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Free prostate specific antigen in blood
Last reviewed: 06.07.2025
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The content of free prostate-specific antigen in the blood serum is normally more than 15% of the total prostate-specific antigen. Half-life is 7 hours.
Clinical PSA increases significantly when determining its various forms, the ratio of which corresponds to the type of pathological process occurring in the prostate gland. In the blood serum, prostate-specific antigen is in two forms: free and associated with various antiproteases. Most of the prostate-specific antigen is in a complex with a 1 -antichymotrypsin. An insignificant part of the prostate-specific antigen is associated with a 2 -macroglobulin and is not determined by conventional ELISA methods. The level of free prostate-specific antigen changes depending on both the individual characteristics of the organism and the type of prostate disease. In prostate cancer, not only does the production of prostate-specific antigen increase in tumor cells, but the synthesis of a 1 -antichymotrypsin also increases significantly, resulting in an increase in the amount of associated and a decrease in the content of the free fraction of prostate-specific antigen with an increase in the total concentration of this antigen. As a result, the content of the free fraction of prostate-specific antigen in the blood serum in prostate cancer is significantly lower than in the norm and in a benign process. This is the basis for differential diagnostics of cancer and hyperplasia of this organ.
The essence of the study is the parallel determination of total prostate-specific antigen and free fraction of prostate-specific antigen and calculation of their ratio (free PSA/total PSA) x 100%.
Determination of the free fraction of prostate-specific antigen is indicated when the concentration of total prostate-specific antigen in the blood increases. If the ratio is below 15%, an ultrasound examination and biopsy are required. If this indicator is above 15%, observation and a repeat examination after 6 months are necessary.
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