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WHAT IS PSA?
(Text from the American Foundation for Urologic Disease.)
Prostate-specific antigen (PSA) is a protein originally found in semen, the fluid that carries sperm. Normally, PSA is made in cells of the prostate called epithelial cells. Epithelial cells make some of the semen that comes out of the penis at the time of sexual climax (orgasm). PSA helps to keep the semen in its liquid form.
PSA is made only by prostate cells. Small amounts of the protein get into the circulation and can be measured in the blood. Certain prostate conditions, including prostate cancer, can cause high levels of PSA in the blood. PSA itself does not have any known effect outside the prostate; even high levels of PSA are not directly harmful. High PSA levels in the blood, however, may be a sign of a prostate problem, perhaps even a serious one. In addition to prostate problems, some medical treatments can affect PSA levels.
The growth of the prostate, as well as the production of PSA itself, is controlled by male hormones. Any treatment that lowers male hormone levels will lead to a drop in PSA blood levels.
How Is PSA Measured?
As with many other routine blood tests, PSA is measured from a small sample of blood. Once a blood sample is taken, the level of PSA in the sample is measured by an accurate laboratory method called an immunoassay. The results are usually reported in ng/ml, shorthand for nanograms per milliliter.
What Is a "Normal" PSA?
There are several different ways to measure PSA. Most physicians think that the "normal range" is between 0 and 4.0 ng/ml for the most common PSA tests. (Because some PSA tests have different normal ranges, you should check with your physician on this point.) The blood level of PSA is often high in men with prostate cancer. However, many men with a PSA level higher than 4 ng/ml do not have prostate cancer or any other important prostate problem. Likewise, a PSA value lower than 4 ng/ml does not rule out the possibility of cancer. The lab's "normal" upper level is simply a cutoff point used to separate men who are less likely to have prostate cancer from those for whom further prostate cancer testing may be appropriate, depending upon the circumstances.
What Causes PSA to Rise?
The level of PSA in the bloodstream may be elevated by an process that leads to an increase in the number of cells making PSA or to a breakdown of the normal barriers in the prostate that prevent much PSA from getting into the bloodstream. The most common condition leading to a high PSA is benign (noncancerous) enlargement of the prostate, called benign prostatic hyperplasia (BPH). BPH is very common in men over the age of 50 and may lead to difficulty with urination. Infection or inflammation in the prostate, called prostatitis, may also cause elevation of PSA by damaging the PSA barrier in the prostate. In addition, some diagnostic tests, such as a needle biopsy of the prostate, may increase PSA levels for several weeks.
It does not appear that a routine digital rectal examination (DRE) of the prostate by the doctor's finger causes an elevation of the PSA.
Causes of an Elevated PSA
- Benign prostatic hyperplasia (BPH)
- Prostate cancer
- Prostate infection
- Prostate biopsy
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Both BPH and prostate cancer are common in men over the age of 50. In addition, there is a lot of overlap in blood PSA levels between men with BPH and those with early prostate cancer. These factors limit the usefulness of PSA as a tool for detecting curable prostate cancer. Many patients who have a PSA level higher than 4 ng/ml will eventually be found not to have prostate cancer. These men have a "false-positive" test. If PSA is tested on men with BPH but no prostate cancer, as many as one-third to one-half of such men will have an elevated PSA. Their PSA results, however, are generally in the 4 to 10 ng/ml range.
In addition to false-positive tests, the PSA may be falsely negative -- that is, normal even when prostate cancer is present. Some 30 to 40 percent of patients with early-stage prostate cancer have a normal PSA. Repeating PSA tests once every year may be useful to find some of the cancers in men who have a normal PSA at first.
False-negative and false-positive findings limit the value of PSA testing. Despite this, PSA testing has led to an increase in the detection of prostate cancer.
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