Prostate-specific antigen (PSA) is a protein that is secreted by the cells of the prostate gland and is found in semen, the medium that carries sperm. PSA is the substance that liquefies semen, allowing sperm the freedom to swim. Small amounts of PSA are detectable in normal human blood and a serum PSA level of below 4 ng/mL is considered normal, while a level between 4 and 10 ng/mL is considered suspicious and requires repeat testing.
High levels of PSA are seen in prostate cancer. However, PSA is not a definitive test for prostate cancer and the only reliable way to confirm a diagnosis of this condition is with a biopsy of the prostate tissue to check whether cancer cells are present.
If a patient's PSA level falls within the borderline range of between 4 and 10 ng/mL, doctors may consider using special types of PSA tests to help determine whether or not a patient needs to undergo a biopsy procedure.
Examples of the further PSA tests that may be performed include:
Two forms of PSA are present in the blood. One form is attached to proteins in the blood, while the other isunattached and circulates freely. The amount of free PSA compared to the total PSA is referred to as the percent-free PSA (fPSA) and this is lower in men who have prostate cancer than in those without the condition.
A lower fPSA indicates a greater likelihood that prostate cancer is present and that a biopsy should be performed. The usual cut-off for fPSA that indicates a biopsy is 10% or less and doctors also recommend that a biopsy should be considered if the fPSA is between 10% and 25%.
This improves the detection of cancer and helps to avoid unnecessary prostate biopsies. Although the test is widely used, there is disagreement among experts about the optimal cut-off value for indicating a biopsy and this may vary according to the original PSA value.
This is a test that measures how fast the PSA level rises over time. The PSA level usually slowly increases as a man ages and research suggests that this increase is faster in men with prostate cancer. However, studies have not shown PSA velocity to be a better diagnostic tool than PSA level and the American Cancer Society guidelines do not recommend including this test as part of the screening process for prostate cancer.
PSA density (PSAD) is a test used to adjust for the fact that PSA levels are higher in men who have larger prostates. The volume of the prostate gland is measured using transrectal ultrasound and the PSA value is divided by the prostate volume to give the PSAD. The greater the PSAD, the more likely it is that a patient has prostate cancer. However, the PSAD has not proven to be as useful as fPSA.
Older men usually have higher PSA levels than younger men, whether cancer is present or not. A raised PSA level may be of more concern in a man aged 50 years than in a man aged 80 years, for example. It has therefore been suggested that PSA results be considered in the context of results for men of the same age group. This age-specific interpretation of PSA results, however, has not proved effective and is not currently recommended as a reliable method for determining prostate cancer risk.
Reviewed by Sally Robertson, BSc