Osmolality revisited--deriving and validating the best formula for calculated osmolality

Area of Science:

• Clinical Chemistry
• Toxicology
• Internal Medicine

Background:

• Accurate serum osmolality calculation is crucial for diagnosing and managing various clinical conditions, including hyperglycemia and intoxication.
• The osmol gap is a valuable tool for detecting unmeasured osmotically active substances, but its accuracy can be affected by the presence of common compounds like ethanol.

Purpose of the Study:

• To derive and validate formulas for calculating serum osmolality and the osmol gap.
• To assess the ability of these formulas to detect unexplained compounds, even in the presence of ethanol.

Main Methods:

• In vitro experiments were conducted to determine the relationship between serum osmolality and various solutes (sodium, potassium, urea, glucose, ethanol, methanol, ethylene glycol).
• Formulas were developed and tested for accuracy in predicting osmolality in normal individuals and patients.
• The osmol gap was calculated in the presence and absence of ethanol to evaluate the detection of other osmotically active compounds.

Main Results:

• Experimental data indicated that factors of 1.15 for glucose and 1.20 for ethanol are necessary for accurate osmolality prediction.
• These factors were validated in normal subjects and a patient database, showing good performance regardless of ethanol presence.
• The mean osmol gap in healthy subjects without ethanol was within the reference interval, and in patients with ethanol, it was also within acceptable ranges using the derived formulas.

Conclusions:

• Applying correction factors of 1.20 for ethanol and 1.15 for glucose is essential for accurate osmolality and osmol gap calculations.
• The study provides validated formulas for osmolality and osmol gap, enhancing the detection of toxic substances.
• Further patient data are needed to verify the proposed factors for methanol and ethylene glycol.