Prostate specific antigen (PSA) is a protein made by cells in the prostate gland and found in the blood. It is found in two forms in the blood:
- bound PSA which is bound to proteins in the blood
- free PSA which isn't bound to proteins.
The standard PSA test measures both of these together to give the total PSA.
If a man’s PSA level is between 4 - 10 ng/ml doctors sometimes call it a borderline result. This is because, although the PSA is higher than average, most men who have a PSA of between 4 and 10 won't have prostate cancer. This is especially the case for men whose prostate feels normal on digital rectal examination (DRE). The free PSA test is a test doctors sometimes use to help them decide how likely it is that a man has prostate cancer when the total PSA result is borderline.
The proportion (percentage) of free PSA that makes up the total PSA is not the same for all men. It is believed that the higher the percentage of free PSA a man has, the less likely it is that he has prostate cancer. Some studies have shown that if the free PSA makes up more than a 25% of a man's total PSA he is at low risk of having prostate cancer. And, that men with a lower percentage of free PSA are at higher risk of having prostate cancer.
However, there are drawbacks to the free PSA test and not all doctors believe that it is helpful. The test can't say whether a man definitely has prostate cancer or not. It can only give an idea how likely this is. There also isn't enough evidence for doctors to be able to agree what percentage of free PSA should be used to say whether a prostate cancer is likely.
The free PSA test can't diagnose or completely rule out prostate cancer. Its main purpose is to try to help some men who have borderline PSA results avoid having a biopsy of the prostate gland if it is unlikely that they have cancer.