Bladder shape impact on the accuracy of ultrasonic estimation of bladder volume

Objective: Variations in bladder shape can lead to errors in ultrasonic estimation of the bladder volume. The purposes of this study were to compare the accuracies of various formulas to estimate bladder volume from sonographic measurements and to assess the impact of bladder shape on the accuracy of bladder volume estimation. Study design: Twenty-four healthy volunteers (13 men, 11 women) and 55 spinal cord injury patients (48 men, 7 women) underwent ultrasonographic measurements of dimensions before and after voiding. Bladder shape was classified as cuboid, ellipsoid, or triangular prism. Ten formulas from the literature were applied to estimate bladder volume, using the volume voided or catheterized as the standard, and then linear regression was used to obtain optimal correction coefficients for the whole data set as well as each of the three bladder shapes Setting: Rehabilitation hospital affiliated with a medical college. Results: The most accurate of the 10 formulas tested was height (H) x transverse depth (Dt) x weight (W) x 0.7 (mean error 17.4% ± 11.6%). Linear regression analysis yield optimal correction coefficients of .72 for the whole data set and .89, .81, and .66 for cuboidal, ellipsoid, and triangular prism-shaped bladders, respectively. The mean error for the estimation of bladder volume using .72 as the correction coefficient was 16 9% ± 11.9% and decreased to 12.7% ± 10.1% (p < .0001, paired t test) when the bladder shape was taken into account. Conclusion: Using bladder shape for reference and applying the corresponding correction coefficient to volume calculations will improve the accuracy of the estimation.