A study on prostate-specific antigen (PSA) tests released in yesterday's Journal of the American Medical Association confirms what ACSH's Dr. Elizabeth Whelan has been saying since 1992 -- PSA tests promise more than they can deliver. For years, men have been conditioned to think that if their PSA result is below 4.0 ng/mL, they do not have to be concerned about prostate cancer. But in fact, there is no cutoff PSA value that is reliable for accurately ruling out cancer in some patients and detecting it in others.
The problems surrounding the PSA test boil down to two everyday words that take on a special meaning in a medical context: sensitivity and specificity. Sensitivity refers to the ability of a test to correctly identify people who have the disease. Specificity, on the other hand, refers to the ability of a test to rule out a disease in people who do not have it. A test that is only sensitive is a bit like a judge who finds all defendants guilty; all those who were truly criminals would in fact be brought to justice, but some innocents would be punished as well. A judge who only cared about specificity would find all defendants innocent, which would eliminate false imprisonment but would let some of the guilty go free. To be a truly valuable diagnostic tool, a judge (or a prostate cancer screening test) must be both sensitive and specific. The PSA is neither.
The study indicates that a clear-cut decision rule for biopsies based on PSA results is out of the question. The researchers found that PSA cutoff values of 1.1, 2.1, 3.1, and 4.1 ng/mL corresponded to sensitivities of 83.4%, 52.6%, 32.2%, and 20.5%, and to specificities of 38.9%, 72.5%, 86.7%, and 93.8%, respectively. The widely accepted 4.1 ng/mL cutoff value would have a 6.2% false positive rate (in other words, 6.2% of men testing positive would undergo unnecessary biopsies) but would have a 20.5% false negative rate (20.5% of men testing negative would erroneously think they were safe). Evidently, PSA is not a definitive screening tool.
The JAMA study should not be interpreted as a reason to forego PSA testing altogether. On the contrary, PSA should continue to be an important part of prostate cancer screening. However, patients should set aside the notion that a reading below 4.0 means that they are in the clear. Physicians and patients together can use a combination of PSAs, digital rectal examination, family history, and personal preference to determine whether a biopsy is needed.
Mara Burney is a research intern at the American Council on Science and Health.
Read ACSH's booklet on Risks for Prostate Cancer.