Osmolality In Clinical Chemistry | Advanced instruments
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Osmolality In Clinical Chemistry

Osmolality is a fundamental measurement of the total solute concentration of body fluids, including but not limited to, whole blood, serum, plasma, urine, and stool. It is a valuable clinical tool in the diagnosis and treatment of patients.

Osmolality is a quick and effective test to help:

  • Evaluate the body’s water balance and its ability to produce and concentrate urine
  • Investigate low sodium levels (hyponatremia)
  • Detect the presence of toxins in the body
  • Monitor the electrolyte balance of patients receiving osmotically active drug therapies (e.g. mannitol used to treat cerebral edema).

It can also help monitor the effectiveness of a treatment or a condition found to be adversely affecting a person’s osmolality.

Calculated vs 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.

Calculated osmolality formulas (using sodium, BUN, glucose) do not account for all osmotically active substances potentially present in body fluids, such as:

  • Methanol
  • Ethanol
  • Isopropanol
  • Ethylene Glycol
  • Propylene Glycol
  • Sorbitol
  • Mannitol

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 
  • Tight tolerances at clinically relevant decision points 


Calculated Osmolality: 

  • 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 


  1. 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.

Physician Application Notes

Three physicians partnered with Advanced Instruments to share their experiences regarding the utility of osmolality testing and the implications of this critical test on quality and cost of patient care.

The Value of Osmolality Testing
Dr. Neville R. Dossabhoy is a consulting nephrologist in Louisiana.

The Value of Osmolality in Emergency Medicine
Dr. David Schindler is an attending physician in the Department of Emergency Medicine at Cornwall Community Hospital in Cornwall Ontario, Canada and Massena Memorial Hospital in Massena, New York.

The Value of Osmolality in Nephrology
Dr. Namrata Sharma Goel is a Nephrologist at the Kidney Health Center in Houston, Texas.

I’d like to start a conversation about osmolality

*The views and opinions expressed in the application notes are those of the authors and are intended for educational purposes.


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