Ionized calcium test?

Is anyone familiar with this? My IBD kitty has been losing weight (a pound since December) and was vomiting again. The vet has done bloodwork and everything (kidney, thyroid, etc.) is normal except for an elevated calcium level. The vet said that this could be indicative of cancer, but I hope not. The vet has had me double her prednisone during the past couple of weeks, and she has stopped the vomiting and gained back two ounces, but she wants to recheck her calcium anyway. This time she is doing the ionized calcium test to make sure that the first calcium reading was accurate (Huh? Do they have faulty equipment or something?). She has to draw a large amount of blood, which sounds scary, and then freeze it and send it off to a larger lab to be tested. She is also going to do some x-rays also to see if there is some kind of mass in her GI tract. My cat is spending tonight at the vet because the vet wants to put her under anesthesia in the morning to do both procedures. I don't think it is normally done this way, but my kitty is a VERY difficult patient. She gets very agitated and stressed out and won't let them do anything. I am hoping everything goes alright. Has anyone else had this kind of test done?

Until someone with actual experience with having this test replies, there are a couple of references you might read through:

1. Scroll down a little on this page to the sections on "High Calcium Levels"....in there, there's a heading on Ionized Calcium: http://www.felinecrf.org/diagnosis_calcium_phosphorus_pth.htm#ionised_calcium
2. One of the links suggested in there is this...it's a little technical...look for the section "Why should I measure serum ionized calcium?"    https://www.animalhealth.msu.edu/Sections/Endocrinology/Calcium.php#05

Thanks for the info. No mass was found today, so I am still waiting for the results of the ionized calcium test, which could take up to three days. The vet further explained the purpose of the test. She said that a high calcium level detected in the blood can sometimes be erroneous, but since the high calcium (total serum?) level was detected on two separate visits/blood draws, she wanted to do the more advanced testing to confirm whether or not there is a problem. Everything else in her bloodwork is still totally normal.

My cat was starving when I picked her up today. The vet did the procedure later in the day than she intended, so by the time I was able to get her back, the poor thing had not had anything to eat in more than 24 hours. She wolfed her food down, but fortunately it did not come back up. The increased prednisone is still working, but I kind of wonder if it is just masking the symptoms of something more serious going on.