Current guidelines for the early detection of prostate cancer recommend a biopsy for men whose P.S.A. rises rapidly, no matter what the initial level. But a new study says that the practice does not help patients find aggressive cancers and that it results in many unnecessary biopsies. P.S.A., or prostate-specific antigen, rises with age, and what is considered normal varies. In general, a level under 4 nanograms per milliliter is considered safe. But even with a normal reading, an increase of 0.35 nanograms per year is widely believed to be high enough to require a biopsy. Researchers examined the records of 5,519 men with a base-line P.S.A. under 3. They followed them for seven years with yearly tests and a biopsy if the level rose above 4.