Which statement is supported by the notes regarding osmolality testing?

• A. Serum is the best specimen for osmolality.
• B. Plasma is the best specimen for osmolality.
• C. Urine is the best specimen for osmolality.
• D. CSF is the best specimen for osmolality.

Answer: (A)

Osmolality testing looks at how many dissolved particles are in a body fluid, so you want a specimen that best represents the circulating solute load. Serum is preferred because it reflects the body's systemic solutes after blood has clotted and been separated, avoiding interference from anticoagulants used with plasma. Urine shows what the kidneys are concentrating or diluting and CSF has its own distinct composition, so neither is the standard for assessing overall systemic osmolality. Therefore, serum is the best-supported specimen for osmolality testing.