Understanding Serum Osmolarity and the Osmolal Gap

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Serum osmolarity is a measure of the different solutes in plasma. It is primarily determined by sodium, glucose, and blood urea nitrogen (BUN). Understanding a patient's osmolarity is a crucial initial step in diagnosing disorders of water balance, notably hyponatremia, and in evaluating patients for potential toxic alcohol ingestions.

What is Serum Osmolarity?

Osmolality and osmolarity refer to the concentration of dissolved particles in a fluid. While osmolality is expressed as osmoles per kilogram of water (mOsm/kg), osmolarity is expressed as osmoles per liter of solution (mOsm/L). In the human body, because fluid is primarily water, the terms are often used interchangeably in a clinical context, though laboratory measurements are technically osmolality.

The primary contributors to serum osmolarity are:

• Sodium (Na⁺): The predominant extracellular cation, driving the vast majority of osmolarity. This is why sodium is multiplied by 2 in the formula (to account for accompanying anions like chloride and bicarbonate).
• Glucose: Especially important in the setting of hyperglycemia.
• Urea (BUN): An "ineffective osmole" because it freely crosses cell membranes and does not shift water between the intracellular and extracellular compartments, but it is included in the total laboratory measurement.

The Calculation Formula

Units: Sodium in mEq/L, Glucose in mg/dL, BUN in mg/dL. The constants (18 and 2.8) convert conventional US units (mg/dL) into SI units (mmol/L).

Clinical Applications

1. Evaluating Hyponatremia

The first step in evaluating a low serum sodium level (<135 mEq/L) is to check the serum osmolarity to determine if it is a true hypotonic hyponatremia:

• Hypotonic Hyponatremia (< 275 mOsm/kg): "True" hyponatremia. The next step is to assess the patient's volume status (hypovolemic, euvolemic, or hypervolemic).
• Isotonic Hyponatremia (275-295 mOsm/kg): "Pseudohyponatremia." Usually caused by a laboratory artifact in the setting of severe hypertriglyceridemia or hyperproteinemia (e.g., multiple myeloma).
• Hypertonic Hyponatremia (> 295 mOsm/kg): Often caused by severe hyperglycemia. The high glucose draws water out of the cells into the extracellular space, diluting the sodium. Other causes include the administration of mannitol.

2. The Osmolal Gap

The calculated osmolarity is an estimate. The laboratory can directly measure the serum osmolality using freezing point depression. The difference between the measured and calculated value is the osmolal gap.

A normal osmolal gap is < 10 mOsm/kg. An elevated gap indicates the presence of unmeasured, osmotically active substances in the blood. The most common cause is ethanol intoxication. Other critical toxicologic causes include ingestions of methanol, ethylene glycol, or isopropyl alcohol.

Limitations

The calculation assumes standard physiological conditions and does not account for ethanol unless it is manually added to the formula (Ethanol / 4.6). Furthermore, while a high osmolal gap is highly specific for toxic alcohol ingestion, a normal gap does not completely rule out a late-presenting toxic alcohol ingestion, because the parent alcohols may have already been metabolized into acids (which raise the anion gap, not the osmolal gap).

References

• Fagugli, R. M., & Franciosi, S. (2014). Disorders of Sodium and Water Balance. Nephrology Dialysis Transplantation, 29(1), i18-i23.
• Kraut, J. A., & Madias, N. E. (2007). Serum Anion Gap: Its Uses and Limitations in Clinical Medicine. Clinical Journal of the American Society of Nephrology, 2(1), 162-174.
• Purssell, R. A., Pudek, M., Brubacher, J., & Abu-Laban, R. B. (2001). Derivation and Validation of a Formula to Calculate the Contribution of Ethanol to the Osmolal Gap. Annals of Emergency Medicine, 38(6), 653-659.