On formula for the calculation of serum osmolality

Q; Why serum sodium is multiplied by 2 in the formula for the calculation of serum osmolality (Sosm)? 

 Calculated Sosm = (2 x serum [Na]) + [glucose]/18 + [BUN]/2.8 + [ethanol]/3.7 


 Answer: 

The serum sodium is multiplied by two in the formula to calculate serum osmolality is to account for accompanying anions (usually chloride and bicarbonate).

Moreover the divisors - 18, 2.8 and 3.7 - in the formula are to convert units of mg/dL to mosmol/kg 

When international units (mmol/L) are used, formula for 
Calculated Sosm = (2 x serum [Na]) + [glucose] + [urea] + (1.25 x [ethanol])


#electrolytes


References: 

1. Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med 2008; 8:5. 

2. Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001; 38:653.