Osmolarity Versus Osmolality. Ethanol level.

Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.

Osmosis does not continue indefinitely but stops when the solutes on both sides of the membrane exert an equal osmotic force. This force is called osmotic pressure.The osmotic pressure depends on the number of particles dissolved in a unit volume of solvent and not on the valence,  weight,  or shape  of  the  particle.  For  example,  an  atom  of  Na +   exerts  the  same osmotic  pressure  as  an  atom  of  Ca 2+   with  a  valence  of  2.  Osmotic  pressure is expressed as  osmoles  (Osm).  One  milliosmole  (mOsm)  is  1/1000  of  an  osmole. 

    Osmolarity refers to the number of mOsm in 1 L of solution, whereas osmolality is the number of mOsm in 1 kg of water. However, osmolality is the preferred physiological term because the colligative property depends on the number of particles in  a given weight (kg) of water.
    The osmolality of plasma is largely a function of Na +  concentration and its anions  (mainly Cl− and HCO 3−) with contributions from glucose and urea nitrogen. Since  each Na +  is paired with a univalent anion, the contribution from other cations such  as  K + ,  Ca 2+ ,  and  Mg 2+   to  the  osmolality  of  plasma  is  generally  not considered.  Therefore, the plasma osmolality is calculated by doubling Na +  and including the  contribution  from  glucose  and  urea  nitrogen  (generally  expressed  as  blood  urea 
nitrogen or BUN), as follows:

Plasma osmolality (typical in the US) = 2[Na+] + [Glucose]/18 + [ BUN ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.

where  18  and  2.8  are  derived  from  the  molecular  weights  of  glucose  and  urea, respectively. Because serum glucose and urea concentrations are expressed as mg/dL,  it  is  necessary  to  convert  these  concentrations  to  mOsm/L  by  dividing  the molecular weights of glucose (180) or urea nitrogen (28) by 10. Normal serum values are Na +  = 142 mEq/L, glucose = 90 mg/dL, and urea nitrogen = 12 mg/dL. The normal range is between 280 and 295 mOsm/kg H2O.

Calculated osmolarity = 2 Na + Glucose + Urea ( all in mmol/L).

The normal range is between 280 and 300 mOsm/l.

If the patient has ingested ethanol, the ethanol level should be included:

calculated osmolality:= 2[Na+] +[Glucose]/18 + [ BUN ]/2.8 + [Ethanol]/3.7

Based on the molecular weight of ethanol the divisor should be 4.6 but empiric data shows that ethanol does not behave as an ideal osmole.