From the Guidelines
The formula for calculating serum osmolality is 2[measured Na (mEq/l)] + glucose (mg/dl)/18, as stated in the protocol for the management of adult patients with hyperglycemic hyperosmolar syndrome (HHS) 1. This equation is used to estimate the effective serum osmolality, which is an important factor in the diagnosis and management of HHS.
- The calculation is based on the measured sodium level and glucose concentration in the blood.
- Sodium is multiplied by 2 to account for its accompanying anions.
- Glucose is divided by 18 to convert its units from mg/dL to mmol/L or mOsm/L. It is worth noting that this formula does not include blood urea nitrogen (BUN) as a component, unlike other formulas for calculating serum osmolality.
- However, the formula provided in the study 1 is specific to the context of HHS and is used to determine the effective serum osmolality, which is a critical parameter in the diagnosis and management of this condition. The normal range for serum osmolality is typically around 285-295 mOsm/kg, but in the context of HHS, an effective serum osmolality of 320 mOsm/kg or higher is considered diagnostic 1.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Serum Osmolality Formula
The formula for calculating serum osmolality is as follows:
- Posm = 2 [Na(+)] + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 2
- Osmolality = 1.86 Na + (Glu/18) + (BUN/2.8) + 9 3
- Predicted osmolality excluding ethanol can be calculated with the formula: 2 Na (mEq/L) + (Urea [mg/dL])/2.8 + (Glucose [mg/dL])/18 4
Effective Osmolality
Effective osmolality or tonicity can be calculated using the formula:
- Eosm = 2 [Na(+)] + glucose/18 2
Osmolal Gap
The osmolal gap is the difference between measured and calculated osmolality, and can be used to diagnose disorders related to sodium, glucose, and water balance, kidney diseases, and small molecule poisonings 5, 6
- The osmolal gap can be calculated using the formula: Osmolal gap = (Ethanol [mg/dL])/3.7 - 0.35 or, in SI units: Osmolal gap (mOsm/kg) = 1.25 (Ethanol [mmol/L]) - 0.35 4