Is a Calculated Serum Osmolality of 291 mOsm/kg Normal?
Yes, a calculated serum osmolality of 291 mOsm/kg is normal and falls within the established reference range of 275-295 mOsm/kg. 1
Normal Reference Range
- The normal serum osmolality range is 275-295 mOsm/kg, making your value of 291 mOsm/kg well within normal limits. 1
- For clinical decision-making, hyperosmolality requiring intervention is defined as >300 mOsm/kg (or >295 mOsm/kg when using calculated osmolarity), while hyposmolality is defined as <275 mOsm/kg. 1
- Research in heart failure patients suggests that being close to the upper limit of normal range (292-293 mOsm/kg) may actually represent optimal osmolality in terms of cardiovascular prognosis. 2
Important Caveats for Interpretation
Before concluding that your osmolality is truly normal, you must verify that serum glucose and urea (BUN) are within normal ranges, as elevated levels of either can falsely elevate calculated osmolality and require normalization before proper interpretation. 1, 3
Key considerations:
- If you used the ESPEN-recommended formula [Osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14], the action threshold is >295 mmol/L rather than >300 mOsm/kg used for directly measured osmolality. 1
- Calculated osmolality can systematically differ from directly measured osmolality, with research showing mean biases ranging from underestimation to overestimation depending on the formula used and the clinical context. 4, 5, 6
- In hyperosmolar states, calculated values tend to overestimate true osmolality, while in hypoosmolar states they may underestimate it. 6
When to Take Action
No immediate intervention is needed at 291 mOsm/kg, as this falls comfortably within the normal range and well below the intervention thresholds. 1
Thresholds requiring action:
- >300 mOsm/kg (measured) or >295 mOsm/kg (calculated): Indicates low-intake dehydration requiring increased oral fluid intake or IV/subcutaneous fluids if the patient appears unwell. 1
- <275 mOsm/kg: Indicates hyposmolality requiring investigation for SIADH, overhydration, or other causes of hypotonic hyponatremia. 1, 3
Common Pitfalls to Avoid
- Do not rely on clinical signs such as skin turgor, mouth dryness, or urine color to assess hydration status, as these are highly unreliable, especially in older adults. 1, 7
- Do not assume calculated osmolality is interchangeable with measured osmolality in all clinical situations—direct measurement remains the gold standard (Grade B recommendation with 94% consensus from ESPEN). 1
- Always check sodium, glucose, and urea levels to ensure proper interpretation of the osmolality value. 1, 7, 3