Hi. Interesting. My Mayo care team said I do not have to worry about empty stomach or not ? Possible me (and my wife) got that wrong but my monthly testing has gone well every month the last 2.5 years. Also I have had minor surgeries, procedures , stents , bloodwork's etc where I was not able to do meds exactly 12 hours apart. Often a bout a 3 hour delay or early. I ALWAYS ask a doctor ahead of time when this happens they always say that is fine. So we do want to do 12 hours but from time to time off by a few hrs is np. Someone (not a doctor , a lab person ) told me TAC (my most important med) stays in our system for 72 hours. I have insomnia every night and do my meds 3 AM and 3 PM.
And my monthly bloodwork has been great with no signs of liver rejection for over 2 years. Best of luck.