One possibility is that a service that is preventative and covered 100% becomes diagnostic when a condition is reported.
I had that happen a few years ago with a colonoscopy.
It was covered 100% as a routine preventative exam.
But polyps were found so it got billed as diagnostic and I ended up paying about half the cost due to deductible and coinsurance.
When is this country going to wake up to medical billing being a colossal scam?

