By David Douglas

NEW YORK (Reuters Health) Jun 27 - Men with increased body mass index (BMI) generally have relatively low prostate specific antigen (PSA) levels even when prostate findings are abnormal -- and thus cancers may be missed or not detected promptly -- according to US and Canadian researchers.

The finding is probably due to a dilution effect. "Obese men have lower PSA values likely due to excess blood volume," senior investigator Dr. Stephen J. Freedland told Reuters Health.

Dr. Freedland of Duke University School of Medicine, Durham, North Carolina and colleagues analyzed data on 535 participants in a free prostate screening program and report the findings in the May issue of Urology.

Most of the men (73%) had elevated BMIs and 27% were obese. In all, 51 men (10%) had a serum PSA level of more than 4.0 ng/mL and 8 (2%) had a level above 10 ng/mL. In total, 61 men (12%) had an abnormal digital rectal examination (DRE).

Compared to men with normal weight, overweight men had 5% lower PSA values. For mildly obese men the difference was 14%, and in moderately and severely obese men, the values were 29% lower.

While BMI had an inverse relationship with PSA, there was no significant association between BMI and DRE findings, the researchers report.

"Thus, when interpreting a PSA value in an obese man," concluded Dr. Freedland, "we should adjust the value we call abnormal downwards to reflect this diluted PSA measurement. If we do not, we may be missing cancers in obese men."

Urology 2008;71:787-791.