Higher Blood Volume in Large
Men Dilutes Prostate Blood Test

It turns out that larger men have more blood, which dilutes the levels of the protein called PSA — a key indicator of prostate trouble, says a report in the Journal of the American Medical Association (JAMA).

In recent years, physicians have learned that they need to adjust the results of blood tests to properly diagnose prostate cancer in obese men, but now researchers believe they know why.

The new study does not definitively prove why overweight and obese men with prostate cancer tend to score lower on the PSA (prostate specific antigen) test, but it does give physicians an idea about what may be going on, says study co-author Dr. Stephen Freedland, at Duke University.

"PSA is not a bad test for obese men," says Dr. Freedland. "We just need to know how to use it. If we use it correctly, it will be just as good as in normal-weight men."

PSA Levels Equal Diagnostic Challenge
Prostate cancer strikes one in six men, mostly those over the age of 65, according to the Prostate Cancer Foundation.

Older men often routinely undergo a PSA blood test that looks for an antigen made by the prostate that helps it function. The antigen leaks into the bloodstream at a steady rate, and the amount is higher in men with prostate cancer, explains Dr. Freedland.

Recent research has revealed that PSA levels are 20 percent to 25 percent lower in overweight and obese men than in men of normal weight. This can translate into diagnostic problems with bigger men, says Dr. Freedland.

"You may call him normal and not worry about him and wait another year or two or longer, and that gives time for the cancer to grow," he notes.

According to Dr. Freedland, it is not clear if there is any impact on PSA levels from being underweight.

In the new study, researchers tested a theory that the reason PSA levels are lower in larger men is because the antigen gets diluted in larger volumes of blood.

"It's like taking a little bit of a drug and putting it in a cup of water versus a bowl of water," says Dr. Freedland.

The study authors looked at the records of about 14,000 men with prostate cancer who underwent removal of their prostates between 1988 and 2006.

The researchers found that men who were fatter had larger blood volumes, which the study authors said supports their theory that dilution caused their PSA levels to lower.

More Study Needed, Experts Say
The same thing could hold true for blood markers of other cancers, says Dr. Freedland.

"As we develop blood tests for other cancers, we should be keeping this in mind," he adds.

However, Dr. Nelson Stone, at Mount Sinai School of Medicine in New York City, is skeptical of the results, because the research just focused on men with advanced prostate cancer. He believes this and other factors may have skewed the results.

"I don't think they've proved their point," he says. But he adds, "We need to be a little bit more careful when we evaluate a patient who is larger, and we look at his PSA and compare to a patient who is thinner. The PSAs are not the same."

Always consult your physician for more information.

Online Resources

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American Urological Association, Inc.

CDC - Men's Health

Journal of the American Medical Association (JAMA) - Obesity-Related Plasma Hemodilution and PSA Concentration Among Men With Prostate Cancer

Men's Health Network

National Prostate Cancer Coalition

NIH - Prostate Cancer

Prostate Cancer Foundation