Prostate specific antigen (PSA) is a protein manufactured exclusively by the prostate gland; PSA is produced for the ejaculate where it liquifies the semen and allows sperm cells to "swim" freely. It is also believed to be instrumental in desolving the cervical mucous cap, allowing the entry of sperm.

High levels of PSA are associated with benign prostatic hyperplasia and prostate cancer. It is normally present in the blood at very low levels. Increased levels of PSA suggest the presence of prostate cancer. PSA can be also elevated due to prostate infection, irritation, benign prostatic hypertrophy (enlargement) or hyperplasia (BPH), recent ejaculation, digital rectal exam etc.

The U.S. Food and Drug Administration (FDA) has approved the PSA test for annual screening of prostate cancer in men of age 50 and older. PSA levels between 4 and 10 ng/mL (nanograms per milliliter) are considered to be suspicious and should be followed by rectal ultrasound imaging and, if indicated, biopsy. PSA is false positive-prone: 7 out of 10 people in this category will still not have prostate cancer.

The current guidelines of the American Cancer Society recommend that men over age 50 should be "offered" a prostate exam every year. These guidelines may be changing. A new European study has shown that a thorough screening for prostate cancer every 4 years is adequate. The screening comprises a PSA blood test, a digital rectal exam, and a transrectal ultrasound. "Very few, if any, aggressive prostate cancers escape (this) screening..."