This test checks to see whether the bladder empties fully by passing a flexible tube called a catheter through your urethra and into your bladder after you’ve urinated. The catheter drains the urine that remains in your bladder and measures it. Another way to test postvoid residual urine is with a bladder scanner that uses ultrasound, a test that uses sound waves to look at how much urine is left in your bladder after you go. The bladder scanner is a painless procedure. Ultrasound gel is placed on your lower abdomen and the machine calculates the volume of urine left in your bladder.