I always had the same thing happen to me, and then I learned a little about why I was getting hosed:
First, you might be paying out of pocket if you haven't met your deductible for the year. a ppo can have a higher deductible per year, but they'll cover more (I used to get all my HRT covered, my endo, and the bloodwork). HMO's won't cover as much: only some of my hrt is covered, endo is not covered, bloodwork can be covered depending on where I go.
Where you go to get your blood work done is important. When I was on a ppo, I could get my blood work done at the hospital where my endo is. On an hmo, I have to pay out of pocket for that. My HMO though, will let me take me my doctor's orders to Quest Diagnostics and get the work done there. They haven't been complaining about that, or refusing to pay, so I'm assuming that it's okay. *knocks on wood*