Serum Osmolality Calculator
Calculate serum osmolality and osmotic gap for electrolyte and fluid balance assessment
Calculate Serum Osmolality
Normal range: 136-145 mEq/L (equivalent to mmol/L)
Normal range: 7-20 mg/dL (2.5-7.1 mmol/L)
Normal range: 70-99 mg/dL (3.9-5.5 mmol/L) fasting
Include when alcohol intoxication or poisoning is suspected
For osmotic gap calculation (normal range: 280-295 mOsm/kg H₂O)
Example Calculation
Diabetic Patient Example
Sodium (Na): 142 mEq/L
BUN: 28 mg/dL
Glucose: 450 mg/dL (diabetic ketoacidosis)
Alcohol: 0 mg/dL
Calculation
Osmolality = 2×142 + 28/2.8 + 450/18 + 0/3.7
Osmolality = 284 + 10 + 25 + 0
Osmolality = 319 mOsm/kg H₂O (High)
Normal Laboratory Values
Serum Osmolality
280-295 mOsm/kg H₂O
Sodium
136-145 mEq/L
BUN
7-20 mg/dL
Glucose (fasting)
70-99 mg/dL
Osmotic Gap
-14 to +10 mOsm/kg H₂O
Clinical Conditions
High Osmolality
- • Dehydration
- • Diabetes insipidus
- • Hyperglycemia
- • Hypernatremia
- • Alcohol intoxication
- • Kidney insufficiency
Low Osmolality
- • Fluid overload
- • SIADH
- • Hyponatremia
- • Paraneoplastic syndrome
High Osmotic Gap
- • Methanol poisoning
- • Ethylene glycol poisoning
- • Salicylate toxicity
- • Mannitol therapy
Understanding Serum Osmolality
What is Serum Osmolality?
Serum osmolality is a measure of the concentration of dissolved particles (osmotically active substances) in blood serum. It reflects the body's water-electrolyte balance and is crucial for maintaining proper cell function and fluid distribution.
Clinical Significance
- •Evaluates fluid and electrolyte disorders
- •Diagnoses poisoning or overdose
- •Monitors diabetes complications
- •Assesses kidney function
Formula Explanation
Osmolality = 2×Na + BUN/2.8 + Glucose/18 + Alcohol/3.7
- 2×Na: Sodium and its associated anions (mEq/L)
- BUN/2.8: Blood urea nitrogen contribution (mg/dL)
- Glucose/18: Glucose contribution (mg/dL)
- Alcohol/3.7: Ethanol contribution when present (mg/dL)
Osmotic Gap
The osmotic gap is the difference between measured and calculated osmolality. An elevated gap suggests unmeasured osmotically active substances.