What is Prostate Specific Antigen (PSA)?

Prostate-specific antigen (PSA) is a substance made by the normal prostate gland. Although PSA is mostly found in semen, a small amount is also present in the blood. Most men have levels under 4 nanograms per milliliter (ng/ml) of blood. However, when prostate cancer develops, the PSA level usually rises above 4ng/ml of blood.

However, the equation of elevated levels of PSA in the blood and the presence of prostate cancer is not entirely straightforward. With a PSA level between 4 and 10 there is a 25% chance (1 in 4) of having prostate cancer. If it is more than 10, your chance of having prostate cancer is over 50% and increases further as your PSA level increases. However, it should also be noted that 15% of men over 50 with PSA levels below 4 will prove to have prostate cancer when a biopsy is taken. There are other factors which also affect PSA antigen levels and you can learn about these here.

Other Complicating Factors

PSA levels can also be affected by a number of other factors, such as:

  • PSA levels will increase if you have an enlarged prostate. The most common cause of this being benign prostatic hyperplasia (BPH) and this is something many men develop as they grow older.
  • The levels of PSA will also increase if you have an infection of the prostate (called prostatitis.
  • The level of PSA will increase naturally as a man ages and it's important to check the current level against the expected level for the age you currently are
  • The levels of PSA will generally increase after ejaculation and it is recommended to refrain from ejaculating for two days before a PSA test.
  • Certain medicines can also affect PSA levels (especially 5-α-reductase inhibitors such as finasteride.
  • A number of herbal preparations (especially those targetted at prostate cancer) may also potentially mask an elevated PSA level.

PSA Tests — Latest Developments

Percent-free PSA

Biochemically, there are two major types of PSA in the male bloodstream. One circulates freely and the other is attached (complexed) to other blood proteins. The latest development is PSA analysis is the Percent-free PSA Test. This measures how much PSA circulates free compared to the total PSA level. It has been found that the percentage of free PSA is lower in men who have prostate cancer than in men who do not.

General practice is for a man with a PSA level in the 4–10 ng/ml level and a free PSA level of less that 10% (which means that the likelihood of having prostate cancer is about 50%) should have a biopsy as a matter of course. In fact, many doctors recommend biopsies for men whose percent-free PSA is 25% or less. However, a recent study found that if men with borderline PSA results had prostate biopsies only when their percent-free PSA was 25% or less then almost 20% of unnecessary prostate biopsies could be avoided, and about 95% of cancers would still be detected.

PSA Velocity

For anyone having regular (yearly) PSA screening this can be a very useful test. This test measures the overall change in PSA levels over time. As a general rule, if the PSA levels go up faster than 0.75 ng/ml per year this is considered a high rete of velocity and even if the overall level is less than 4ng/ml a biopsy is indicated.

PSA Density

For men with an existing enlarged prostate the PSA test may be problematic and another test can be used. First the doctor determines the overall volume of the prostate gland, using transrectal ultrasound. The PSA level is then determined and is divided by the prostate volume. A high PSA density is supposedly indicative of a greater likelihood of prostate cancer but this test has not proven to be very accurate and determination of Percent-free PSA may be a better indicator.

Age-specific PSA Ranges

Because of the increase in overall PSA levels with age, a borderline PSA level may be of concern to a man of 50 but would lie in the normal range for a man of 80. For this reason, some doctors have suggested comparing PSA results with results from other men of the same age. However, this is a new technique and is not well proven at this time.