Julie,
It is standard to have levels drawn every third dose, so that would be before the fourth dose. That is a trough to make sure the levels are high enough, but more importantly not too high. Tobra is extremely dangerous to kidney function. I have been put into renal failure on tobra and vanc because my levels weren't checked often enough. I would not stand for any less. Don't need kidney problems on top of everything else. Good luck with treatment...I hope he feels like new again soon. I am currently on vanc and have been on it going on my 4th week.