Honey will be at the vet all day for the ACTH test. It's a serious of blood tests. They take a baseline test, then inject him with hormone, then take another test, then inject him again. It's a hassle, but not particularly painful. Unless Kelly just HATES getting blood drawn, or injections.
The urine cortisol/creatinine test simply sent us in that direction. It told us that my dogs liver levels were off. I don't see how a reputable vet would ever diagnose Cushings off a urine test.
You can *conclusively* confirm the existence of the tumor with an MRI. Since my girl's ACTH tests kept coming back inconclusive, then negative, then inconclusive (oh, yeah, it was fun...we did a quite a few of these!), I would have gone this route before starting the meds. The most commonly used medicine, Lysodren, is a really strong drug and can have severe side effects. I wouldn't have put my girl on it until I was convinced it was necessary. Sometimes, vets are pretty cavalier about Lysodren. But when I read about the "crash" that can happen, I got pretty concerned.
If my dog definitively had Cushings, I would have used it. Surgery has its own risks. But I was going to be positive that this med was necessary. Turns out, we got a couple negative ACTH test results in a row, so we just stood back and retested every year til, well, she passed away from heart disease at 12.
I like this website for accurate info on Cushings:
http://www.kateconnick.com/library/cushingsdisease.html