Normal levels for a prostate-specific antigen test vary by age, as older men commonly have slightly higher levels than younger men, explains MedlinePlus. A normal level is considered 4.0 nanograms per milliliter of blood or 2.5 nanograms per milliliter of blood for men in their 50s or younger.
Some conditions, such as a prostate infection, large prostate and urinary tract infection, can cause abnormal results, according to MedlinePlus. Recent prostate tests or placement of a catheter can also affect prostate-specific antigen test results. This test is done to screen individuals for prostate cancer, as well as to screen individuals for prostate cancer recurrence. The test helps detect cancer while it is still in early stages.