Which of the following specimens is preferred for osmolality measurements?

• A. Serum
• B. Plasma
• C. Whole blood
• D. Lymph

Answer: (A)

Osmolality reflects the concentration of osmotically active small particles in the fluid, so you want a specimen that is free of cells and free of added substances that could alter solute content. The measurement, typically by freezing point depression, is most accurate when the sample is a cell-free serum. Serum is obtained after blood clots, leaving the liquid without cells and without anticoagulants that would add extra solutes; this makes the osmolality more representative of the circulating extracellular fluid. Whole blood contains cells that can release intracellular solutes if they lyse, skewing the result. Lymph is not a representative sample of systemic plasma osmolality and is impractical for routine testing. Therefore serum is the preferred specimen for osmolality measurements.