Like breast cancer screening in women this technique is subject to over diagnosis. 18% of men who are diagnosed and opt for radical prostatectomy do not have the illness. When their extracted gland can be examined in full no cancer is found.
Individual measurements alone are not an accurate it is better to measure the trend. Even If there is an increase if it is low the man may die with the disease not from it.
I have a friend who was recommended for biopsy and treatment when 64. He is now 71, has passed through his panic and is living comfortably with an above average PSA. If he had had radiation treatment he would never have known if he had dangerous prostate cancer or not.