What Is Serum Osmolality?
Osmolality is a special variation of molality and is a measure of the number of dissolved, osmotically active, particles in a fluid, and is an indicator of your water-electrolytes balance. In the case of serum osmolality, we measure substances such as sodium, urea nitrogen, glucose, alcohol, and potassium. Sodium is the main component of serum osmolality, whereas potassium can be omitted, as it has a minuscule influence.
Patients who have altered levels of osmotically active substances will have an altered serum osmolality and an increased osmolar gap. These patients include the diabetic (raised glucose levels), the kidney insufficient (raised urea nitrogen levels), and those with sodium disorders - many elderly people and people suffering from hypothalamus disorders.
Clinicians measure the serum osmolality quite often when it comes to fluid and electrolyte disorders. Still, there are clinical situations in which this measurement is crucial — in patients where poisoning or overdose is suspect. Substances such as salicylates, ethyl alcohol (food alcohol), methyl alcohol, or propylene glycol affect the serum osmolality and osmotic gap.
How to Use the Calculator
To use the serum osmolality calculator, input the following values:
- Na (mEq/L): Sodium concentration in the blood.
- Blood Urea Nitrogen (mg/dL): An indicator of kidney function.
- Glucose (mg/dL): Blood sugar levels.
- Serum Alcohol Concentration (mg/dL): Optional; used in cases of suspected alcohol-related imbalances.
- Measured Serum Osmolality (mOsm/kg H₂O): Optional; used for calculating the osmotic gap.
Click "Calculate" to get:
- Serum Osmolality
- Serum Osmotic Gap
Serum Osmolality and Gap Calculations
Serum Osmolality Formula
Serum osmolality = 2×Na + BUN/2.8 + glucose/18 + alcohol/3.7
- Na: Sodium concentration [mEq/L]
- BUN: Blood urea nitrogen [mg/dL]
- Glucose: Blood sugar [mg/dL]
- Alcohol: Serum alcohol concentration [mg/dL]
Osmolality versus osmolarity
By definition, we measure serum osmolality in the laboratory (e.g., using osmometer or freezing point depression). Its units are mOSm/kg H₂O. Osmolarity is the one that we calculate from other parameters and is given in mOsm/L H₂O.
Osmolarity is a little less than the osmolality of a given solution. In osmolality, the weight of the soluble substances doesn't include the solvent weight. In the case of osmolarity, the volume covers the volume of solutes. The divisor in osmolarity is bigger, so the result is slightly lesser than for osmolality. But at very low concentrations (500mOsm), the changes are negligible.
Thus, in practice, both terms are used interchangeably. But technically, what we calculate here, despite the name of serum osmolality calculator, is osmolarity.
Osmotic Gap Formula
An osmotic gap is a difference between the measured osmolality and the calculated one (osmolarity to be precise).
osmotic gap = measured osmolality − counted osmolality
The normal range of the osmotic gap is -14 to +10 mOsm/kg.
An increase in the serum osmolar gap indicates the presence of other osmotically active substances, like mannitol, salicylates, or alcohol. It's also an indicator of a state of hypertriglyceridemia or hypergammaglobulinemia.
Why Use This Calculator?
- Quick Diagnosis: Provides immediate insights into hydration, kidney function, and possible poisoning.
- Precision: Accurately differentiates between calculated and measured osmolality.
- Versatility: Useful in assessing electrolyte disorders, alcohol toxicity, or osmotic imbalances.