Plasma Osmolarity Calculator – Osmolal Gap
Calculate plasma osmolarity and osmolal gap from lab values
How to Use
- Enter sodium level in mEq/L (normal range: 135-145 mEq/L)
- Enter glucose level in mg/dL (normal range: 70-100 mg/dL)
- Enter BUN (Blood Urea Nitrogen) level in mg/dL
- Optionally enter measured osmolality to calculate osmolal gap
- Click calculate to see results and interpretation
What is Plasma Osmolarity?
Plasma osmolarity (or serum osmolality) is a measure of the concentration of solutes in blood plasma. It reflects the balance of water and electrolytes in the body and is crucial for maintaining proper cellular function.
Osmolarity is typically measured in milliosmoles per kilogram (mOsm/kg) and the normal range is 275-295 mOsm/kg. Values outside this range can indicate various medical conditions requiring evaluation and treatment.
Calculation Formula
The calculated plasma osmolarity uses the following formula:
Osmolarity = 2 × Na + Glucose/18 + BUN/2.8
Where:
- Na = Sodium in mEq/L (multiplied by 2 to account for accompanying anions)
- Glucose in mg/dL (divided by 18 to convert to mmol/L)
- BUN (Blood Urea Nitrogen) in mg/dL (divided by 2.8 to convert to mmol/L)
Understanding Osmolal Gap
The osmolal gap is the difference between measured osmolality (from a laboratory test) and calculated osmolarity:
Osmolal Gap = Measured Osmolality - Calculated Osmolarity
A normal osmolal gap is typically less than 10 mOsm/kg. An elevated gap suggests the presence of unmeasured osmotically active substances in the blood.
| Osmolal Gap | Interpretation | Possible Causes |
|---|---|---|
| < 10 mOsm/kg | Normal | No significant unmeasured osmoles |
| > 10 mOsm/kg | Elevated | Toxic alcohols (methanol, ethylene glycol), ketones, lactic acidosis |
| Negative | Low | Laboratory error, pseudohyponatremia, ethanol |
Clinical Applications
Plasma osmolarity and osmolal gap calculations are used in various clinical scenarios:
- Evaluating fluid and electrolyte imbalances
- Diagnosing hyponatremia and hypernatremia
- Detecting toxic alcohol ingestion (methanol, ethylene glycol, isopropanol)
- Assessing diabetic ketoacidosis and other metabolic conditions
- Monitoring patients with SIADH (syndrome of inappropriate antidiuretic hormone)
- Guiding fluid resuscitation and replacement therapy
- Investigating unexplained altered mental status
Interpreting Results
Interpretation of osmolarity results requires clinical context:
High Osmolarity (>295 mOsm/kg):
- Dehydration or volume depletion
- Hypernatremia (elevated sodium)
- Hyperglycemia (elevated glucose)
- Uremia (elevated BUN)
- Toxic alcohol ingestion
- Mannitol or other osmotic agents
Low Osmolarity (<275 mOsm/kg):
- Overhydration or excessive fluid intake
- Hyponatremia (low sodium)
- SIADH (syndrome of inappropriate ADH)
- Psychogenic polydipsia
- Certain medications (diuretics, antidepressants)
Limitations and Considerations
Important considerations when using this calculator:
- This formula provides an estimate; direct measurement is more accurate
- The formula may underestimate osmolarity in certain conditions
- Ethanol contributes to osmolarity but is not included in this formula
- Laboratory errors can affect both measured and calculated values
- Clinical correlation is essential for proper interpretation
- Serial measurements are often more useful than isolated values
- Always consult with a healthcare provider for medical decision-making
Frequently Asked Questions
- What is the difference between osmolarity and osmolality?
- Osmolarity is measured in osmoles per liter (mOsm/L) and osmolality in osmoles per kilogram (mOsm/kg). In clinical practice, the terms are often used interchangeably as the difference is minimal in dilute solutions like plasma. Osmolality is the more accurate term for laboratory measurements.
- When should I be concerned about an elevated osmolal gap?
- An osmolal gap greater than 10 mOsm/kg should prompt evaluation for toxic alcohol ingestion (methanol, ethylene glycol), severe ketoacidosis, or lactic acidosis. If there's suspicion of toxic ingestion, seek emergency medical care immediately.
- Can this calculator diagnose medical conditions?
- No. This calculator is an educational tool that provides estimated values based on laboratory results. Medical diagnosis requires comprehensive clinical evaluation by a qualified healthcare professional, including physical examination, medical history, and additional testing.
- Why use calculated osmolarity when it can be measured directly?
- Calculated osmolarity is useful for several reasons: it's quickly available from routine lab tests, it helps identify the osmolal gap when compared to measured values, it can guide initial assessment when measured osmolality is not immediately available, and it's a useful screening tool. However, measured osmolality is more accurate for final clinical decisions.