What is Plasma Osmolality?
Plasma osmolality measures the concentration of dissolved particles (solutes) in the blood plasma. It is a critical indicator of the body's water balance and is tightly regulated between 275 and 295 mOsm/kg. The hypothalamus senses changes in osmolality and triggers thirst or antidiuretic hormone (ADH) release to maintain homeostasis.
The three major contributors to plasma osmolality are sodium (with its associated anions), glucose, and urea (measured as BUN). Sodium and its counter-ions account for approximately 90% of plasma osmolality under normal conditions.
Osmolality Formula
Where Na is in mEq/L, Glucose in mg/dL, and BUN in mg/dL. The result is in mOsm/kg.
The sodium is multiplied by 2 to account for associated anions (chloride and bicarbonate). Glucose is divided by 18 (its molecular weight divided by 10) and BUN is divided by 2.8 (urea nitrogen molecular weight divided by 10) to convert from mg/dL to mOsm/kg.
Interpreting Results
| Range (mOsm/kg) | Classification | Clinical Significance |
|---|---|---|
| < 275 | Hypo-osmolar | Water excess, hyponatremia, SIADH |
| 275 – 295 | Normal | Normal water balance |
| 296 – 320 | Hyperosmolar | Dehydration, hyperglycemia, uremia |
| > 320 | Critical | Risk of altered consciousness, seizures, coma |
Osmolality Balance Diagram
Osmolal Gap
The osmolal gap is the difference between the measured serum osmolality (from the lab) and the calculated osmolality. A normal osmolal gap is typically less than 10 mOsm/kg.
| Osmolal Gap | Interpretation |
|---|---|
| < 10 mOsm/kg | Normal — no significant unmeasured osmoles |
| 10 – 20 mOsm/kg | Mildly elevated — consider ethanol, mannitol, or lab error |
| > 20 mOsm/kg | Significantly elevated — strongly consider toxic alcohol (methanol, ethylene glycol, isopropanol) |
An elevated osmolal gap is a critical finding in the emergency department, as it may indicate poisoning with methanol or ethylene glycol. These toxic alcohols are not captured in the standard osmolality formula and create a gap between measured and calculated values.
Osmolality vs Osmolarity
Osmolality is measured per kilogram of solvent (water), while osmolarity is measured per liter of solution. In clinical practice, the two terms are often used interchangeably because the difference is negligible at normal physiologic concentrations (approximately 1–2%).
- Osmolality (mOsm/kg): More accurate because it is unaffected by temperature and solute volume. Measured by freezing-point depression or vapor-pressure osmometry.
- Osmolarity (mOsm/L): Calculated from known solute concentrations. Slightly lower than osmolality due to the volume occupied by proteins and lipids.
Clinical Conditions
Hypo-osmolality (Low Osmolality)
- SIADH (Syndrome of Inappropriate ADH Secretion): Excess ADH causes water retention, diluting plasma
- Psychogenic polydipsia: Excessive water intake overwhelms kidney excretion capacity
- Hyponatremia: Most common cause; low sodium directly reduces osmolality
- Adrenal insufficiency: Cortisol deficiency impairs free water excretion
Hyperosmolality (High Osmolality)
- Diabetic ketoacidosis (DKA): Markedly elevated glucose raises osmolality
- Hyperosmolar hyperglycemic state (HHS): Extreme hyperglycemia with osmolality often >320
- Diabetes insipidus: Lack of ADH (central) or kidney response to ADH (nephrogenic) causes water loss
- Dehydration: Loss of free water concentrates all solutes
- Uremia: Severe kidney failure raises BUN substantially
Worked Example
A patient presents with Na = 140 mEq/L, Glucose = 100 mg/dL, BUN = 14 mg/dL:
Osm = 280 + 5.56 + 5.0 = 290.6 mOsm/kg
This falls within the normal range (275–295 mOsm/kg). If the measured osmolality from the lab was 295 mOsm/kg, the osmolal gap would be 295 − 290.6 = 4.4 mOsm/kg, which is also normal (<10).
Frequently Asked Questions
Why is sodium multiplied by 2?
Sodium exists in solution with associated anions, primarily chloride and bicarbonate. Each sodium ion is paired with an anion, effectively doubling its osmotic contribution. The factor of 2 accounts for both cations and their accompanying anions.
Can ethanol affect osmolality?
Yes. Ethanol is an osmotically active substance that is not accounted for in the standard formula. Each 100 mg/dL of ethanol adds approximately 22 mOsm/kg to the measured osmolality. Some formulas include an ethanol correction: add (Ethanol / 4.6) to the calculated osmolality.
When should I order a serum osmolality?
Serum osmolality is indicated when evaluating hyponatremia, suspected toxic alcohol ingestion, altered mental status of unknown etiology, polyuria/polydipsia workup, and monitoring of mannitol therapy. It is also useful in distinguishing between dilutional and depletional hyponatremia.
What is the effective osmolality (tonicity)?
Effective osmolality, or tonicity, excludes freely permeable solutes like urea. Since BUN crosses cell membranes freely, it does not contribute to water shifts between compartments. Tonicity = (2 × Na) + (Glucose / 18). This is the clinically relevant value for assessing cell hydration.