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The PSA Test: How Does It
Work?
Who Should Have It Done?
How Accurate Is It?
For over ten years, medical researchers have known about Prostate Specific
Antigen -- PSA -- as a "marker" for prostate cancer. It has only
been in the last several years, however, that testing for this substance
in men's blood has been used to help assess a man's risk for the disease.
According to William
J. Catalona, M.D., Washington University School of Medicine, St. Louis,
in a report published recently by the Journal
of the American Medical Association, "An elevated PSA does not mean
the presence of cancer, but rather indicates the presence of prostatic
disease and the need for further evaluation. Cancers smaller than 1 cm
usually do not elevate PSA levels. More than 90 percent of cancers
detected through PSA-based screening have the potential to become clinically
important, judged by their size and grade. The goal of PSA screening,"
he stressed, "should be to detect the cancer before the PSA level rises
above 10 ng/L."
WHAT IS IT?
PSA is a protein excreted only by the prostate, a gland located at the
neck of the bladder that surrounds the urethra, the tube through which
urine and sperm exit. The prostate is one of the male sex glands
that works in response to hormones to secrete fluids that help transport
sperm during ejaculation. A healthy prostate is approximately the
size of a walnut and weighs about an ounce.
For most of a man's life, this gland does its job without notice.
Then, somewhere around age 50, the gland suddenly has a growth spurt.
It can become enlarged enough to squeeze the urethra, playing havoc with
a man's ability to urinate.
In most cases, this is caused by a condition called Benign Prostatic
Hyperplasia (BPH) rather than cancer. But, it is during this same
time frame when a man's risk for developing prostate cancer begins increasing.
The disease rarely exhibits any symptoms in its early stages so, by the
time symptoms do appear, the cancer is likely to have already spread into
other parts of the body.
Measuring a man's PSA level involves a simple blood test.
Generally, the higher the level of PSA in the blood, the more likely it
is that prostate cancer will be found. Cancerous prostate tissue
secretes about 10 times as much PSA as normal tissue does. It is
measured in nanograms -- billionths of a gram -- per milliliter of blood.
Levels below 4 ng/mL are considered the upper range of normal. (If
the test turns up more than 10 ng/mL, there is a 67 percent chance that
the man has prostate cancer.)
There are several things other than cancer that can drive the levels
up: prostate enlargement, some medications, infection, or a recent biopsy.
Despite earlier warnings, a digital exam does not provoke additional secretion
of the protein. The drug finasteride (ProscarTM ) -- prescribed for
an enlarged prostate -- can also skew a PSA test by reducing the level.
The change in PSA levels has also turned out to be an important diagnostic
tool to detect the new -- curable -- prostate cancers. Tracking year-to-year
changes in the PSA level can alert the physician that additional tests
may be appropriate.
HOW ACCURATE IS IT?
As pointed out earlier, no single test is absolutely fail-safe in the
detection of prostate cancer. In recent studies, one fourth of men
with mild PSA level elevations (4 to 10 ng/mL) were found to have cancer,
although it sometimes took 2, 3 or even 4 biopsy sessions to diagnose the
cancer through a biopsy; most of them were confined to the prostate.
The other three quarters were false-positive results. There were
also some false-negative cases -- men with cancer that did not elevate
the PSA.
Rates of false-positive PSA screening test results increase with age
since about 10 to 12 percent of all men over the age of 50 have elevated
PSA levels. The test also fails to detect 20 to 40 percent of the
smaller cancers that occur near the rectal surface of the prostate and
can be detected by DRE.
WHO SHOULD HAVE THE PSA TEST?
While the results of a number of studies to determine the overall value
of prostate cancer screening are still years away, the American
Cancer Society and the American Urological
Association now recommend PSA testing -- along with a digital rectal
exam -- beginning at age 50, and at age 40 for men in the high risk categories
and those with a strong family history of the disease.
Preliminary studies -- reported in both the New
England Journal of Medicine and the Journal
of the American Medical Association -- found that measuring PSA levels,
when done along with a DRE, increases the chances of detecting PC by about
70 percent, compared to using the DRE alone, and more than doubles the
percentage of cancers that are confined to the prostate at the time of
diagnosis. |
