In almost all blood collection settings, patients will be positioned in one of two positions: supine or sitting.
The patient's posture prior to and during blood collection can affect laboratory values obtained from certain tests. Differences have been noted in certain laboratory values in patients who have been in a recumbent or supine position, as opposed to those who have been standing or ambulatory for long periods of time. This is due to shifts in body fluids from the blood vessels into the interstitial space. Fluids tend to stay within the bloodstream when the patient is supine. This tends to dilute the blood. There is a shift of fluids into the interstitial spaces upon standing or sitting for prolonged periods of time, which tends to concentrate the blood.
The laboratory tests that are the most affected by this phenomenon of positioning include proteins and components that are highly-bound to proteins:
To minimize variability, attempts should be made to have patients avoid prolonged standing prior to the venipuncture. Approximately 20 minutes are required to equalize the fluid shifts related to changes in position.