There seems to be a common progression of tests for urinary issues... from least invasive to most. This makes sense:
The first test is very simple and non-invasive. You are told to arrive with a full bladder. You are sent to pee on the toilet and then return to the exam room. The nurse thyen does either an ultrasound bladder scan or inserts a straight catheter to see how much residual pee is in your bladder. If it's less than about 30ml, the tests are done.
The next stage is a Uroflow test. again, you are supposed to arrive with a full bladder. You are again sent to the bathroom and instructed to pee on/into a commode fitted with a special funnel and flow sensor that measures and graphs the amount of pee you pass vs time from start to finish. The equipment looks like this:
https://img.techpowerup.org/201107/uroflow-commode-00.jpg
I have what's called a "shy bladder"... I just can't pee when someone is watching me. So when I was asked to do this test, even though alone in the bathroom, I knew there was a nurse just outside watching the display of my pee flow, and I just dribbled a bit. So the urolologist ordered the next stage.
A full Urodynamics test followed. As I and others have described, this test is more invasive. You get seated in a special chair, legs spread wide, undressed from the waist down and a straight catheter inserted to be sure you are empty to start off. Small ( Fr 8 ) catheters are inserted in your urethra and vagina or rectum to measure the pressures. EMG electrodes, similar to EKG ones, are stuck near your anus and connected to the recording 'puter. The test is then started and saline is very slowly pumped into your bladder via the catheter at roughly 1ml per second. At this rate it takes about 5 minutes to fill your bladder to feel an urge to pee. The saline may have a dye added so the bladder shows up better on x-rays which can be taken during the filling. When you feel the need to pee, you tell the nurse. The filling continues yntil you are no longer able to hold the solution and you start to leak. The filling then stops and you are told to pee into the funnel of the Uroflow apparatus. The measurement catheters are then removed and a straight catheter is again inserted to drain you of residual solution. And you're done. This is what the exam apparatus looks like:
https://img.techpowerup.org/201107/urodynamics-room-00.jpg
The final step is a video cystoscopy. This is the most invasive, but I've not experienced it.
All in all, the tests are somewhat embarrassing in that one of your body's primal functions are involved, but are not painful. A bit ouchie during the catheter insertions, but nothing more.