Variables
Description
What is this formula?
The Effective Osmolality formula estimates the concentration of osmotically active substances that generate water movement across cell membranes. Unlike total osmolarity, it excludes urea because urea freely crosses most cell membranes and does not contribute significantly to tonicity.
When to use it
Use this formula when evaluating cellular hydration, hyperglycemia, hyperosmolar states, neurologic symptoms, or disorders of water balance.
Example
Data:
Na = 140 mEq/L
Glu = 180 mg/dL
Formula:
EffOsm = 2Na + Glu/18
Substitution:
EffOsm = (2 × 140) + (180/18)
EffOsm = 280 + 10
Result:
EffOsm = 290 mOsm/kg
The effective osmolality is 290 mOsm/kg.
Applications
- Hyperosmolar hyperglycemic state
- Neurology
- Nephrology
- Fluid balance assessment
- Critical care medicine
Note
This formula estimates tonicity rather than total osmolarity. Urea is intentionally excluded because it equilibrates across cell membranes and therefore contributes little to transcellular water shifts. Effective osmolality is often more clinically relevant than total osmolarity when evaluating cerebral edema and cellular hydration.