For osmolality testing, which specimen is described as the best?

• A. Serum
• B. Plasma
• C. Urine
• D. CSF

Answer: (A)

Osmolality reflects the concentration of all osmotically active particles in the extracellular fluid, so we want a specimen that accurately represents circulating solute levels without being altered by additives or unusual compartments. Serum is the best choice because it provides a liquid similar to the extracellular environment after blood has clotted, reflecting key solutes such as sodium, glucose, and urea without the potential interference from anticoagulants or clotting proteins. Urine varies with hydration and renal handling, so it doesn’t mirror systemic osmolality. CSF has a distinct composition and is obtained invasively for different diagnostic reasons, not for assessing overall body osmolality. Plasma requires anticoagulants, which can introduce small changes in osmolality, making serum the more standardized choice for osmolality testing and for calculating the osmolar gap.