If your prostate is blocking the opening from your bladder, this will make passing urine a lot slower. There is a machine to test this. The flow test works best if you pass a lot of urine more than 200mls ie a paper cup, so you might be asked to go into the hospital with a comfortably full bladder (but do not overfill your bladder) or you might be given water to drink and wait until your bladder is full. All you have to do is pass urine into a funnel-shaped container and all the measurements are recorded automatically!