Given sodium 142 mmol/L, glucose 130 mg/dL, and urea nitrogen 18 mg/dL, the calculated plasma osmolality is approximately:

Given sodium 142 mmol/L, glucose 130 mg/dL, and urea nitrogen 18 mg/dL, the calculated plasma osmolality is approximately:

• A. 290
• B. 291
• C. 295
• D. 298

Answer: (D)

Calculated plasma osmolality is driven by the major osmoles in serum: sodium (counted as 2 Na+ to reflect accompanying anions), glucose, and urea nitrogen. The standard equation is Osm = 2 × Na + glucose/18 + BUN/2.8, where concentrations are in mmol/L for Na, and mg/dL for glucose and BUN. Plugging in the values: 2 × 142 = 284; glucose term 130/18 ≈ 7.22; BUN term 18/2.8 ≈ 6.43. Adding them gives 284 + 7.22 + 6.43 ≈ 297.65, about 298 mOsm/kg. This is an approximate value because the formula ignores small contributions from other solutes and from measurement rounding, but it closely matches the given answer.

Calculated plasma osmolality is driven by the major osmoles in serum: sodium (counted as 2 Na+ to reflect accompanying anions), glucose, and urea nitrogen. The standard equation is Osm = 2 × Na + glucose/18 + BUN/2.8, where concentrations are in mmol/L for Na, and mg/dL for glucose and BUN.

Plugging in the values: 2 × 142 = 284; glucose term 130/18 ≈ 7.22; BUN term 18/2.8 ≈ 6.43. Adding them gives 284 + 7.22 + 6.43 ≈ 297.65, about 298 mOsm/kg.

This is an approximate value because the formula ignores small contributions from other solutes and from measurement rounding, but it closely matches the given answer.