PSA TEST: PROSTATE-SPECIFIC, NOT CANCER-SPECIFIC
Posted on March 27, 2009, under Men's Health-Erectile Dysfunction.
PSA is an enzyme whose purpose seems to be to break down coagulated semen (this is especially useful in some animals. PSA is made almost exclusively by the prostate; and it is prostate-specific, not cancer-specific. In other words, you can have prostate cancer and still have a low PSA level. This is why a blood test alone isn’t enough, why a digital rectal exam is also a must. And, just because you have a high PSA does not necessarily mean you have prostate cancer—many men with high PSA levels don’t.
However, when PSA is elevated, over a level of four nanograms per milliliter, this means you have some sort of prostate trouble—maybe BPH, maybe cancer, maybe an infection. Maybe the prostate has undergone some trauma, like a needle biopsy, or you’ve had a TUR procedure for BPH, or even a vigorous rectal exam—all of these can elevate PSA. (Conversely, taking the drug finasteride to treat BPH can artificially lower the PSA reading.
Gram for gram, PSA levels in the blood are about ten times higher in cancerous tissue than benign tissue. PSA is normally secreted and disposed of through tiny ducts in the prostate. But in prostate cancer, the ductal system doesn’t drain into the urethra. The PSA builds up, leaks out of the prostate and shows up in the bloodstream. That’s why it has proven to be such a good marker for cancer.
But the PSA test is far from perfect. Just as high readings can signify a problem other than cancer, low readings can also be misleading. The bottom line is that even if your PSA is low, you could still have cancer. About a quarter of men who turn out to have prostate cancer have a low PSA level, less than 4 nanograms per milliliter. About 25 percent of men with a PSA between 4 and 10 turn out to have cancer. In men with a PSA over 10, about 65 percent are found to have cancer.
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