It's been a couple of years, so you decide to see your primary-care physician for a physical. You feel fine, but it's the responsible thing to do. You get your blood pressure measured and your blood drawn. Within a few days you'll get the lab report that will give you the readout on the amount of cholesterol and sugar in your blood. (This drill is so routine that you and your doctor don't even discuss the implications of a possible bad test result.) If you've entered your middle years, he'll probably ask if you want the lab to test your blood for PSA, a screening test that can tell you if you're at an elevated risk for prostate cancer.