Looking for more details? Read our comprehensive Serum Osmolarity guide
Why Calculate Serum Osmolarity?
Serum osmolarity helps evaluate the concentration of solutes in the blood. It is a critical tool for assessing hydration status, investigating the causes of hyponatremia (low sodium), and identifying the presence of unmeasured toxic osmotically active substances (such as toxic alcohols) when compared to the measured osmolality to find the osmolal gap.
Formula
Note: Na⁺ is in mEq/L or mmol/L. Glucose and BUN are in mg/dL. The divisors (18 and 2.8) are used to convert mg/dL to mmol/L.
Interpretation (General Guidelines)
- Normal Range: ~ 275 - 295 mOsm/kg
- High Osmolarity: > 295 mOsm/kg (Hypertonic state, dehydration, hyperglycemia, uremia)
- Low Osmolarity: < 275 mOsm/kg (Hypotonic state, overhydration, true hyponatremia)
Limitations
Calculated osmolarity is an estimate of measured osmolality:
- Osmolal Gap: The difference between measured osmolality (from the lab) and calculated osmolarity is the osmolal gap. A normal gap is < 10 mOsm/kg.
- Unmeasured Osmoles: The calculated formula does NOT account for toxic alcohols (ethanol, methanol, ethylene glycol) or other substances like mannitol. If these are present, the measured osmolality will be significantly higher than the calculated osmolarity, resulting in a high osmolal gap.
How to Use
Enter the patient's serum Sodium, Glucose, and BUN. The tool will calculate the estimated serum osmolarity. Entering ethanol (optional) will account for toxic ingestions.
Case Example
Patient: Routine labs show Na: 140, Glucose: 90, BUN: 14.
Calculation: (2 * 140) + (90 / 18) + (14 / 2.8) = 280 + 5 + 5 = 290 mOsm/kg.
Interpretation: Normal serum osmolarity (reference range 275-295 mOsm/kg).
Frequently Asked Questions
It is the difference between the measured serum osmolality (from the lab) and the calculated osmolarity. A gap >10 suggests the presence of unmeasured osmotically active substances, like ethanol, methanol, or ethylene glycol.