Modeling the effects of obesity and weight gain on PSA velocity.

BACKGROUND: Hemodilution theory states that higher blood volume in obese men effectively dilutes circulating PSA levels resulting in lower PSA test results. Here we apply hemodilution theory to model the effect of weight gain on PSA velocity.
METHODS: Hemodilution formulas were used to model PSA velocity for a series of plausible scenarios in which initial weight and weight gain were varied. The formulas were also applied to published summary data on weight, weight change and PSA velocity from the Prostate Cancer Prevention Trial (PCPT).
RESULTS: Under hemodilution theory, PSA velocity is understood to be influenced by total circulating PSA mass (ng of PSA) and BMI at the initial test and total circulating PSA mass and BMI at the subsequent test. PSA velocity in a man with a stable BMI of 35 is estimated to be 13% lower than in a man with a stable BMI of 25. A gain of 4 BMI units is predicted to attenuate PSA velocity by as much as 25%. When applied to summary data from the PCPT, the formulas estimate that a 10 pound weight gain causes a -0.028 ng/ml change in PSA, which closely matches PCPT results where a 10 pound weight gain caused a -0.024 ng/ml change in PSA. We provide software to implement the hemodilution formulas to model PSA velocity for different weights and changes in weight through time (http://www.eheintl.com/psa.jsp).
CONCLUSION: Stable obesity and weight gain both independently attenuate PSA velocity, potentially obscuring clinically relevant changes in circulating PSA.