For osmolality determination, which specimen is listed in the material?

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

Answer: (A)

Osmolality reflects how concentrated a fluid is by counting solute particles per kilogram of water, and urine directly shows how the kidneys concentrate or dilute filtrate in response to hydration and ADH. Urine osmolality is highly informative about renal concentrating ability and water balance, making it the specimen most commonly listed for osmolality determination in standard materials. While plasma/serum osmolality are used to assess overall osmotic status and to calculate the osmolal gap, and CSF osmolality is only used in specific contexts, urine provides the most direct and frequently pertinent information for this test.