The test looks for a protein called prostate-specific antigen, or PSA, which is often elevated in men with prostate cancer. However, the new study shows that men whose PSA value is abnormally high one day frequently have a normal result on a later retest.
Since PSA levels in blood may vary from day to day, "it's probably prudent to recheck a level" at least six weeks later before advising a patient to have a biopsy, says study author Dr. James Eastham, a surgeon at Memorial Sloan-Kettering Cancer Center in New York City. Biopsies are costly and carry the risk of bleeding, infection and discomfort.
PSA is a protein in blood that surges in men with prostate cancer, as well as those with other, benign prostate conditions. In about 10 percent of men with cancer, the protein remains in the normal range. Although PSA testing is often encouraged for men over 50, its value as an early-detection tool isn't certain, and evidence is mixed about whether the exam can prevent deaths from prostate cancer.
"Even with flaws, it is a very useful tool. You just have to use it in an appropriate way," says Eastham, whose research appears in the May 28 issue of the Journal of the American Medical Association.
More than 220,000 American men will be diagnosed with prostate cancer this year, and nearly 29,000 will die of the disease, according to the American Cancer Society. The tumors are typically slow-growing, however, and most men with prostate cancer die of other causes.
In the latest research, Eastham's group followed 972 men, whose average age was 62, enrolled in a seven-year colon cancer prevention study. Each had five separate PSA tests drawn over a four-year period.
About one in five men had a PSA level considered high -- over 4 nanograms per milliliter of blood -- on at least one reading. Yet on a subsequent test, roughly half of them had fallen back into the normal range for the protein, suggesting the spike wasn't a signal of prostate cancer.
"A single abnormal PSA level should be viewed with caution," the researchers write. Doctors should confirm the reading "before expensive or invasive tests, such as a prostate biopsy, are recommended."