Called WN, had them drop the number and re-add (as others have said to do), but it still didn't work. Same deal for second existing reservation. Many here have said that you should get it if you add the number after the fact, but it didn't work for me. Tried every work around and spent time talking to the WN folks on the phone... to no avail. Only starting getting precheck for reservations booked after adding the KTN to my WN profile.
1 You may drink water, as much water as you can would be helpful, until arrival. 2  Routine medications may be taken, unless you have been instructed otherwise. If you are diabetic, you may take your diabetes medication no less than 4 hours prior to the exam.  Arrive 15-30 minutes before your PET scan.
I had a PET scan in 2007, and had to do a low carb diet for a couple days before the test. I don't remember now what exactly what was on it, the hospital where I was getting the test gave me an information sheet on it. If you Google low carb diet that should give you some ideas.