Scientists have found a way to predict which cases of prostate cancer will turn out to be the most serious — a tool that should help physicians decide when to take aggressive action against this generally slow-growing cancer of old age, according to researchers in a report released today.

The researchers found that a dramatic surge in blood levels of a chemical called PSA in the year before diagnosis was linked to the greatest risk of death.

Patients with steep-rising PSA values probably require especially aggressive treatment and should not be managed by a regime of “watchful waiting,” said the study’s senior author, Dr. William J. Catalona, director of the clinical prostate-cancer screening program at Northwestern Memorial Hospital in Chicago. In addition, such a steep rise in PSA levels should warrant a biopsy even if they do not reach the currently accepted warning threshold, Catalona said.

Prostate cancer is the second-leading cancer killer of men. Each year, about 240,000 men are diagnosed with the disease and nearly 30,000 die of it, according to the American Cancer Society. Prostate cancer mostly strikes men older than 65, and its course is unpredictable. Often it develops so slowly that patients can die of other conditions before the cancer has a chance to kill them. In other cases, its course is quick and more deadly.

The problem for doctors has been their inability to distinguish one from the other at an early stage.

Since the 1990s, doctors have been widely screening men with a blood test for prostate specific antigen, or PSA, a prostate protein that leaks into the bloodstream when the prostate’s biology goes awry.

Men whose blood PSA levels have been rising swiftly might be good candidates for radical prostatectomy and additional treatments such as hormone therapy or chemotherapy. Those whose PSA levels have remained constant may be more suitable for surgery without additional treatments, or a watch-and-wait approach.