Osmolality is a measure of the number of dissolved particles in a given weight of solution. This measurement is in contrast to molality:
- Molality is a concentration with the moles of solute as the numerator per 1 kilogram of solution (moles/kg).
- The osmolality of a solution is based on the number of dissociated particles in the solution. It counts each ion that dissociates from salt in solution independently whereas molality counts the salt as a whole.
- An example would be a 1 molal solution of NaCl in 1 kg of water would be a 2 osmolal solution because Na+ and Cl- dissociate in solution and are counted independently.
In healthy individuals, the osmolality of serum is tightly regulated and can be estimated using the following formula:
- Calculated Osmolality = [1.86 x sodium concentration (mmol)] + [glucose concentration (mg/dL) / 18] + [BUN concentration (mg/dL) / 2.8]
Osmolality can also be measured by an osmometer. A normal reference range is typically 275 – 295 mOsm/kg.
Patients may have an increase in their measured osmolality whereas their calculated osmolality may appear lower. This discrepancy is often called an osmolal gap and may indicate additional dissolved particles that are in the patient's serum that are not accounted for in the calculated osmolality. A common reason for a significant osmolal gap are dissolved alcohols such as ethanol, methanol, isopropanol, or ethylene glycol. This discrepancy can be calculated by the following equation:
- Osmolal Gap = [Measured Osmolality] – [Calculated Osmolality]
Here is a case example of this:
A 26-year-old patient was found unconscious and was transported by ambulance to the ED. Their serum laboratory results were as follows:
- Serum sodium: 142 mmol/L
- Serum glucose: 88 mg/dL
- BUN: 15 mg/dL
- Measured serum osmolality: 316 mOsm/kg
- Calculated Osmolality
- = 1.86(142) + (88/18) + (15/2.8)
- = 264.12 + 4.89 + 5.36
- = 274 mOsm/kg
- Subtracting the measured serum osmolality by the calculated osmolality, we obtain an osmolal gap of:
In normal individuals, the osmolal gap would be <10 mOsm/kg. This gap is significant enough to indicate that other dissolved particles are present in this patient's serum. Serum ethanol was ordered for this patient with a result of 198 mg/dL which likely accounts for this gap.
