Serum Osmolality of 289-293: Normal Hydration Status
A serum osmolality of 289-293 mOsm/kg indicates normal hydration status and optimal electrolyte balance, particularly when the value is in the upper-normal range (292-293 mOsm/kg). 1, 2
Normal Range and Clinical Significance
Normal serum osmolality ranges from 275-295 mOsm/kg, with your values of 289-293 falling comfortably within this range 3, 4, 2
Values of 292-293 mOsm/kg represent the optimal plasma osmolality level, particularly for cardiovascular prognosis 2
The action threshold for dehydration is >300 mOsm/kg (directly measured) or >295 mmol/L (calculated), so values of 289-293 are well below the dehydration cutoff 1, 3
What These Values Tell You
Sodium, chloride, bicarbonate, and potassium are the primary contributors to serum osmolality, and your values indicate these electrolytes are in appropriate balance 1, 4
The patient has adequate fluid intake and is not experiencing low-intake dehydration 1
There is no evidence of hyperosmolar states such as diabetic ketoacidosis (which typically shows osmolality <320 mOsm/kg) or hyperosmolar hyperglycemic state (≥320 mOsm/kg) 3
Clinical Context and Interpretation
Research in heart failure patients demonstrated that the third quartile (mean 293 mOsm/kg, 95% CI 292.72-293.3) had the best cardiovascular prognosis compared to all other quartiles, suggesting this upper-normal range is actually protective 2
Interpretation requires confirming that serum glucose and urea are within normal ranges, as abnormalities in these values affect osmolality interpretation 1, 3
Do not rely on clinical signs like skin turgor, mouth dryness, or urine color to assess hydration status, as these have been shown to be unreliable, especially in older adults 1, 3
Important Caveats
If using calculated osmolarity rather than direct measurement, be aware that calculated values tend to overestimate osmolality in lower ranges and underestimate in higher ranges 5, 6
The ESPEN-recommended calculation formula (osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14) has 90% sensitivity and 68% specificity for detecting dehydration, with an excellent negative predictive value of 98% 7
In older adults, monitor serum osmolality periodically when they contact the healthcare system, if clinical condition changes unexpectedly, or when malnourished or at risk of malnutrition 1, 8