Prostate-specific antigen (PSA) is a protein that liquifies semen in preparation for ejaculation. It is also believed that PSA helps sperm to penetrate the mucous layer that caps the female cervix. Normal PSA levels are 4 nanograms per milliliter (ng/mL) or lower. A measurement of PSA is generally considered one of the most effective tests for the presence of prostate cancer
PSA is found in a number of body fluids, but the most important PSA level for a prostate test is in the blood, which is called serum PSA. A serum PSA level with a range of 4-10 ng/mL might indicate the presence of prostate cancer. Further testing must be done to confirm both the initial PSA level and the presence of prostate cancer. Testing might include digital rectal examination, ultrasound imaging and prostate biopsy to evaluate prostate size and firmness and to look for tumors and cancer cells. It is also possible to have normal PSA levels and still have prostate cancer.
Other factors can influence PSA levels. For example, normal PSA levels are generally expected to be much less than 4 ng/mL, but an older man might have PSA levels on the upper limit of normal. This is because of the damage that occurs to the prostate with age. Enlargement of the prostate not caused by prostate cancer, called benign prostatic hypertrophy, might yield PSA levels a little higher than might be considered unsuspicious in a younger man who has no prostate problems. Other factors that raise PSA levels include recent ejaculation, prostate infection or irritation and digital rectal examination, although a digital rectal examination raises PSA substantially only in men who already have higher-than-normal PSA levels.
Most PSA is bound to the protein of blood serum, but there is a percentage that is unbound to blood serum protein. This is called free PSA. The usual PSA test measures the total amount in the blood without differentiating between free and total PSA. In men whose PSA levels are above 4 ng/mL, measuring the percentage of free PSA in the total level might prevent an unnecessary prostate biopsy. Free PSA levels of more than 25 percent of the total count are considered within normal limits.