The free:total PSA ratio is useful in assessing the risk of prostate cancer in patients with slightly high PSA levels, between 4 and 10 nanograms per millilitre (ng/mL). When the total PSA result is in this grey area, the free:total PSA ratio is used to determine whether this increase could be caused by prostate cancer (25% of cases) or by other factors (benign prostatic hyperplasia, recent sexual activity, intense physical exercise or other factors (75% of cases)). PSA exists in the blood in either a protein-bound or a free form (free PSA). Total PSA measures both forms, while free PSA measures only the free form. In cancer, the fraction (ratio) of the free form decreases. A low free:total PSA ratio is therefore indicative of a higher cancer risk, while a higher ratio points more to benign prostatic hyperplasia. It is important to know that a normal result does not completely rule out the possibility of prostate cancer. Also note that results can vary from one laboratory to another, depending on the testing technique used. It is therefore recommended to always use the same laboratory for monitoring.