Osmolality describes how concentrated a fluid is with dissolved particles, such as salts, sugar and the waste product urea. Tests can measure it in blood, urine or stool. The sample type changes what the result means.
Blood osmolality helps assess the balance between water and dissolved substances. Urine osmolality shows how concentrated the urine is. Looking at both can help a clinician understand how the kidneys are responding to the body’s water balance.
Dehydration can raise blood osmolality, but so can increases in substances such as glucose (sugar) or sodium (a component of salt). One result cannot identify the cause. A urine result also needs the blood results and the reason for testing.
Check whether the sample was blood, urine or stool. Stool testing answers different questions, usually about persistent diarrhea. Follow the testing team’s preparation instructions; do not deliberately restrict water or drink extra water to change the result.

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