Management of Hyposmolality

Diagnosis and Assessment

Directly measured serum or plasma osmolality should be used as the primary indicator of hydration status, particularly in older adults 1, 3
An action threshold of serum osmolality <275 mOsm/kg indicates hyposmolality 4
Where directly measured osmolality is not available, calculated osmolarity using the equation: osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L) can be used 1, 3
Evaluate for symptoms of hyposmolality which may include general weakness, confusion, headache, and nausea at serum sodium concentrations of 125-130 mEq/L 4
Life-threatening manifestations may occur when serum sodium levels drop below 120 mEq/L 4
Assess volume status to differentiate between euvolemic, hypovolemic, and hypervolemic hyposmolality 4

For patients with Syndrome of Inappropriate ADH (SIADH), free water restriction (<1 L/day) is the first-line treatment for asymptomatic mild SIADH 4
Hypertonic 3% saline IV is indicated for life-threatening or acute symptomatic severe hyponatremia (<120 mEq/L) 4
For patients with low-intake dehydration (hypernatremia/hyperosmolality), encourage increased oral fluid intake with preferred beverages (tea, coffee, fruit juice, water) 5
For older adults who appear unwell with serum osmolality >300 mOsm/kg, subcutaneous or intravenous fluids should be offered in parallel with encouraging oral fluid intake 5
For volume depletion (hypovolemic hyposmolality), oral rehydration solution (ORS) should be administered for mild to moderate dehydration 6
For severe dehydration, isotonic intravenous fluids such as lactated Ringer's and normal saline should be administered 6

Hydration status should be reassessed regularly until corrected, then monitored periodically 5
All older persons should be screened for hydration status when they contact the healthcare system, if clinical condition changes unexpectedly, and periodically when malnourished or at risk of malnutrition 7
Older persons and their informal carers may use appropriate tools to assess fluid intake, but should also ask healthcare providers for assessment of serum osmolality periodically 2, 5

Do not rely on clinical signs like skin turgor or dry mouth to assess hydration status, especially in older adults 2
Do not use bioelectrical impedance for hydration assessment 2
Assessment of fluid intake is often highly inaccurate in older adults, particularly in residential care settings 5
Do not use oral rehydration therapy or sports drinks for low-intake dehydration 5