From the Guidelines
Serum osmolality should be ordered when evaluating sodium abnormalities, particularly in cases of hyponatremia, to help distinguish between true and pseudohyponatremia or pseudohypernatremia, and to calculate the osmolal gap, as recommended by the most recent expert consensus statement 1. When diagnosing hyponatremia, measuring serum osmolality helps to differentiate between hypotonic, isotonic, and hypertonic states.
- The normal serum osmolality range is 275-295 mOsm/kg.
- A calculated serum osmolarity (using the formula: 2×Na + glucose/18 + BUN/2.8) that differs from measured osmolality by more than 10 mOsm/kg indicates an elevated osmolal gap, suggesting the presence of unmeasured osmotically active substances like alcohols, mannitol, or glycine. In hyponatremia with normal or elevated osmolality, consider pseudohyponatremia from hyperlipidemia or hyperproteinemia, or true hypertonic hyponatremia from hyperglycemia or mannitol administration.
- In hypernatremia, osmolality measurement confirms hyperosmolar state and helps guide fluid replacement therapy. Measuring both parameters provides crucial diagnostic information that directs appropriate treatment of sodium disorders by identifying the underlying pathophysiologic mechanism, as supported by previous guidelines 1. The most recent study 1 emphasizes the importance of measuring serum sodium, serum osmolality, and urine osmolality as an initial biochemical work-up in patients with suspected nephrogenic diabetes insipidus, which can be applied to the evaluation of sodium abnormalities in general.
From the Research
Serum Osmolarity and Osmolality in Sodium Abnormalities
- Serum osmolality is a critical parameter in the evaluation of sodium abnormalities, including hyponatremia and hypernatremia 2, 3, 4, 5.
- Measurement of serum osmolality is used to determine the effective serum tonicity, which helps in the differential diagnosis of hyponatremia 2.
- Serum osmolality is also used to classify sodium disorders based on volume status (hypovolemia, euvolemia, and hypervolemia) 3.
- In patients with normal or elevated effective serum osmolality (280 mOsm/kg or greater), pseudohyponatremia should be excluded 2.
- Urine osmolality is used to determine whether water excretion is normal or impaired in patients with hypo-osmolar state (serum osmolality less than 280 mOsm/kg) 2.
Ordering Serum Osmolarity and Osmolality
- Serum osmolality should be ordered in patients with sodium abnormalities, including hyponatremia and hypernatremia, to determine the underlying cause and guide treatment 2, 3, 4, 5.
- Serum osmolarity is not directly mentioned in the studies, but serum osmolality is a critical parameter in the evaluation of sodium abnormalities.
- The studies suggest that serum osmolality should be ordered as part of the initial laboratory evaluation in patients with sodium abnormalities, along with other parameters such as urine osmolality and urine sodium concentration 2, 3, 4, 5.