Low PSA, typically 4 nanograms or less per milliliter of blood, suggests that prostate cancer is not present. A PSA test measures the levels of prostate specific antigen, a protein produced by the prostate, in the blood, according to Health.com.
High levels of PSA are often present in men with prostate cancer, indicates the National Cancer Institute; however, there are other non-cancerous conditions that may result in high PSA, and some prostate cancer patients do not have elevated PSA levels. Therefore, other tests, such as a digital rectal exam, are often conducted together with a PSA test to assist doctors in establishing the nature of the problem.
Low PSA levels generally indicate that a man does not have prostate cancer, but the PSA test is not 100 percent accurate because there are various factors that can cause a man's PSA levels to fluctuate, states the National Cancer Institute. Urinary tract infections, prostate biopsies, prostatitis or prostate surgery may cause elevated PSA levels. Conversely, some drugs may lower PSA levels in men. These and other fluctuations can lead to false positives or false negatives. If prostate cancer is suspected, a doctor may conduct a biopsy to sample and test prostate tissue.