In TOS McCoy was a Lt Cmdr and Crusher was a full Cmdr. It seems that to be Chief Medical officer you need to be a reasonably high-ranking officer. The exception to this is The Doctor (EMH) in VOY, but I think we will ignore him, because he's a hologram and VOY was in a desperate situation. So, is it actually referenced or written anywhere that there is a minimum ranking the CMO needs to be (and if so, what is this ranking) or do we have to go by anecdotal evidence from the shows and make an educated guess based on that?
Starfleet CMOs have held a range of ranks but typically at least Lieutenant or Lt Cmdr (e.g. as head of department) except where the medical department consists solely of the officer themself.
- Katherine Pulaski - Commander
- Doctor Carter - Commander
- Christine Chapel - Commander
- Leonard McCoy - Lieutenant Commander
- Beverley Crusher - Lieutenant Commander
- Julian Bashir - Lieutenant (Junior Grade)
- Elizabeth Lense - Lieutenant (Junior Grade)
Additionally, various civilians have worked as Starship CMOs without holding any formal rank;
- EMH - No rank
- Phlox - No rank
- Philip Boyce - No rank
The only semi-canon reference I've been able to find to CMO rank is from the "Star Fleet Officer Requirements" manual, written to coincide with the release of Star Trek III.
The implication is that assuming a role of dept head implies an increase in rank to Lt. Cmdr