Every time someone leaves your employment on their own accord, but stays in the area and the field, they’ve fired you as a manager.
Sure, people change employers for a variety of other reasons, but if they don't relocate and don't change professions, it should be a red flag. You've got to ask this question in the exit interview: what could I have done to keep you?
If their departure was for more money, don't sweat it. Chances are the difference is sizable and a chasm beyond which you can bridge. But nobody changes jobs for a dollar an hour more unless there's something else you're not providing.
Phlebotomy staffing is one of the most difficult skill sets to maintain in healthcare. Most labs have a revolving door in the phlebotomy department, and struggle to find and keep qualified personnel. If you're losing your best to other employers in the same community and industry, something you can control is probably causing the exodus.
Not everyone will be frank with you when you ask "what could I have done to keep you," but not asking only keeps the revolving door spinning.