Water in the body moves readily from the blood vessels into the space between the cells in the body. Posture has a strong effect on how much water flows into or out of the blood vessels. Research has shown that “Immediately upon assuming a new posture the plasma begins to shift and continues to shift for 20 to 40 minutes until a new equilibrium is obtained”. To get consistent results, USADA requires athletes to remain in a seated position for a minimum of 20 minutes prior to the blood draw.
In order to promote good practice and consistency, as well as assist all National Anti-Doping Organizations in the development of systems and protocols, the World Anti-Doping Agency has issued a guideline document on blood sample collection. This document is in accordance with the World Anti-Doping Code and other International Standard documents. The USADA blood sample collection process conforms with these WADA documents.
 Sawka MN, “Body fluid responses and hypohydration during exercise-heat stress.” In Human Performance Physiology and Environment Medicine at Terrestrial Extremes. Pandolf KB, Sawka MN, and Gonzales RR, eds. (Cooper Publishing Group, Traverse City, MI, 2001) p 239