Serum sodium is 130 mmol/L, glucose 540 mg/dL and urea nitrogen 28 mg/dL. What is the calculated serum osmolality?
Answer and explanation
Correct answer: C. 300 mOsm/kg
- Step 1: Osmolality = 2 x Na + glucose/18 + BUN/2.8.
- Step 2: 260 + 30 + 10 = 300 mOsm/kg.
Answer
300 mOsm/kg.
Why not the others: '260 mOsm/kg' counts sodium only and leaves out glucose and urea; '828 mOsm/kg' adds glucose and urea in mg/dL without converting; '290 mOsm/kg' leaves out the urea term.
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