Context: Recent studies have suggested that obese men have lower serum prostate-specific antigen (PSA) concentrations than nonobese men. Because men with higher body mass index (BMI) have greater circulating plasma volumes, lower PSA concentrations among obese men may be due to hemodilution. Objective: To determine the association between hemodilution and PSA concentration in obese men with prostate cancer. Design, Setting, and Participants: Retrospective study of men who underwent radical prostatectomy for prostate adenocarcinoma from 1988 to 2006, using data from the databases of the Shared Equal Access Regional Cancer Hospital (n=1373), Duke Prostate Center (n=1974), and Johns Hopkins Hospital (n=10 287). Multivariate linear regression models adjusting for clinicopathological characteristics were used to analyze the main outcome measures. Main Outcome Measures: Associations between BMI and mean adjusted PSA concentrations, mean plasma volume, and mean adjusted PSA mass (total circulating PSA protein, calculated as PSA concentration multiplied by plasma volume), assessed by determining P values for trend. Results: After controlling for clinicopathological characteristics, higher BMI was significantly associated with higher plasma volume (P<.001 for trend) and lower PSA concentrations (P≤.02 for trend) in all cohorts. In 2 of the 3 cohorts, PSA mass did not change significantly with increasing BMI. In the third cohort, higher BMI was associated with increased PSA mass (P<.001 for trend), but only between BMI category less than 25 and the other categories. Conclusions: In men undergoing radical prostatectomy, higher BMI was associated with higher plasma volume; hemodilution may therefore be responsible for the lower serum PSA concentrations among obese men with prostate cancer. Prospective studies are needed to evaluate this association in screened populations.
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