The osmolal gap is the difference between a patient's measured serum osmolality and that estimated from knowledge of the patient's sodium, glucose and SUN levels. An increased osmolal gap may be an indication of unmeasured osmotically active compounds, pseudohyponatremia or laboratory error.


osmolal gap in mOsm/kg H2O =

= (measured osmolality in mOsm/kg H2O) - (calculated osmolality in mOsm/kg H2O)


calculated osmolality in mOsm/kg H2O=

= (2 * (sodium in mmol/L)) + (glucose in mmol/L) + (urea in mmol/L)



• The normal serum osmolality is 275-295 if <= 60 years of age and 280-301 if > 60 years of age.



• A normal osmolal gap: < 10

• A significant osmolal gap: > 10


If the measured osmolality is normal but there is a significant osmolal gap present (> 10), then consider one of the following:

(1) a low serum sodium, probably is due to artifact caused by severe hyperlipidemia or hyperproteinemia ("pseudohyponatremia" due to decreased serum water content)

(2) an underyling disease such as ketoacidosis or chronic renal failure

(3) an unusual osmotically active substance is present (ethanol, methanol, mannitol, ethylene glycol, propylene glycol, ethyl ether, isopropyl alcohol, acetone, paraldehyde, or an osmotic contrast dye)

(4) use of plasma rather than serum, with anticoagulant in collection tube providing osmotically active material

(5) laboratory error.


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