Plasma Osmolality Calculator
Calculate plasma osmolality using the Smithline-Gardner formula for electrolyte balance assessment
Calculate Plasma Osmolality
Normal range: 135-145 mEq/L
Normal range: 70-99 mg/dL
Normal range: 6-24 mg/dL
For osmolal gap calculation (mOsm/kg H₂O)
Plasma Osmolality Results
Formula (mg/dL): (2 × Na) + (Glucose ÷ 18) + (BUN ÷ 2.8)
Formula (mmol/L): (2 × Na) + Glucose + BUN
Calculation: Enter values to see calculation
Example Calculation
Patient values: Na = 140 mEq/L, Glucose = 90 mg/dL, BUN = 14 mg/dL
Calculation: (2 × 140) + (90 ÷ 18) + (14 ÷ 2.8) = 280 + 5 + 5 = 290 mOsm/kg H₂O
Interpretation: Normal osmolality (275-295 mOsm/kg H₂O)
Osmolality Reference Ranges
Normal
275-295 mOsm/kg H₂O
Proper electrolyte balance
Low (Hypo-osmolality)
<275 mOsm/kg H₂O
May indicate overhydration, SIADH
Elevated
295-320 mOsm/kg H₂O
May indicate dehydration
Critical
>320 mOsm/kg H₂O
Requires immediate attention
Associated Conditions
High Osmolality
- • Diabetes mellitus
- • Diabetes insipidus
- • Dehydration
- • Hypernatremia
Low Osmolality
- • SIADH
- • Overhydration
- • Hyponatremia
- • Psychogenic polydipsia
Osmolal Gap
Normal Gap
<10 mOsm/kg H₂O
No significant unmeasured osmoles
Elevated Gap
>10 mOsm/kg H₂O
May suggest:
- • Toxic alcohol ingestion
- • Ketoacidosis
- • Lactic acidosis
- • Uremia
Understanding Plasma Osmolality
What is Plasma Osmolality?
Plasma osmolality measures the concentration of solutes in plasma, expressed as milliosmoles per kilogram of water (mOsm/kg H₂O). It reflects the body's electrolyte-water balance and indicates conditions affecting fluid homeostasis.
Osmolality vs. Osmolarity
- • Osmolality: Osmoles per kg of water (temperature-independent)
- • Osmolarity: Osmoles per liter of solution (temperature-dependent)
- • Medical practice prefers osmolality for accuracy
The Smithline-Gardner Formula
For mg/dL units:
Osmolality = (2 × Na) + (Glucose ÷ 18) + (BUN ÷ 2.8)
For mmol/L units:
Osmolality = (2 × Na) + Glucose + BUN
Clinical Applications
- • Evaluate dehydration status
- • Diagnose diabetes insipidus
- • Assess electrolyte disorders
- • Monitor fluid therapy
- • Detect toxic ingestions (osmolal gap)
⚠️ Medical Disclaimer
This calculator is for educational and reference purposes only. It should not replace professional medical judgment, laboratory interpretation, or clinical assessment.
Healthcare providers must:
- Verify all laboratory values and calculations independently
- Interpret results in clinical context with patient history
- Consider sample timing, collection methods, and laboratory variability
- Correlate with physical examination and other diagnostic tests
- Follow institutional protocols for abnormal values
- Consider factors affecting osmolality: medications, disease states, sample handling
Always consult with a physician or qualified healthcare provider for proper medical interpretation and treatment decisions. This calculator does not diagnose medical conditions.