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Cells in the prostate gland produce a protein called prostate-specific antigen (PSA). Semen contains a large volume of PSA, and smaller amounts can be found in the blood. By monitoring blood levels of PSA, it's possible to detect prostate cancer in its earliest stages. PSA is measured in nanograms per milliliter, written as ng/mL. What do the numbers mean?

• 4 ng/mL or lower: A normal PSA level for most men, but in a minority of cases, prostate cancer may still be present.
• Between 4 and 10 ng/mL: Suggests the possibility of prostate cancer. At these levels, there is approximately a 25% chance that you have prostate cancer.
• Above 10 ng/mL: There is a 50% chance that prostate cancer is present. The higher the PSA rises above 10 ng/mL, the greater the chance that you have prostate cancer.

It's important to remember that PSA levels can rise for reasons other than prostate cancer, including aging, infection, the use of certain herbal supplements, and an enlarged prostate (not due to cancer). Your PSA level may also increase after ejaculation, so doctors advise men to refrain from ejaculation at least two days prior to a PSA exam—and be sure to inform your doctor about all medications and herbal supplements you're taking.

If a PSA test or digital rectal exam raises concerns, your doctor will recommend a biopsy to establish a diagnosis. After the biopsy, the pathologist will look for cancer cells in the tissue samples taken from the prostate.