The accuracy of portable abdominal ultrasound equipment in measuring postvoid residual volume

This study examines the accuracy of a new portable abdominal ultrasound machine (Bladder Scan, BVI 2500) used to measure postvoid urine residual (PVR). Using this machine, we started prospectively measuring residual urine in the first 80 women undergoing uroflowmetry in our urodynamic unit. Ultrasound PVR measurements were done immediately prior to catheterization while the patient was in the supine position. The catheterized postvoid residual was used as the gold standard. Eighty paired measurements were done on 78 women. Compared to catheterized PVR, ultrasound PVR measurements tend to underestimate and correlated poorly with the actual residual volume. The reading was considered accurate if it was within 25% of the catheterized PVR. The ultrasound PVR measurements were most accurate (60.6%) when the readings were below 50 ml and least accurate (10%) when readings were higher than 150 ml. Readings between 50 and 150 ml were 27% accurate. A measurement of zero '000' was common (44%), usually reflecting volumes of less than 50 ml (84% of cases). However, it could indicate that the bladder had been missed altogether. Partial measurement of the bladder volume, where the lateral bladder borders are missing, produces readings much below the actual volume: a 'tip of the iceberg' phenomenon. We therefore advocate caution when interpreting PVR measurements from portable abdominal ultrasound machines, and if an accurate measurement of PVR is necessary, catheterization remains a more reliable method.