In the osmolality calculation, which substance is multiplied by two?

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Multiple Choice

In the osmolality calculation, which substance is multiplied by two?

Osmolality reflects the number of dissolved particles that contribute to osmotic pressure. When salts dissolve, they break into two particles. In blood, the dominant contributor is sodium salts, so the sodium term is doubled to approximate the total particle count from those salts. That’s why the calculation uses 2 × [Na+], plus separate terms for glucose and urea (which don’t dissociate into ions). The common formula is Osmolality ≈ 2[Na+] + [glucose]/18 + [BUN]/2.8. Glucose and urea contribute as their actual concentrations, not doubled, because they are non-electrolytes.

Osmolality reflects the number of dissolved particles that contribute to osmotic pressure. When salts dissolve, they break into two particles. In blood, the dominant contributor is sodium salts, so the sodium term is doubled to approximate the total particle count from those salts. That’s why the calculation uses 2 × [Na+], plus separate terms for glucose and urea (which don’t dissociate into ions). The common formula is Osmolality ≈ 2[Na+] + [glucose]/18 + [BUN]/2.8. Glucose and urea contribute as their actual concentrations, not doubled, because they are non-electrolytes.

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