Which formula is most accurate in predicting plasma osmolality?

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Multiple Choice

Which formula is most accurate in predicting plasma osmolality?

Plasma osmolality is determined by the number of osmotically active particles in the blood, with the dominant contributor being sodium and its accompanying anions. For every Na+ ion, there are accompanying anions (like Cl− and HCO3−), so the osmotic effect of sodium-containing species is roughly twice the sodium concentration. That’s why the sodium term is multiplied by 2. Glucose and urea (often reported as BUN) also contribute to osmolality, but they are present in different units. Glucose is typically in mg/dL, so dividing by 18 converts it to mmol/L. BUN is in mg/dL as well, so dividing by 2.8 converts it to mmol/L. Adding these components together gives a close estimate of plasma osmolality: 2 × Na + (glucose ÷ 18) + (BUN ÷ 2.8). Other formulas either skip the doubled sodium contribution or use improper units or components, making them less accurate. This combination—2 × Na, glucose/18, and BUN/2.8—best matches the actual osmolality.

Plasma osmolality is determined by the number of osmotically active particles in the blood, with the dominant contributor being sodium and its accompanying anions. For every Na+ ion, there are accompanying anions (like Cl− and HCO3−), so the osmotic effect of sodium-containing species is roughly twice the sodium concentration. That’s why the sodium term is multiplied by 2.

Glucose and urea (often reported as BUN) also contribute to osmolality, but they are present in different units. Glucose is typically in mg/dL, so dividing by 18 converts it to mmol/L. BUN is in mg/dL as well, so dividing by 2.8 converts it to mmol/L. Adding these components together gives a close estimate of plasma osmolality: 2 × Na + (glucose ÷ 18) + (BUN ÷ 2.8).

Other formulas either skip the doubled sodium contribution or use improper units or components, making them less accurate. This combination—2 × Na, glucose/18, and BUN/2.8—best matches the actual osmolality.

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