Accuracy of Bladder Scanner in Measuring Bladder Volumes in Postpartum Women

Importance: Ultrasound bladder scanners may provide a less invasive method to measure postpartum urinary volume, but their accuracy must be validated. Objective: The objective of this study was to evaluate the accuracy of a bladder scanner in measuring bladder volumes in postpartum women. The secondary objective was to evaluate the effect of obesity on scanner accuracy. Study Design: This prospective cohort study included women older than 18 years who gave birth vaginally at term gestation. After delivery, we obtained 3 sequential measurements of the bladder volume using an ultrasound bladder scanner. We then measured true bladder volume by transurethral catheterization. The primary outcome was the absolute difference between the bladder scanner volume and the catheterized volume. A Wilcoxon signed-rank test was used to compare absolute median difference between the bladder scan volume and true catheter volume. Repeated-measures analysis of variance and linear regression were used to evaluate the effect of obesity on the accuracy of the bladder scanner. Results: We enrolled 70 patients (61.4% nulliparous, 38.6% multiparous). One delivery was vacuum-assisted, 4 were forceps-assisted, and 65 were spontaneous vaginal. The median age was 34 years, and median body mass index was 30.5. Median difference between bladder scanner and catheter volume was 42.7 mL (P < 0.001), with the scanner underestimating true volume 82.9% of the time. The scanner was less accurate in patients with a body mass index ≥ 30 (P = 0.001). Conclusions: The bladder scanner is less accurate than catheterized urine volume. However, the median difference between the bladder scanner and the catheterized volume is 42.7 mL, making it suitable for clinical use. Accuracy deteriorates in obese patients.