plasma osmolar gap calculation

The plasma osmolal gap is the difference between the measured and calculated plasma osmolality. The calculated plasma osmolality is determined as follows:

Osmolality=2×sodium+glucose18+urea2.8+EtOH4.6Osmolality = 2\times sodium + \frac{glucose}{18} + \frac{urea}{2.8} + \frac{EtOH}{4.6}

Plasma Osmolality (mOsm/kg H2O) = (2 × Serum Sodium [mEq/L]) + Plasma Glucose (mg/dL)/18 + Blood Urea Nitrogen (mg/dL)/2.8 + EtOH/4.6

A high osmolal gap (>10 mOsm/kg H2O) indicates the presence of unmeasured osmoles such as methanol or ethylene glycol, which are metabolized to organic acids, thereby increasing the anion gap. A high osmolal gap is also seen in pseudohyponatremia, in which plasma osmolality is normal while calculated osmolality is low.

  • anion gap metabolic acidosis

  • metformin associated lactic acidosis MALA

    • This patient’s history is most consistent with metformin-associated lactic acidosis (MALA). The patient has severe metabolic acidosis with very large anion gap, no osmolar gap (plasma osmolar gap calculation), and very increased serum lactate—that is, she has severe lactic acidosis. However, aside from the lactate level itself, there is no evidence of shock of any type, nor is there evidence of infection. Osmolar gap is normal, ruling out concomitant methanol or ethylene glycol ingestion. Ketones are not measured, but the delta anion gap:delta bicarbonate ratio is 1.95, most consistent with pure lactic acidosis.