Why Glucose is Divided by 18 in the Osmolality Formula
The glucose value is divided by 18 to convert glucose from mg/dL to mmol/L and account for its molecular weight, ensuring accurate calculation of its osmolar contribution in mOsm/kg. 1, 2
The Mathematical Basis
The division by 18 represents a unit conversion factor that accomplishes two things simultaneously:
- Converts glucose concentration from mg/dL to mmol/L: The molecular weight of glucose is 180 g/mol, which equals 18 mg/mmol when working with deciliter units 3
- Each mmol of glucose contributes 1 mOsm to osmolality: Since glucose does not dissociate in solution (unlike sodium chloride which dissociates into 2 particles), one mole of glucose equals one osmole 3
The Standard Formula for Effective Osmolality
The American Diabetes Association recommends the formula: Effective serum osmolality = 2[measured Na (mEq/L)] + glucose (mg/dL)/18 1, 2
This formula specifically calculates "effective" osmolality (tonicity) because:
- Sodium is multiplied by 2 to account for its accompanying anion (chloride or bicarbonate), representing the total osmolar contribution of sodium salts 3
- Glucose divided by 18 converts the glucose concentration to its osmolar contribution 1, 2
- Urea is excluded from effective osmolality because it freely crosses cell membranes and does not create an osmotic gradient, though it's included in total osmolality calculations 3
Clinical Application in HHS Management
For your elderly patient with T2D, impaired renal function, and HHS:
- The diagnostic threshold for HHS is effective osmolality ≥320 mOsm/kg, calculated using this formula 1, 2
- Monitor osmolality every 2-4 hours during treatment to ensure the decline does not exceed 3 mOsm/kg/h, which is critical in elderly patients with renal impairment 1, 4
- The glucose/18 component will decrease as treatment progresses, contributing significantly to the overall osmolality reduction as insulin therapy lowers glucose from >600 mg/dL toward the target of 250-300 mg/dL 1, 5
Common Pitfall to Avoid
Do not use total osmolality formulas that include BUN/2.8 when assessing treatment targets in HHS - the effective osmolality formula (which excludes urea) is what determines the diagnostic threshold of ≥320 mOsm/kg and guides fluid management decisions 1, 2. In elderly patients with impaired renal function, elevated BUN will artificially inflate total osmolality calculations but does not contribute to the hyperosmolar state requiring correction 4.
Alternative Formula Context
While some sources describe total osmolality as: 2[Na+] + glucose/18 + BUN/2.8 3, the American Diabetes Association specifically uses the simpler formula without BUN for HHS diagnosis and management because only effective osmoles (those that don't freely cross cell membranes) create the dangerous osmotic gradient 1, 2.