Time required for resetting postural effects on serum constituents in healthy individuals

We investigated postural effects on concentrations of commonly measured serum analytes and sought for optimal duration of sitting before venipuncture in an outpatient setting.

Seven healthy volunteers changed posture serially at 30-minute (min) intervals from lying, sitting, slowly walking, sitting, and to lying again. At the end of the first lying posture, wing-needle was inserted into cubital vein and 3mL of blood was drawn every 5min. All the serum was stored at -80°C until the time of collective measurements for 16 major chemistry analytes two weeks later.

Concentrations of macromolecular analytes (albumin, total cholesterol, alkaline phosphatase, etc.) in supine position were increased (on average 5-7%) after 30min of sitting and further increased (10-14%) during 30min of walking. After slow walking, time required for return of values to the levels of sitting posture took approximately 20min. Concentrations of small molecules (sodium, urea, etc.) did not show any variation by the postural changes. Potassium and inorganic phosphate showed walking-related increase in concentrations. Creatinine was lowered in sitting posture, which implicates postural changes in renal flow.

It is generally recommended to sit for 20min or more before venipuncture, although the effect of prior physical movement remains a little longer for potassium and inorganic phosphate.