Prostate Cancer: Interpreting the PSA Test
PSA - what is it?
PSA or Prostate Specific Antigen is a protein
which is secreted into ejaculate fluid by the
healthy prostate. One of its functions is to aid
sperm movement. Normally, only very low levels
of the enzyme are able to enter the bloodstream.
However, because in cancer, the normal structure
of the tissue is disrupted, considerably more PSA
is able to leak into the blood stream, and for this
reason, a raised level of PSA in blood (or serum)
is an indicator for the presence of prostate cancer.
How good is the PSA test?
When we take any test, it is preferable to get a
definite answer: "yes, you have the cancer" or
"no, you don't". Unfortunately the PSA test is
not that good. At best, it is an indicator of the
probability or risk that you have prostate cancer.
This can be very helpful, because it guides the
decision about whether you should have further
tests. However it does mean that the
interpretation of PSA levels is not necessarily
What conditions other than
cancer cause the PSA level
The prostate typically enlarges as men get older.
Because small amounts of PSA are produced by
the healthy prostate, its blood level tends to rise
as men grow older. This is shown in Table 1 below.
Conditions affecting the prostate other than aging
and cancer can cause the PSA in the blood stream
to rise. These include infection and the quite
common condition of benign prostatic
In the United States, about two thirds of older
men who have a raised PSA actually have BPE,
not prostate cancer z . This
- Suggested upper limits of PSA
for different age groups
Age (years) Serum PSA (ng/ml)
40 - 49
50 - 59
60 - 69
70 - 79
Note: These levels are used only as a guide for it is
possible to have prostate cancer and have a PSA
level in the normal range, although this is
means that if you have a raised PSA, you do not
necessarily have prostate cancer one estimate for
this figure is that cancer is found in only one man
in three with an abnormal PSA result. If you have
a rectal examination at the same time, and it also is
abnormal, you're chances of having prostate cancer
are higher, however (one in two).
Other ways of measuring PSA have been
developed in an effort to make the test more
specific for prostate cancer. One of these is called
the "free to total" PSA and is a ratio (expressed as
a per cent). Much of the PSA in the blood is
bound to protein, including that produced by
cancer cells. But men with BPE have higher levels
of free (unbound) PSA. This means that if the
total PSA level is high, the free to total ratio will
give an idea of whether this is due to benign
disease of cancer. This test is not yet widely used
What is a normal PSA level?
Most authorities agree that if you have a PSA
greater than 4 ng/ml, you should have further
investigations. Depending on your age and family
history, your doctor may refer you directly to a
Urologist, or may repeat the test before referring
you for further investigation.
If cancer is present, the level of PSA in the blood
rises as the tumour grows. This means that lower
levels of PSA are found in association with small
tumours which may be still confined to the
prostate gland (ie. haven't spread to the lymph
glands or bones). The PSA level, together with
the cancerous characteristics of the tumour cells
themselves (called "grade") can indicate the risk
that a tumour has grown beyond the prostate
How fast do cancers grow?
Most prostate cancers grow slowly. It can take 5
-10 years after the PSA rises above 2.5ng/ml, for
it to "appear clinically", that is, cause
symptoms4. The median survival time (period
for which 50% of men survive with treatment)
after the PSA starts to rise, is thought to be 10 -
17 years For this reason, a PSA which starts to
rise in an older man, say 75 - 80 years, is usually
not considered to be a threat. In a man just over
50, however, it should be investigated further.
These figures are presented as a guide only - the
outlook for anyone diagnosed with prostate
cancer takes into account many clinical factors
such as the nature of the tumour cells, or tumour
grade, the stage of the disease, other illnesses and
It is clear then, that PSA levels can mean
different things at different times. Nevertheless,
PSA levels are useful, because they indicate the
risk of cancer in those who haven't been
diagnosed with it, and may therefore indicate the
need for further investigations. Typically the
next step in the investigation at this stage is a
biopsy (removal of prostate tissue with a
needle guided by an ultrasound probe in the
rectum). If you have had a PSA test, and know
the result, it is important to discuss all of the
factors involved with your medical adviser, to
help determine your level of risk and the need for
Series - available from your state cancer
organisation - ph: 13 1120.
1. Should I be
2. Interpreting the PSA test
3. After the
4. Monitoring after treatment
6. Useful resources
Sexual function after treatment
Prostate Cancer Tests: the Decision is Yours. A
guide for men considering having a test for
prostate cancer. Anti-Cancer Council of Victoria
(ACCV). A booklet which explains what to
consider before taking a test. Available from
ACCV: ph: 131120.
Prostate Cancer Screening: questions and
answers. Commonwealth Department of Health
and Family Services, 1996. Brochure.
Your Prostate, Your Choices. G. Hirst,
S. Wilde. 1999 Bantam Books. This
comprehensive book explains some of the
complex questions that make decisions about
testing and treatment so difficult.
Prostate Cancer Screening: summary of the
review. Australian Health Technology Advisory
Committee (AHTAC) 1996. AGIP Catalog no
9605282. This is a summary of the evidenced-based review on prostate cancer screening. Full
review also available (AGIP cat no 9605290).