US in feline renal disease

Comparison of ultrasonographic findings in cats with and without azotaemia

Christopher R. Lamb, Helen Dirrig, Stefano Cortellini

Background
Azotemia in cats can result from acute kidney injury (AKI) or chronic kidney disease (CKD), with various causes such as ureteral obstruction, nephrolithiasis, and pyelonephritis. Ultrasonography (US) is commonly used to assess renal abnormalities in azotemic and non-azotemic cats. However, the correlation between specific US findings and azotemia remains unclear, and the accuracy of subjective renal size assessment by radiologists has been questioned.

Methods
This retrospective case-control study analyzed renal US findings in 238 azotemic and 270 non-azotemic cats from a veterinary hospital database. Cats with pre-renal azotemia or urethral obstruction were excluded. US parameters—including renal size, echogenicity, renal pelvis dilation, and perinephric fluid—were extracted from archived images and reports. Statistical tests were performed to determine associations between US findings and azotemia.

Results
Azotemic cats had significantly lower body weight, body condition scores, and urine specific gravity compared to non-azotemic cats. The most strongly associated US finding was perinephric fluid (OR 26.4, 95% CI 3.4–207.7). Other findings significantly linked to azotemia included small kidneys, hyperechoic renal cortex, loss of corticomedullary differentiation, renal calculi, enlarged kidneys, and dilated renal pelvis. Subjective assessment of renal size was often inaccurate, with radiologists more likely to report abnormal kidney size in azotemic cats despite no objective differences in renal length.

Limitations
The study relied on retrospective data, limiting the ability to establish causation. The azotemic group included cats with varied underlying conditions, and specific diagnoses were not always confirmed. Subjective bias in US interpretation was noted, particularly in renal size assessment.

Conclusions
Perinephric fluid emerged as the US finding most strongly associated with azotemia and may be a more reliable indicator of renal dysfunction than renal size or echogenicity. Subjective assessments of renal size were prone to bias, highlighting the need for objective measurements in clinical practice. While US is valuable for evaluating individual cases, it is not recommended as a general screening tool for CKD due to the prevalence of US abnormalities in non-azotemic cats.

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