Definition

Prostate-specific antigen (PSA, APS, hK3, KLK2A1), also known as kallikrein-3 (KLK3) or P-30 antigen, is a glycoprotein enzyme encoded by the KLK3 gene. PSA is a member of the kallikrein-associated peptidase family and is secreted by prostate epithelial cells. PSA is produced for semen, which liquefies semen in semen coagulate, allowing sperm to swim freely. It is also believed to help dissolve cervical mucus, allowing sperm to enter the uterus. A small amount of free PSA is present in the serum of healthy men with a healthy prostate, but in the case of prostate cancer or other prostate diseases, free PSA is usually elevated with prostate-specific antigen (PSA) and is not the only indicator of prostate cancer, but prostatitis or benign prostatic hyperplasia may also be detected. Wang separated PSA from prostate extract in 1979[1], and used it for the first time in clinic in 1986, and soon replaced prostate Acid phosphatase as the most important prostate cancer. PSA is a Serine protease with a molecular weight of 34000, a sugar content of 7%, and a pl=6.9. The concentration of PSA in semen is 0.2~0.5mg/ml, and the concentration in serum is only one-millionth of that in semen.

Most PSA in the blood binds to serum proteins. A small amount does not bind to a protein and is called free PSA or FPSA. In men with prostate cancer, the ratio of free (unbound) PSA to total PSA decreases.

Fig. 1 Pentameric assembly 1 of PDB entry 3qum coloured by chain and viewed from the front.

Function

The level of free PSA refers to the ratio of the protein prostate-specific antigen (PSA) in the blood that is not attached to other proteins. The risk of cancer increases if the free-form PSA’s rate of total PSA is less than 25%. The lower the rate, the greater the likelihood of developing cancer. According to the Journal of American Medical Association, the likelihood of prostate cancer is listed based on the percentage of free PSA in men with a total PSA between 4-10 ng/mL.

Fig. 2 The relationship between FPSA ratio and prostate cancer probability

Compare FPSA and TPSA

Comparing the test results of total PSA (TPSA) and free PSA (FPSA) can give doctors an idea of how likely a person is to develop prostate cancer.

The range of free PSA may vary, but in general, a higher ratio of free PSA to total PSA indicates a lower risk of prostate cancer. On the other hand, a lower ratio of free PSA to total PSA indicates a higher risk of prostate cancer.

However, PSA testing alone does not confirm the diseases, as other factors can also affect PSA levels. A digital rectal examination (DRE) and prostate biopsy may also be necessary.

FPSA and Prostate Cancer

The expression of PSA protein is not specific to prostate cancer. Specific prostate epithelial cells in a variety of conditions, including normal and proliferative states. However, under normal circumstances, almost all PSA proteins are secreted into semen, and in prostate cancer, PSA proteins leak into the bloodstream. Therefore, serum PSA measurement is of great significance for the observation of prostate cancer. It is widely used as a blood marker for prostate cancer to screen for and detect recurrence after treatment. The higher the serum PSA level, the more likely it is to be prostate cancer. The detection rate of prostate cancer with PSA 4.1 ~10 ng/ml is 20 ~ 30%, and the detection rate of prostate cancer with PSA≥10 ng/ml is 30 ~ 50%. However, it is important to note that about 15% of people with normal PSA and induration of the prostate found on digital rectal examination (DRE) were found to be cancerous.

 Clinical Application

 FPSA test is mainly suitable for untreated people with TPSA value of 2-20 ng/ml. The FPSA/TPSA ratio of people with prostate cancer is significantly lower, and the FPSA/TPSA ratio of people with benign prostatic hyperplasia is significantly higher. The FPSA/TPSA cutoff value of 0.15, below which prostate cancer is highly suspected, has a diagnostic sensitivity of 90.9%, specificity of 87.5%, and accuracy of 88.6%, which is significantly better than TPSA alone. The main advantage of the PSA test is to detect  prostate cancer before symptoms or changes appear in your body that could become prostate cancer. It can also detect prostate cancer before it spreads to other parts of the body.

Alpha Lifetech Inc. is dedicated to developing FPSA diagnostic regents. Alpha Lifetech Inc. is a reputable supplier focusing on research, manufacture and sales of In Vitro Diagnostic (IVD) regents. All the IVD regents offered by Alpha Lifetech Inc. have undergone strict QC validation and are certified by the COA (certificate of analysis). Meanwhile, we can provide customized services according to customers' requirements. Alpha Lifetech Inc. is committed to supply high-quality, high-sensitivity antigen and antibody products for scientific research and industrial customers. In addition to regular small packages, Alpha Lifetech Inc.'s large-scale fermentation platform also allows us to provide raw material-grade IVD regents for the majority of industrial clients.

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