What is the Difference Between Free PSA and Total PSA?

The main difference between free PSA and total PSA lies in their binding to other proteins in the blood. Prostate-specific antigen (PSA) is a protein produced by prostate cells, and it circulates in the blood in two ways: bound to other proteins or unbound (free).

Free PSA: This is the unbound PSA that circulates freely in the blood. A free PSA test measures the percentage of unbound PSA in the blood.
Total PSA: This measured amount includes both free and bound PSA in the blood.

Doctors often use both tests together to determine the likelihood of prostate cancer. A higher ratio of free PSA to total PSA indicates a lower risk of prostate cancer, while a lower ratio indicates a higher risk. If a man's total PSA level is between 4 and 10 ng/ml, doctors may test the blood for free PSA. A percent-free PSA above 25% is considered normal, while a lower percentage may warrant further investigation, such as a biopsy.

In summary, the main difference between free PSA and total PSA is their binding status in the blood. Free PSA circulates freely, while total PSA includes both free and bound PSA. The ratio of free to total PSA can help determine the risk of prostate cancer, and both tests are often used together for a more accurate assessment.