Question 1

Why is urea excluded from the effective osmolality?

Accepted Answer

Urea is an ineffective osmole because it freely crosses cell membranes and therefore does not create an osmotic gradient across the majority of body cells, although it does contribute to the measured osmolality.

Question 2

What is the target rate of osmolality correction in HHS?

Accepted Answer

The effective osmolality should decline at 3–5 mOsm/kg/hour. A faster decline increases the risk of cerebral oedema. The fluid infusion rate is adjusted to achieve this target.

Question 3

How does effective osmolality differ from measured osmolality?

Accepted Answer

Measured osmolality (by laboratory osmometer) includes urea, ethanol, and other solutes. Effective osmolality only includes Na⁺ and glucose. The difference (osmolar gap) is used to screen for toxic alcohol ingestion.

Question 4

Should I use effective osmolality or corrected sodium to guide HHS treatment?

Accepted Answer

Both are complementary. Effective osmolality tracks the osmotic gradient across cells, while corrected sodium tracks the true sodium concentration. Many clinicians monitor both during HHS management.