Prostate Cancer Tests Vary in Specificity, Sensitivity

Q:  For checking a man's prostate, a blood test to determine the prostate-specific antigen (PSA) has been the gold standard of the exam. Now in the past several years, a new reading called free prostate is being used. Just how much more accurate is this than the former PSA, and can it also sometimes be a false positive like the PSA? It is claimed that the free prostate can more accurately determine the possible presence of cancer. Is this true?

A:  Yours is a complicated but important question since prostate cancer is the leading cancer in men, with about 180,000 new cases diagnosed every year. Patients often have no symptoms and are taken by surprise when a nodule is discovered on exam or a high PSA is found.

William See, MD, Professor and Chairman of Urology at the Medical College of Wisconsin, explained to me that there are now at least six different ways to look at serum PSA: total PSA, free PSA, age-adjusted PSA, ethnically adjusted PSA, PSA velocity and PSA density. Each of these has unique characteristics.

Some tests are more sensitive for identifying patients with cancer and others are more specific, meaning that fewer patients without cancer test false positive. Unfortunately, none of the available tests is perfect. All will miss a percentage of cancers (false negative), and all will incorrectly identify some patients who prove not to have cancer (false positive).

The Total PSA test, which measures nanograms of PSA per milliliter of blood, is a more sensitive test. The drawback is that the more sensitive the test, the more likely that the result is a false positive.

The Free PSA test, which measures the percentage of PSA that is not bound to proteins in the blood, is more specific. This means that fewer patients without cancer test false positive.

To give you an example, I have a 65-year-old patient whose PSA went from 3 to 9 in the past year. Since he was double the Total PSA threshold of 4, he underwent a biopsy, which fortunately was negative. So the abnormal PSA was a "false positive" and NOT cancer.

If his Free PSA had been measured at 30% he might have avoided immediate biopsy. The free PSA threshold of less than 25% is more specific for a diagnosis of cancer and avoids the need for biopsy in about 20% of patients who would otherwise undergo this procedure based upon total PSA alone.

Determining the risk of a given patient harboring prostate cancer is not always easy and involves more than just going by one PSA value.

"Dear Doctor" is a compilation of patient questions answered by doctors from the Medical College of Wisconsin.