It is the time required for the PSA level to double. It is dependent to a large extent on the baseline PSA level. For instance, a PSA level of 0.1 could easily double to 0.2 without much change in the total PSA. However, it would require a much greater increase in PSA for the level to double from 4 to 8. PSA doubling time is most useful in evaluating the aggressiveness of a cancer that has recurred after radical prostatectomy or hormonal therapy.