The prostate specific antigen (PSA) test is generally regarded as the best screening tool currently available for detecting prostate cancer in its early stages, when there's the greatest chance for successful treatment.

Test results report the level of PSA detected in the blood. According to the American Cancer Society, a PSA level ranging from 0 to 4 nanograms per milliliter (ng/mL) is considered normal for an average man. A PSA level of 4 to 10 ng/mL is considered slightly elevated; levels between 10 and 20 ng/mL are considered moderately elevated; and anything above that is considered highly elevated.

As a general guideline, a reading between 4 and 10 ng/mL indicates a 20 percent to 50 percent chance that cancer is present, while a measurement above 10 ng/mL is considered a strong indication of prostate cancer.

However, while the test is best method currently available for early detection of prostate cancer, it is far from perfect.

Tests of a significant number of healthy men yield elevated PSA readings. These "false positives" are fairly common in men with benign prostatic hyperplasia (BPH), a prostate infection, or who ejaculated within two days before taking a PSA test.

For these men, false-positives often lead to additional medical procedures such a biopsies, with significant financial costs and creating unwarranted anxiety for the patient and his family.

On the other hand, PSA tests sometimes produce readings in the normal range when prostate cancer is actually present.

Certain hair-growth and BPH medications, and some herbal supplements, can produce lower PSA levels. PSA readings are believed to miss about 25 percent of all tumors.

Researchers are working on ways to improve the accuracy of the PSA test, and some doctors use "age-adjusted PSA levels" -- defining a different level as normal for different age groups -- to determine if a biopsy is needed.

Others may rely more on "PSA velocity" -- with a sharp rise in the PSA level over a short period of time raising the suspicion of cancer. Another more controversial measure is "PSA density," which considers the relationship of the PSA level to the size and weight of the prostate.

But until a better screening tool is developed, the PSA test will continue to be the best hope for early detection of prostate cancer.