There are two ways:
Some researchers think catheters measure urine in the bladder better than an ultrasound, while others think the two tests work equally well.
- Catheter: A nurse slides a thin, flexible tube called a catheter through your urethra (where pee comes out) and into your bladder, then takes out the urine.
- Ultrasound: A machine uses sound waves to show live images of your bladder so your doctor can see how much urine is there.