Which formula best approximates serum osmolality?

Which formula best approximates serum osmolality?

• A. Serum osmolality ≈ 2 Na + BUN/2.8 + glucose/18
• B. 2 Na + Ca/2 + glucose/15
• C. Na + BUN/2.8 + glucose/18
• D. 3 Na + BUN/2.8 + glucose/18

Answer: (A)

Serum osmolality reflects the number of osmotically active particles in plasma, and the main contributors are sodium with its accompanying anions, plus glucose and urea (BUN). The best approximation uses 2 × [Na+] to capture sodium and its counterions, plus glucose and BUN converted to millimoles per liter. Glucose is converted from mg/dL by dividing by 18, and BUN by dividing by 2.8, because 1 mmol of glucose is 180 mg and 1 mmol of urea is 60 mg. Adding these together gives osmolality in mOsm/kg: 2 Na + glucose/18 + BUN/2.8. This aligns with how osmolality is calculated from the major serum solutes, which is why it’s the most accurate choice. Other formulas that replace or modify these components or use different conversion factors don’t reflect the actual contributions or unit conversions for common serum measurements.

Serum osmolality reflects the number of osmotically active particles in plasma, and the main contributors are sodium with its accompanying anions, plus glucose and urea (BUN). The best approximation uses 2 × [Na+] to capture sodium and its counterions, plus glucose and BUN converted to millimoles per liter. Glucose is converted from mg/dL by dividing by 18, and BUN by dividing by 2.8, because 1 mmol of glucose is 180 mg and 1 mmol of urea is 60 mg. Adding these together gives osmolality in mOsm/kg: 2 Na + glucose/18 + BUN/2.8. This aligns with how osmolality is calculated from the major serum solutes, which is why it’s the most accurate choice. Other formulas that replace or modify these components or use different conversion factors don’t reflect the actual contributions or unit conversions for common serum measurements.