Objectives: To provide a mathematical background for understanding the phenomenon of analyte hemodilution using a kinetic analysis. Methods: The first assumption for this analysis is that change in concentration of any analyte, such as prostate-specific antigen (PSA), is due to the flux of the analyte from an organ into the blood minus its flux from the blood. What results is a relatively simple differential equation that emphasizes the importance of plasma volume, organ mass, and two rate constants. Results: The analyses demonstrate how serum PSA can be affected by plasma volume as well as body mass and how hemodilution due to obesity can be at least partly corrected for by expressing PSA in units of total mass or total mass density. Conclusions: At a time when obesity is prevalent, expressing analytes in units of total mass may make them relate more closely to disease status and prognosis.