Nov. 20, 2007 -- Obese men may have prostate cancer despite relatively low values on a prostate cancer screening test called the PSA test, a new study suggests.
The PSA test detects PSA -- prostate-specific antigen -- in the blood. Only prostate cells give off PSA. PSA levels can rise as prostate tumors grow.
Earlier studies showed that obese men have slightly lower PSA concentrations than lean men do. It's not clear why this is so. New findings from Duke University researcher Stephen J. Freedland, MD, and colleagues now suggest an answer.
"Being a big guy, you have enormous blood volume, so PSA is diluted," Freedland tells WebMD. "Obese men having these lower PSA values due to the dilution factor means we are going to miss some of them early on."
The researchers reviewed medical records from some 14,000 men with prostate cancer who had their prostate glands removed. Based on the men's body mass index, and adjusting for differences in the men's clinical conditions and prostate pathology, the researchers used pre-operative PSA test values to calculate the actual amount of PSA in the men's blood.
They found that obese men had lower PSA values than did non-obese men even though they had equal or even higher amounts of PSA in their blood.
"What this suggests is if we use the same PSA threshold for obese and normal-weight men, we may be missing some cancers," Freedland says. "For example, a PSA score of 4.1 in an obese man would be diluted down to a 3.3."
Alan R. Kristal, DrPH, one of the researchers investigating the link between obesity and prostate cancer, strongly disagrees with Freedland's calculation. Kristal is associate head of the cancer prevention program at Seattle's Fred Hutchinson Cancer Research Center.
"This is an over-interpretation of the data. Most studies show the difference is much smaller," Kristal tells WebMD. "Doctors will look not just at the PSA level but the speed of PSA increase over time, the PSA relative to other men the same age, findings from a digital rectal exam, and other information before they make a decision on whether to do biopsy. These teeny-tiny differences due to obesity have no impact."
It's an important dispute to resolve. As Freedland points out, there soon will be other blood tests for other kinds of cancer.
"This issue is something we need to keep in mind as we move forward with blood-based cancer screening," Freedland says.
Freedland and colleagues report their findings in the Nov. 21 issue of The Journal of the American Medical Association.