Prostate Cancer Testing
How Effective is PSA Testing?
As men age, their chance of getting prostate cancer increases. Men over the age of 50 are often routinely tested for their level of Prostate Specific Antigen, a marker of prostate cancer. Ideally, we would like such a marker to be totally trustworthy, as depicted in the figure on the right: If the measured PSA level is below 4 ng/ml (nanogram per mililiter), the patient is free of cancer. If it is above, we would be sure that the patient has cancer, and subsequently gets treated. Unfortunately, the situation is not as clear as we would ideally like. Many patients who test greater than 4 ng/ml find, after undergoing painful biopsies, that they do not have detectable cancer. Likewise, patients whose level is below 4 ng/ml, find they have cancer.
A study (published in the New England Journal of Medicine, Vol 350, p 2239, 2004) demonstrates very clearly that people with low PSA values are also in danger of having prostate cancer. In the figure to the right, their results (full line) show the percentage of patients with prostate cancer, as a function of the measured PSA level. (The broken lines depict the range of results within the Margin of Error). Even at the very lowest PSA level, 7% of the patients are found to have this cancer! At 3, this value rises to 27%. The conclusion is that while high levels of PSA certainly indicate a higher probability of cancer, a low level (below 4 ng/ml) does not assure the absence of it. So how does the PSA result impact a decision to do further biopsies?
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