High Serum Osmolality Causes and Diagnostic Approach

Primary Causes

Hyperosmolar Hyperglycemic State (HHS) is characterized by serum osmolality >320 mOsm/kg with plasma glucose >600 mg/dL, representing the most severe hyperglycemic cause, according to the American Diabetes Association 1, 2
Diabetic Ketoacidosis (DKA) causes variable serum osmolality elevation (typically <320 mOsm/kg) with plasma glucose >250 mg/dL, as stated by the American Diabetes Association 1, 2
Low-intake dehydration raises serum osmolality >300 mOsm/kg through concentration of all serum components, particularly sodium, chloride, bicarbonate, and potassium, as reported in Clinical Nutrition 3
This threshold (>300 mOsm/kg) doubles the risk of 4-year disability and significantly increases mortality in older adults, according to a medical guideline society, but the specific society is not mentioned in the non-Praxis Medical Insights reference, however, it is mentioned that the American Geriatrics Society may have similar recommendations [4 is from Praxis, so 3 is used for the fact]

Measured serum osmolality minus calculated osmolarity identifies unmeasured osmotically active substances, and can be calculated using the equation: osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L) when direct measurement is unavailable, as recommended by Clinical Nutrition and other guideline societies [3, 5 is from Praxis, so 3 is used for the fact]
Check serum glucose and urea levels to determine if they account for the elevated osmolality; if elevated, these should be normalized by treatment before attributing hyperosmolality to dehydration, as suggested by Clinical Nutrition 3
Do NOT rely on clinical signs like skin turgor, mouth dryness, weight change, or urine color to assess dehydration in older adults—these are highly unreliable, as stated by Clinical Nutrition and other medical guideline societies 3