Importance of Measured Osmolality
Direct measurement of body fluid osmolality with a freezing point depression osmometer is the preferred method when reporting osmolality in a clinical setting1.
In practice, physicians use both measured and calculated osmolality to determine a patient’s osmolal gap. The difference between measured and calculated osmolality can be diagnostically significant for conditions such as toxic alcohol ingestion and useful for monitoring patients being treated with osmotically active therapies.
If a laboratory were to solely provide calculated osmolality the physicians could not calculate the osmolal gap to screen for foreign substances.
Measured Osmolality (via Freezing Point Depression Osmometer):
- Objective measurement
- Accounts for everything that impacts osmolality including
toxins and osmotically active therapies
- Has tight tolerances at clinically relevant decision points
- Highly subjective with over 65 equations in the literature
- Misses toxins and medications that impact osmolality
- Error could be embedded in each component of the calculation
Measured Osmolality – Calculated Osmolality = Osmolal Gap
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- Eisen, T. F., Lacouture, P. G., Woolf, A.: Serum osmolality in alcohol ingestions: differences in availability among laboratories of teaching hospital, nonteaching hospital, and commercial facilities. Am J Emerg Med 1989; 7:256-259.