At an Independent Clinical ("Reference") Laboratory
Reference labs are usually private, commercial facilities that do high volume routine and specialty testing. Most of the tests performed are referred from physician's offices, hospitals, and other healthcare facilities like nursing homes. While most hospitals try to do as many tests as possible in-house, reference labs are used for specialized tests that are ordered only occasionally or that require specialized equipment.
Since reference labs are some distance away, you may never step into a reference lab – unless you visit one of their phlebotomy stations to have your blood drawn – although you probably have heard of them. Many employee health insurance plans require that you use a specific lab, which is often indicated on your insurance card.