Subtle errors of bladder wall thickness measurement have a significant impact on the calculation of ultrasound-estimated bladder weight. A pilot study.

ABSTRACT

AIMS: Ultrasound-estimated bladder weight (UEBW), is an emerging diagnostic tool, which has been used in both males and females with lower urinary tract dysfunction. The currently acknowledged UEBW calculation methods rely on the accurate measurement of bladder wall thickness (BWT). We aim to identify if subtle errors in BWT measurement have a significant impact on UEBW calculations. MATERIALS AND METHODS: Twenty patients were randomly selected from an overactive bladder patient cohort. The primary endpoint was to identify the range of false BWT measurements outside which significant changes in UEBW calculation occur. We used the Kojima method and a semi-automatic 3-D model that is based on Chalana's principle. Measurements were performed using the correct BWT and a series of faulty calculations from +0.5 mm to -0.5 mm using steps of 0.05 mm from true BWT. The effect of a fixed 0.5 mm BWT error was checked in bladder volumes above and below 250 ml and in three UEBW groups (<35 gr; 36-50 gr; >51gr). RESULTS: BWT measurement errors above 0.25 mm cause statistically significant changes in UEWB calculation when a 3-D model is used and errors above 0.15 mm when Kojima's method is used. At a fixed BWT error of 0.5 mm and bladder volume <250 ml, there is a 23.76% deviation from true UEBW, while at volumes >250 ml the deviation is 32.72%. The deviation is inversely proportional to the UEBW result, and heavier bladders deviate less. CONCLUSIONS: UEBW is a promising diagnostic tool, but small errors in BWT measurement might cause significant deviation from the true values. A 3-D calculation model appears to minimize such risks.