Variables
Description
What is this formula?
Plasma Tonicity estimates the concentration of effective osmoles that generate water movement across cell membranes. It reflects the osmotic forces that influence cellular hydration and volume.
When to use it
Use this formula when evaluating disorders of sodium balance, hyperglycemia, cerebral edema risk, dehydration, or disturbances of cellular water distribution.
Example
Data:
Na = 140 mEq/L
Glu = 180 mg/dL
Formula:
Ton = 2Na + Glu/18
Substitution:
Ton = (2 × 140) + (180/18)
Ton = 280 + 10
Result:
Ton = 290 mOsm/L
The estimated plasma tonicity is 290 mOsm/L.
Applications
- Hyponatremia evaluation
- Hypernatremia evaluation
- Hyperglycemic emergencies
- Neurology
- Critical care medicine
Note
Plasma tonicity is closely related to effective osmolality and excludes substances such as urea that freely cross cell membranes. It is often more clinically relevant than total osmolarity when assessing transcellular water shifts, cerebral edema, and cellular hydration status.