Prostate Cancer (cont.)
What are false-positive
elevations in the PSA test?
False-positive elevations in the PSA are increases in the
PSA that are caused by conditions other than prostate cancer. For example,
benign prostatic hypertrophy (BPH) and infection or inflammation of the
prostate (prostatitis) from whatever cause can elevate the PSA. Note also that
even a rectal examination or an ejaculation within the prior 48 hours can sometimes
elevate the PSA. False-positive elevations are usually in the 4 to10 range,
but they can go as high as 25 or 30. At these higher levels, however, caution
in the interpretation of the test is warranted because a prostate cancer may
well be present. Non-prostatic diseases or infections, medications, foods,
smoking, and alcohol do not cause false-positive elevations of the PSA.
The ability of the PSA test to detect prostate cancer
(called the sensitivity of the test) is high. The reason for this is that most
patients, although not all, with prostate cancer have a borderline or an
abnormally elevated PSA. The ability of the test to exclude other diagnoses
(called the specificity of the test), however, is lower because of the other
conditions that can cause false-positive elevations of the PSA.
What refinements have
been made in the PSA test?
Recently, several refinements have been made in the PSA
blood test. The purpose of these refinements is to help doctors to better
assess a borderline or an elevated PSA. The goal is to determine more
accurately who has prostate cancer and who has a false-positive elevation of
the PSA from another condition. In other words, the purpose of the improvements
is to improve the sensitivity and the specificity of the test.
One refinement is called the PSA ratio. This ratio is
determined by dividing the amount of PSA that circulates freely in the blood
stream by the amount of PSA that is bound to proteins in the blood stream.
Research has shown the PSA that circulates freely in the blood tends to be
associated with benign prostatic hypertrophy (BPH) whereas the PSA that is
bound to protein tends to be linked with prostate cancer. Thus, a high PSA
ratio suggests a false-positive elevation of the PSA and weighs against the
diagnosis of prostate cancer. In contrast, a high PSA with a low PSA ratio
favors the diagnosis of prostate cancer.
Another recent modification of the PSA test is based on
the observation that as men age, the amount of PSA in the blood can normally
rise without the presence of a prostate cancer. Thus, doctors can use what is
referred to as an age-specific PSA, especially to evaluate borderline values.
In the age-specific PSA, the normal values are adjusted for the age of the
patient. Accordingly, the age-specific normal ranges are 0 to 2.5 for men in
their 40s, 0 to 3.5 in their 50s, 0 to 4.5 in their 60s, and 0 to 6.5 for men
70 and over. Therefore, as an example, a PSA of 4 would be considered
borderline for men in their 30s and 40s, but could be normal for men in their
50s, 60s, and 70s.
Yet another improvement of the PSA test is called the PSA
velocity or slope. The velocity is calculated as the rate at which the PSA
changes with repeated testing over time. The more rapid the rise in the PSA,
the more likely is the presence of a prostate cancer. The less rapid the rise
in the PSA, the less likelihood there is that a prostate cancer is present.
Next: How is prostate cancer diagnosed? »
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