Comparison of estimations of urinary bladder volume in different scanning positions using 2D linear dimension formula and 3D bladder circumference tracing in client-owned cats

Urinary bladder volume (UBV) can be estimated using point-of-care ultrasound. The purpose of this study was to compare 2 UBV estimation methods, i.e., three-dimensional (3D) bladder circumference tracing and 2-dimensional (2D) linear bladder dimension formula, against actual bladder volumes in awake client-owned cats and identify the best scanning position for UBV estimations. Up to 3 paired sets of orthogonal longitudinal and transverse bladder ultrasound images were acquired by a trained clinician from 21 cats positioned in dorsal, right lateral, and left lateral recumbency. UBV estimation was performed with these images by 2 different observers using both methods. Actual bladder volumes were measured through urethral catheterization and compared to the estimated UBV using Lin's concordance correlation coefficient and Bland-Altman analyses. Considering all positions, both methods showed substantial strength-of-agreement with actual bladder volumes; the 3D bladder circumference method (rho(c) = 0.963, 95% CI: 0.952 to 0.974) with a significant median bias of -4.08 mL (P < 0.001, IQR -7.63 to -0.68 mL, LOA -48.55 to 21.75 mL) and the 2D linear dimension method (rho(c) = 0.974, 95% CI: 0.966 to 0.982) with a median bias of -0.82 mL (P = 0.686, IQR -3.89 to 4.05 mL, LOA -35.23 to 35.21 mL). Scanning in left lateral recumbency provided the strongest strengths-of-agreement and precision against actual bladder volumes for both methods. Regardless of scanning positions, the 2D linear dimension method is more accurate than the 3D bladder circumference method, although both methods are imprecise with increasing volumes and UBV assessment through urinary catheterization remains the gold standard.