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The difference between free and bound PSA

Information on the difference between free and bound PSA. 

What PSA is

PSA stands for prostate specific antigen. It is a protein made by both normal and cancerous cells in the prostate gland.

The PSA test

Doctors measure the amount of PSA in your blood to help diagnose prostate cancer. There is no single PSA level that is considered normal. The reading varies from man to man and the level of PSA in the blood gets higher as men get older. But the normal range is considered to be around 4 nanograms per millilitre or less (usually written as 4 ng/ml).

A raised PSA level can be a sign of: 

  • an infection of the prostate gland (prostatitis)
  • a benign (non cancerous) enlarged prostate
  • prostate cancer

The test can show if your PSA level is raised, but not why. So the PSA test alone is not enough to accurately diagnose prostate cancer.

Higher levels of PSA are more likely to be caused by prostate cancer. Levels higher than 10ng/ml might be a sign of prostate cancer, but can also be due to benign prostate conditions. A PSA level between 4 and 10 ng/ml is usually due to benign prostate disease. If you have a PSA of between 4 and 10, your doctor will probably do more tests to rule out cancer. You might have: 

  • a biopsy of your prostate gland
  • a free and bound PSA test
  • a repeat PSA test a few months later

Testing free and bound PSA

A standard PSA test measures the total PSA in the blood. But PSA has different forms. It can either be:

  • bound – attached to a protein in the blood
  • free – not attached to a protein

We know from research that the proportions of free and bound PSA are different in men with prostate cancer, compared to men who have benign prostate disease. The general opinion is that a higher amount of free PSA in a test means a lower chance of cancer. Many doctors think that if more than a quarter (25%) of the total PSA is free, there is less chance of having prostate cancer. Put another way, men with prostate cancer are thought to have a lower proportion of free PSA.

So it is possible that this test could help doctors find the cause of a raised PSA level. This might mean that some men with benign conditions avoid unnecessary biopsies. But methods of testing free and bound PSA in laboratories vary. And doctors are not agreed on the ratios of free to bound PSA that indicate cancer. So it is not used routinely in diagnosing prostate cancer. You are more likely to have it done if your standard PSA test result was borderline.

Last reviewed: 
12 Mar 2014
  • Cancer and principles and practices of oncology (10th edition), VT Devita, S Hellman and SA Rosenberg. Lippincott, Williams and Wilkins, 2015.

  • Cancer and its management, Tobias and others. 6th Ed, Wiley-Blackwell, 2010

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