Serum Osmolality/Osmolarity

Serum Osmolality / Osmolarity
mEq/L
mg/dL
mg/dL
mg/dL
Calculates expected serum osmolarity, for comparison to measured osmolality to detect unmeasured compounds in the serum.

When to Use

An osmolar/osmolal gap (difference between measured and expected) >10 can be caused by toxic alcohols (e.g., ethanol, methanol, ethylene glycol, isopropyl alcohol*), mannitol, and lorazepam infusions (which contain propylene glycol). *Isopropyl alcohol causes a large osmolar gap but does not cause an anion gap metabolic acidosis.

Formula

Traditional equation: Serum osmolarity, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 4.6) Purssell et al equation: Serum osmolarity, US units = (2 × Na + (BUN / 2.8) + (glucose / 18) + (ethanol / 3.7) Note: the range displayed in the calculator uses the equations above. Osmolar gap equation: Osm gap = measured serum osm - calculated serum osm Note: the osmolar gap displayed in the calculator uses the Pursell et al. equation.

Pearls / Pitfalls

At extremes of serum glucose, such as in hyperosmolar hyperglycemic state (HHS), osmolar gap is skewed, and clinical judgment should be used to make decisions regarding correction. Notes on osmolality versus osmolarity: Osmolality and osmolarity are often used interchangeably in medical practice. Both relate to solution concentration, but: Osmolality is defined as the number of solute particles per kilogram of solvent (in this case, body fluid). Osmolarity is defined as the number of solute particles per liter of solution. Osmolarity is affected by temperature and pressure (due to volume changes), while osmolality is not (mass remains constant). Serum osmolality is what is measured by an osmometer. In dilute solutions like blood plasma, 1 kg of water is approximately equal to 1 L. Therefore, the numerical values of osmolality and osmolarity are nearly identical, making the terms functionally interchangeable in clinical contexts. Notes on osmolar versus osmolal gap: Osmolar gap is the difference between the measured osmolality and the calculated osmolarity . Osmolal gap is the difference between measured and calculated osmolality . Since solute concentrations (e.g., sodium, glucose, BUN) are typically measured in mEq or mg (or mmol) per dL or L (i.e., volume), we typically calculate the expected osmolarity . Thus, in clinical practice, osmolar gap is the standard term used .

Advice

Methanol and ethylene glycol ingestions are treated by blocking alcohol dehydrogenase with fomepizol or ethanol, and often sodium bicarbonate if severely acidotic.

More Information

If values are in SI units, these are already molar (mmol/L for example), so these do not need to be divided by their molecular weight.

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