Doctors call it "PSA velocity," the change in the level of a chemical in the blood called prostate-specific antigen. In recent years, many doctors have come to rely on it as the best indicator for when it's time for an initial biopsy to check for prostate cancer. But PSA velocity has come under challenge. A recent article in the Journal of the National Cancer Institute concludes that a rising PSA is no better sign of incipient prostate cancer than the old signal --- a PSA level over a specific threshold. That's often 4 nanograms of PSA in a thousandth of a liter of blood. (A healthy man's PSA level can range from zero to 10 and beyond.) "We found out that in many cases, to our complete surprise, [PSA velocity] didn't really tell us very much at all," says Andrew Vickers, a biostatistician at Memorial Sloan-Kettering Cancer Center in New York and lead author of the study. "Once you knew what somebody's PSA level was, their change in PSA, or PSA velocity, was essentially uninformative," Vickers says of his results, "particularly for the aggressive cancers, the ones we should really be worrying about."