Serum osmolality calculator online: hyponatremia and toxic alcohol workup
Serum osmolality from sodium, glucose, and urea. Used to differentiate hypo- and hypernatremia and to calculate the osmolar gap when methanol, ethylene glycol, or isopropanol poisoning is suspected. Reference range 275–295 mOsm/kg.
About this calculator
Serum osmolality is the total concentration of osmotically active particles that determines water distribution between intra- and extracellular space. Calculation: Osm = 2 × [Na⁺] + [glucose] + [urea] (all in mmol/L) or Osm = 2 × [Na⁺] + [glucose, mg/dL]/18 + [BUN, mg/dL]/2.8. Reference range 275–295 mOsm/kg. Used in primary care, critical care, nephrology, toxicology, and endocrinology. Hypo-osmolality (<275) with hyponatremia indicates true hypo-osmotic hyponatremia (SIADH, hypovolemic, or hypervolemic) and requires careful correction – rapid correction risks osmotic demyelination syndrome. Hyper-osmolality (>295) occurs in hyperglycemia, hypernatremia, uremia, and hyperosmolar diabetic coma. The main diagnostic use is the osmolar gap: measured minus calculated osmolality. A gap >10 mOsm/kg indicates unmeasured osmotically active substances: methanol, ethylene glycol, isopropanol, acetone, mannitol, or contrast agents. In suspected toxic alcohol poisoning the osmolar gap rises before clinical symptoms – a valuable early marker. The calculator does not replace measured osmolality – in critically ill patients direct measurement by freezing-point depression is recommended.
Source
Formula version: 2024-v1
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