Which analyte increases in the supine posture due to fluid shifts?

• A. Albumin
• B. Hemoglobin
• C. Platelets
• D. Hematocrit

Answer: (A)

Posture alters how fluids are distributed between the compartments of the body. When you lie down, the pressure changes inside the blood vessels and the hydrostatic forces that push fluid into the interstitial space are reduced. This shift moves fluid back into the intravascular (plasma) compartment, increasing plasma volume. Albumin is a major protein confined to the vascular space, so its serum concentration tends to reflect these fluid-shift changes more noticeably. In this context, albumin ends up being the analyte whose measured level rises with the fluid redistribution that occurs in the supine posture. The other listed factors are more influenced by changes in cell mass or dilution of plasma as a whole and do not show the same rise with this fluid shift.