Docs do a lot of testing. Sometimes, we have a diagnosis in mind and are confirming it. Sometimes, we're just screening.
As an OB, I do a boatload of screening tests. Quad screens, pap tests, mamms, FOBTs, 1 hour GTTs. I also do a fair number of diagnostic labs: CBCs, GC/Chlamydias, biopsies.
I have found that people get confused as to what exactly screening means. Screening tests do NOT give you a diagnosis! Instead, they say, "Hey! You might want to investigate a little further." A positive pap test leads to a colposcopy, possibly with biopsy. A positive FOBT leads to a referral to GI for w/u. Many positive screens turn out to be a big fat nothing.
Diagnostic tests, OTOH, lead me to a diagnosis (duh) and treatment. A positive chlamydia test buys you a dose of zithromax and a repeat test in 2 months (as well as offering further STD testing and reminding you that your SO needs treatment too). I positive endometrial biopsy invokes the cancer talk and treatment.
I could now talk about pre- and post-test probabilities, but most people's eyes would glaze over.