Quest is apparently the only show in town for getting medical testing done. This particular site is conveniently located near Fairfax City. They are almost always busy and I've never had less than about a 30 minute wait. You can make appointments on Saturdays but not during the week. If you have to go on a Saturday, make an appointment because they take those first.
The phlebotomists are good for the most part. I've had one that bruised my arm, but my favorite can get blood drawn without my even knowing it. :)