From the Research
The osmolar gap should be calculated in hyponatremia when there is suspicion of the presence of unmeasured osmotically active substances in the serum, particularly in cases of suspected toxic alcohol ingestion, severe hyperglycemia, mannitol administration, or other conditions where osmotically active substances may accumulate. This calculation is crucial because it helps differentiate between true hyponatremia and pseudohyponatremia, guiding appropriate management strategies. To calculate the osmolar gap, subtract the calculated serum osmolality (2 × Na + glucose/18 + BUN/2.8) from the measured serum osmolality, with a normal gap being less than 10 mOsm/kg 1. An elevated osmolar gap (>10 mOsm/kg) in a hyponatremic patient suggests the presence of unmeasured solutes and warrants further investigation to identify the specific substance.
The importance of calculating the osmolar gap in hyponatremia is highlighted by the potential for severe morbidity and mortality associated with this condition, as noted in a study published in JAMA in 2022 1. Additionally, a study published in the journal Nefrologia in 2017 emphasizes the need for a thorough diagnostic approach to hyponatremia, including the measurement of plasma osmolality and the calculation of the osmolar gap 2.
Some key points to consider when calculating the osmolar gap include:
- The formula for calculating the osmolar gap: measured serum osmolality - calculated serum osmolality (2 × Na + glucose/18 + BUN/2.8)
- A normal osmolar gap is less than 10 mOsm/kg
- An elevated osmolar gap (>10 mOsm/kg) suggests the presence of unmeasured solutes and warrants further investigation
- The calculation of the osmolar gap is particularly important in cases of suspected toxic alcohol ingestion, severe hyperglycemia, mannitol administration, or other conditions where osmotically active substances may accumulate, as noted in a study published in FP essentials in 2017 3.
Overall, the calculation of the osmolar gap is a critical step in the diagnosis and management of hyponatremia, and should be performed in all cases where there is suspicion of the presence of unmeasured osmotically active substances in the serum.