by Megan Rauscher

Last Updated: 2007-11-20 16:00:24 -0400 (Reuters Health)

NEW YORK (Reuters Health) – Hemodilution related to larger plasma volume may be responsible for the lower prostate-specific antigen (PSA) levels observed in overweight and obese men as compared with normal weight men, new findings suggest.

Recent evidence has indicated that prostate cancer screening may be adversely affected by increased BMI, the researchers note in the November 21 issue of the Journal of the American Medical Association.

In a retrospective study of nearly 14,000 men with prostate cancer treated by radical prostatectomy, Dr. Stephen J. Freedland from Duke University Medical Center, Durham, North Carolina, and colleagues found that men with a BMI of 35 or greater had 21% to 23% greater plasma volume than normal-weight men.

Obese men also had lower PSA values, as others have shown. In this study, men with a BMI of 35 or greater had 11% to 21% lower PSA concentrations relative to normal-weight men.

"But very interestingly," Dr. Freedland told Reuters Health, "when we looked at the amount of PSA in the blood, we actually saw no association between obesity level and the amount of PSA in the blood. Obese men had just as much PSA in the bloodstream or even slightly more and yet their concentrations were lower."

This suggests that circulating PSA in obese men is diluted by their larger plasma volume.

"The consequence is that we may actually be missing cancers in obese men," Dr. Freedland said. "It’s not that PSA is a bad test for obese men; it’s just that we need to learn how to use it better, and when we do that I think we are going to be able to detect these cancers earlier in obese men."

JAMA 2007;298:2275-2280.