All the sibling comments touch on valid points. I'm going to try and summarize them for clarity.
- The US has a few nationalized (single-payer) insurance programs... TriCare for military, Medicare for retirees, and Medicaid for the indigent.
- Obtaining Medicaid is not easy - you have to be truly indigent in many states (it's federally funded, but states manage their own programs)
- Everybody else is left to buy their own insurance policy. Sometimes this is partially funded by an employer, but not always.
- Due to the high cost of insurance (and health care in general), many plans have very high copays and deductibles.
So, even with an average health instance policy, a person could be stuck with the full cost of that test because of a deductible. In the past, I've had deductibles as high as $5000/individual, $12,000/family (note: I don't know, we're back on a PPO with a $2000 deductible).
When I use a medical service, I usually receive a statement that lists retail prices for services rendered, negotiated fees (between MD and insurance), and sometimes an excess amount that I owe.
If the hospital is "out of network" (no contract between insurance and hospital), I could be due a larger chunk of the bill personally (up to 100%, depending on insurance plan). This is called "balance billing" - google will give you all sorts of insane results from $50,000 helicopter rides to entire hospital centers that don't work with a single insurance provider so they can bill whatever they want.
As noted elsewhere, it's a complete mess and embarrassment for one of the wealthiest nations on the planet.