A New Ultrasound Cutoff for Detecting Feline Urethral Disease

Evaluation of Total Urethral Thickness Using Ultrasonography in Cats

Seungeun Lee; Yoojin An; Yoonju Choi; Sungsoo Kim; Kichang Lee; Hakyoung Yoon

Background

Urethral wall thickening is a potential indicator of pathological changes in the feline lower urinary tract; however, ultrasonographic reference ranges for total urethral thickness in cats have not previously been established. In clinical practice, invasive diagnostic methods such as biopsy or cystourethroscopy are rarely performed due to technical difficulty and risk, potentially leading to underdiagnosis of urethral disease. This study aimed to establish reference values for ultrasonographic total urethral thickness in clinically normal cats, evaluate the influence of demographic and physiological factors, compare measurements with cats showing lower urinary tract signs (LUTS), and determine a diagnostic cutoff value.

Methods

This retrospective multicenter study analyzed ultrasonographic images and medical records from 302 cats examined between April 2021 and May 2025. Total urethral thickness was measured on mid-sagittal ultrasound images at a standardized location cranial to the pelvic symphysis. Cats were classified as clinically normal (n = 240) or as presenting with LUTS (n = 62). Statistical analyses assessed associations between urethral thickness and breed, sex, neutering status, body weight, and bladder volume. Receiver operating characteristic (ROC) analysis with bootstrap validation was used to determine an optimal diagnostic cutoff, and intra- and interobserver reliability were evaluated using intraclass correlation coefficients.

Results

Clinically normal cats had a mean total urethral thickness of 2.20 ± 0.26 mm, with no significant effects of breed, sex, neutering status, body weight, or bladder volume. Cats with LUTS demonstrated significantly greater urethral thickness (2.75 ± 0.51 mm; p < 0.001). Multivariable regression identified LUTS status as the strongest independent predictor of increased urethral thickness. ROC analysis yielded an area under the curve of 0.859, with a bootstrap-corrected value of 0.858. A cutoff value of 2.49 mm provided 75.9% sensitivity and 88.2% specificity for distinguishing cats with LUTS from clinically normal cats. Measurement reliability was excellent for both intraobserver and interobserver assessments.

Limitations

Histopathological confirmation of urethral changes was not performed, limiting interpretation of the underlying causes of urethral thickening. Subclinical urinary tract disease in clinically normal cats could not be fully excluded. The intrapelvic urethra could not be evaluated due to acoustic shadowing, and the number of intact cats was small, reducing power to assess neutering effects. Measurements were derived from existing cine loops rather than independently reacquired scans, potentially inflating reliability estimates.

Conclusions

This study establishes ultrasonographic reference ranges for total urethral thickness in clinically normal cats and demonstrates that cats with LUTS have significantly increased urethral thickness. A threshold of 2.49 mm is proposed as a clinically useful screening cutoff for detecting urethral abnormalities. These findings support ultrasonographic measurement of urethral thickness as a reliable, non-invasive tool for the evaluation and differentiation of feline lower urinary tract disease.

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