Which expression most accurately estimates plasma osmolality?

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Multiple Choice

Which expression most accurately estimates plasma osmolality?

Explanation:
Plasma osmolality is determined by the number of osmotically active particles in the blood, mainly sodium with its accompanying anions, plus the solutes glucose and urea. To estimate it from common lab values, we convert glucose and BUN from mg/dL to their metabolic units (mmol/L) and account for sodium’s accompanying ions by doubling Na. The standard expression is 2 × Na + glucose/18 + BUN/2.8. This combination captures the major contributors to osmolality and uses conversion factors so the result is in mOsm/kg. Other expressions either omit key contributors, double-count ions, or use incorrect unit conversions, leading to poorer estimates.

Plasma osmolality is determined by the number of osmotically active particles in the blood, mainly sodium with its accompanying anions, plus the solutes glucose and urea. To estimate it from common lab values, we convert glucose and BUN from mg/dL to their metabolic units (mmol/L) and account for sodium’s accompanying ions by doubling Na. The standard expression is 2 × Na + glucose/18 + BUN/2.8. This combination captures the major contributors to osmolality and uses conversion factors so the result is in mOsm/kg. Other expressions either omit key contributors, double-count ions, or use incorrect unit conversions, leading to poorer estimates.

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