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King's Coll. Hosp., London, United Kingdom.

Introduction: Catheterisation provides the most accurate measurement of post-void urinary residual (PVR) (1). NICE (2) guidelines recommend using a bladder scan or catheter in patients to measure post void residual, but advises using a bladder scanner in preference, on the grounds of acceptability and fewer adverse events such as urinary tract infections. Despite having less morbidity, the bladder scanner can be inaccurate due to the shape and size of the bladder and also due to user error. There is no consensus regarding how to calculate bladder volume using ultrasound. The most widely used formula involves multiplying the height, width and depth of the bladder by a proportionality constant described by Hwang et al (3).
Objective: To test:

  1. different formulae to calculate the bladder volume using ultrasound and compare them with the actual bladder volume drained via a catheter
  2. the accuracy of patients hearing or feeling “bubbles” at the end of urodynamics testing as a measure of being empty
  3. how good we are at estimating post-void residuals using x-ray at the end of video urodynamics testing

Methods: This was a prospective cohort study of consecutive women who attended our urodynamics clinic between November 2016 - January 2017. All women underwent assessment including history, physical examination, urinalysis, uroflowmetry, videocystourethrography, pressure flow studies and PVR measurement with a catheter. Urodynamics were performed by trained doctors and specialist nurses in keeping with Good Urodynamic Practice. Each patient had a transabdominal bladder scan before uroflowmetry in the supine position using Sonosite®180 plus. The height, width and depth dimensions were obtained in the sagittal and transverse planes. Data were collected using a standardised proforma. Bladder volumes were calculated by:
height x width x depth x proportionality constant (0.52 or 0.625 or 0.65 or 0.7) (3)
After voiding during pressure flow studies, we asked whether the patient heard or felt “bubbles”, to test whether this is an accurate measure of being empty at the end of the investigation. Each patient was fluoroscopically screened to see if she had a residual. If she had, the clinician estimated the volume and an in/out catheter was passed to identify the true residual. Statistical analysis was performed using SPSS version 23.
Results: We assessed a total of 85 patients. All four formulae positively correlated significantly. The formula using 0.52 as the proportionality constant, correlated best (r = 0.938, p<0.001). This was also the only formula which did not show a significant difference between the calculated and actual volume (p = 0.275). Our results are shown in table 1. Hearing “bubbles” at the end of the urodynamics test had a sensitivity of 91%, specificity of 80%, positive predictive value of 93% and negative predictive value of 76%, in predicting an empty bladder. Comparing video estimation with true post void residual was significantly positively correlated (r=0.842, p<0.001). There was no significant difference between the estimated and actual bladder volumes (p=0.579).
Conclusions: This study shows that the bladder scan formula using the proportionality constant of 0.52, to estimate bladder volume, correlated best and was not significantly different from the actual bladder volume. Although the other formulae were also strongly positively correlated, there was a significant difference from the actual volume. More work is needed to produce patient individualised proportionality constants (3). The use of 3D bladder scanners may improve accuracy of volumes. Our clinicians were able to accurately estimate the post void residual on x-ray with no significant difference from the actual volume, which may negate the need to catheterise the patient at the end of the test. Hearing “bubbles” at the end of the test has a positive predictive value of 93%, which may also avoid unnecessary fluoroscopic screening, reducing radiation exposure. This study has identified the best formula to accurately estimate bladder volume and that video estimation and hearing “bubbles” at the end of the test can avoid unnecessary intervention.
References: 1.Neurourology and Urodynamics (2016); 35:137-168 2.NICE guidelines. 2013 3.Urology (2004); 64(5): 887-891

Table 1. Comparing the different formulae in correctly identifying bladder volume

Formula (n)

Pearson correlation coefficient (r)

Difference between calculated and actual volume (p)

h x w x d x 0.52 (n=85)

0.938 (p<0.001)


h x w x d x 0.625 (n=68)

0.937 (p<0.001)


h x w x d x 0.65 (n=68)

0.937 (p<0.001)


h x w x d x 0.7 (n=68)

0.937 (p<0.001)