In the blood, PSA is either bound or free (unbound). Bound PSA means that it is attached to other proteins. PSA that is not attached to other proteins is called free PSA because it circulates freely in the blood.
Percent-free PSA is a ratio that compares the amount of free PSA to the total PSA level. The total PSA level includes the amount of both free and bound PSA in the blood.
Free PSA levels are often higher in those with non-cancerous conditions of the prostate and lower in those with prostate cancer. If you have a total PSA level between 4 and 10, doctors may test your blood for free PSA. A percent-free PSA above 25% is considered normal. Some doctors recommend that if you have a percent-free PSA of 18% or less you should have a prostate biopsy. Other doctors recommend having a biopsy if the percent-free PSA is around 12% or less.
Nomograms
Nomograms are statistical models that predict probable outcome. Prostate cancer nomograms look at PSA test results and several other factors, such as age, ethnicity, family history, urinary symptoms, DRE results and prostate biopsy results. The nomograms are then used to:
- calculate your risk of developing prostate cancer
- determine the need for a biopsy
- determine how aggressive the cancer may be
Researchers are studying nomograms to see how they can help doctors determine your risk of developing prostate cancer.
PSA density
PSA density (PSAD) is the level of PSA in the blood in relation to the size (volume) of the prostate measured during a TRUS. It is calculated by taking the PSA level and dividing by the prostate volume. PSA levels are usually higher in those with enlarged prostates. Some doctors may use PSAD to help decide whether to do a biopsy because a high PSAD (above 0.1) is more often related to prostate cancer.
PSAD of the transition zone (the part of the prostate that surrounds the
urethra
) is the level of PSA in the blood in relation to the volume of the transition zone of the prostate. PSAD of the transition zone is another test that some doctors may use to help decide whether or not to do a biopsy.