In 81 outpatients the postvoiding residual urine (PVR) using real-time B-mode ultrasonography (3.5 MHz transducer) was measured. For the calculation of the bladder volume the formula for an ellipsoid (V = 4/3π × r1 × r2 × r3) was found to be most accurate in predicting the actual volume measured by in-and-out catheterization (r = 0.982). Other volume formulas, using only one diameter of the bladder, were found to be much less accurate. For any arbitrary value of PVR, used in determining clinical management, the incidence of misjudgment by ultrasound was negligibly low. We conclude, that sonographic measurement of the PVR as a quick, noninvasive method, should replace catheterization, if the basic equipment is available. Additional information, e.g., prostate size, bladder configuration, diverticula, etc., can be obtained during the procedure without additional costs or loss of time.
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