagnostic potential in asymptomatic animals. Doppler ultrasonography was feasible but sometimes limited by patient stress, obesity, and altered renal morphology, affecting waveform quality. No significant difference was found between right and left kidney RI values, though some cats showed inter-kidney discrepancies, possibly indicating unilateral disease. Mean RI values were normal but near early CKD values.The significant correlation between elevated RI in the right kidney and increased sSDMA was not observed in the left. The asymmetry is unclear, but extrarenal factors and comorbidities might influence both RI and sSDMA. Despite limitations, creatinine also correlated positively with sSDMA, supporting its reliability in early renal assessment. RI should not be a standalone CKD screening method but a complementary tool, especially with sSDMA. Bilateral assessment is recommended, and interpretation should be cautious with morphological abnormalities or poor patient cooperation. Overall, RI measurement is feasible in unsedated cats and may aid early detection of renal changes in subclinical disease. Keywords: feline, resistive index, doppler, kidney, SDMA.