PSA density … PSA velocity … what does it all mean? Jack L. Mostwin, M.D., professor of urology at Johns Hopkins explains these important terms.
All men, aged 40 or so, undergo PSA (prostate-specific antigen) testing to screen them for prostate cancer. PSA is an enzyme produced almost exclusively by the glandular cells of the prostate and normally only very small amounts of PSA are present in the blood. High levels of PSA can indicate prostate problems, including BPH and prostate cancer. But high PSA levels can also be caused by a variety of daily activities. Recently, researchers have developed several ways to improve the PSA test’s accuracy. These improvements include:
- PSA density -- assessing PSA level in relation to prostate size
PSA density takes the size of a man’s prostate into account when evaluating his PSA level. It is calculated by dividing the PSA value by the size of the prostate (as determined by transrectal ultrasound). This measurement helps doctors distinguish between BPH and prostate cancer: The higher the PSA density, the greater the chance of cancer, because the elevated PSA level is less likely to be the result of daily activities or benign prostate enlargement.
According to several studies, a PSA density greater than 0.15 indicates a higher risk of cancer. PSA density appears most useful in diagnosing prostate cancer in men with PSA levels between 4 ng/ml and 10 ng/ml.
- PSA velocity—monitoring annual changes in PSA velocity
This measurement takes into account annual changes in PSA values, which rise more rapidly in men with prostate cancer than in men without the disease. PSA velocity is especially helpful in detecting early cancer in men with mildly elevated PSA levels and a normal digital rectal exam. It is most useful in predicting the presence of cancer when changes in PSA are evaluated over at least one to two years.
- Percent PSA—measuring the ratio of free to total PSA (percent free PSA or complexed PSA)
PSA in the blood is either bound (attached to proteins) or unbound (free). PSA assays usually measure the total PSA (both free and complexed). Other assays measure the percentage of free PSA or the percentage of complexed PSA.
Compared to men with BPH, men with prostate cancer have a higher percentage of bound PSA and a lower percentage of free PSA. Research suggests that determining the ratio of free to total PSA in the blood helps distinguish between PSA elevations due to cancer and those caused by BPH.
- Age-specific PSA—adjusting the PSA result for a patient’s age.
PSA increases with age because the prostate gradually enlarges as men grow older. Some years ago, researchers suggested adjusting PSA levels to the age of the patient: Higher levels would be considered normal in older men, and lower levels considered normal in younger men. However, there is concern that the use of higher PSA thresholds in older men will miss important cancers. As noted above, physicians should suspect the presence of prostate cancer when levels are above 2.5 ng/ml in men in their 40s.
