Prostate-specific antigen (PSA) is a protein produced by the cells of the prostate gland and is found in semen, the medium that carries sperm. PSA liquefies semen and allows sperm to swim. Small amounts of PSA are detectable in the blood of males and a serum PSA level below 4 ng/mL is considered normal. However, a PSA level between 4 and 10 ng/mL is considered suspicious and may require a repeat test.

PSA is secreted in large amounts by prostate cancer cells. As prostate cancer progresses, the blood levels of PSA begin to rise and an increased PSA level is therefore associated with an increased risk for prostate cancer. However, PSA is not a definitive test for prostate cancer and many men may still develop prostate cancer, despite having blood PSA levels under 4 ng/mL. In around 15% of all prostate cancer cases diagnosed on biopsy, the PSA level is still within the normal value range.

However, it is estimated that men who have a borderline PSA level of between 4 and 10 ng/mL have a 25% risk of developing prostate cancer and this risk is increased further still to 50% when the PSA level is above 10 ng/mL.

Prostate cancer may be suspected based on a patient's symptom profile and details such as family history of the condition, age and ethnicity. As well as a patient's PSA level being tested, a digital rectal examination (DRE) and tissue biopsy are performed to check for the presence of cancer.

Factors affecting levels of PSA include:

• PSA levels may be raised in cases of a non-cancerous enlargement of the prostate called benign prostatic hyperplasia (BPH).
• PSA levels rise with increasing age.
• Infection or inflammation of the prostate gland (prostatitis) may cause a rise in PSA levels.
• Factors such as physical activity and even sex may also increase PSA, which can remain high for a couple of days after ejaculation. Therefore, men are asked to abstain from ejaculation for 2 days before having their blood PSA checked.
• Urological procedures such as prostate biopsy, cystoscopy and even DRE can temporarily increase the PSA level.
• Some hormonal therapies that increase levels of testosterone can raise the blood PSA level.
• The use of 5-alpha reductase inhibitors such as finasteride or dutasteride may lower the levels of male hormones and therefore reduce PSA levels. These are used to relieve urinary symptoms in prostate disease.
• Obesity can also lower blood PSA.
• Blood-thinning medications such as aspirin and cholesterol-lowering drugs such as statins also lower PSA levels.