Calculate plasma osmolality, effective osmolality (tonicity), osmol gap, corrected sodium for hyperglycemia, and free water deficit. Includes osmol gap differential diagnosis for toxic alcohol eval...
Plasma osmolality is a fundamental measurement of the concentration of dissolved solutes in the blood, normally maintained within the tight range of 275–295 mOsm/kg by the interaction of ADH (antidiuretic hormone), thirst mechanisms, and renal water handling. Derangements of osmolality indicate significant electrolyte, glucose, or uremia abnormalities and can signal the presence of toxic unmeasured solutes.
This calculator computes plasma osmolality from sodium, glucose, and BUN using the standard formula, calculates effective osmolality (tonicity) which excludes BUN as a freely permeable solute, determines the osmol gap when a measured osmolality is available, corrects sodium for hyperglycemia, and estimates free water deficit in hypernatremic states. The osmol gap — the difference between measured and calculated osmolality — is a critical screening tool for toxic alcohol ingestion (methanol, ethylene glycol, isopropanol).
Understanding the distinction between total osmolality and effective osmolality is clinically crucial: BUN contributes to total osmolality but not to tonicity because it freely crosses cell membranes and does not drive water shifts. Only effective osmolality determines cellular hydration status and drives the clinical symptoms of hypo- or hyperosmolality. This distinction explains why a patient with severe uremia (high total osmolality) but normal sodium may have no osmolar symptoms.
This calculator provides comprehensive osmolality assessment including calculated osmolality, effective tonicity, osmol gap with differential diagnosis for toxic alcohols, hyperglycemia-corrected sodium, and free water deficit estimation — all essential tools for emergency medicine, critical care, and toxicology. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
Calculated Osm = 2 × Na + Glucose/18 + BUN/2.8 (all in mg/dL). Effective Osm = 2 × Na + Glucose/18. Osmol Gap = Measured Osm − Calculated Osm. Corrected Na = Na + 1.6 × ((Glucose − 100) / 100). Free Water Deficit = 0.6 × Weight × (Na/140 − 1).
Result: Calculated Osm = 290 mOsm/kg, Osmol Gap = 30, Significantly elevated — investigate toxic alcohols
With Na 140, glucose 90, BUN 14: calculated osmolality = 2×140 + 90/18 + 14/2.8 = 290. The measured osmolality of 320 yields an osmol gap of 30, which is significantly elevated and warrants evaluation for methanol or ethylene glycol ingestion.
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The normal osmol gap ranges from −10 to +10 mOsm/kg. Some sources cite a mean of approximately +2. Gaps consistently above +10 warrant clinical evaluation for unmeasured osmoles.
Elevated osmol gap (>10) can be caused by toxic alcohols (methanol, ethylene glycol, isopropanol), ethanol, mannitol, IV contrast, propylene glycol, renal failure, and lactic acidosis. Gaps >25 are highly suggestive of toxic alcohol ingestion.
BUN (urea) freely crosses cell membranes along its concentration gradient. It does not create an osmotic gradient between intracellular and extracellular fluid. Therefore, it does not contribute to "effective" osmolality or tonicity, which determines water movement between compartments.
Corrected Na = Measured Na + 1.6 × ((Glucose − 100) / 100). For every 100 mg/dL glucose above normal, sodium decreases by approximately 1.6 mEq/L due to osmotic water shift from ICF to ECF. This is dilutional, not true hyponatremia.
Yes. A significantly negative osmol gap (<−10) usually indicates laboratory artifact, most commonly due to hypertriglyceridemia or hyperproteinemia causing pseudohyponatremia in some assay methods.
Free water deficit (L) = 0.6 × weight (kg) × (Na/140 − 1). The 0.6 factor represents total body water fraction (0.5 for elderly women). This guides IV free water replacement for hypernatremia, correcting no faster than 10–12 mEq/L per 24 hours.