Serum Osmolality Calculator
Calculate serum osmolality to evaluate hydration status, diagnose hyponatremia types, and assess for unmeasured osmoles including toxic alcohols.
What is serum osmolality and why is it important?
Serum osmolality measures concentration of solutes in blood (mOsm/kg). Normal range 275-295 mOsm/kg. Major contributors: sodium (doubles due to anions), glucose, and BUN (urea). Formula: 2*Na + (Glucose/18) + (BUN/2.8). Used to diagnose hyponatremia types, assess hydration status, detect unmeasured osmoles (alcohols, toxins), calculate osmolal gap. Critical in metabolic emergencies, SIADH, diabetes insipidus, toxicology.
How do you calculate serum osmolality?
Formula: Osmolality = 2 * Sodium + (Glucose/18) + (BUN/2.8). The factor 2 accounts for sodium's anions (Cl-, HCO3-). Divide glucose by 18 to convert mg/dL to mmol/L (molecular weight). Divide BUN by 2.8 for same conversion. Example: Na 140, Glucose 90, BUN 14: Osm = 2*140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290 mOsm/kg (normal). Some formulas add +9 for other solutes.
What is osmolal gap and what does it indicate?
Osmolal gap = Measured osmolality - Calculated osmolality. Normal <10 mOsm/kg. Elevated gap (>10) indicates unmeasured osmoles present: toxic alcohols (methanol, ethylene glycol, isopropanol), ethanol, mannitol, contrast dye, severe hypertriglyceridemia, hyperproteinemia. In poisoning context, osmolal gap >25 suggests toxic alcohol ingestion. Always measure osmolality directly when suspecting toxins - calculation alone misses these dangerous substances.
What causes high and low osmolality?
High osmolality (>295): Hypernatremia, hyperglycemia, uremia, dehydration, diabetes insipidus, toxic alcohols, mannitol. Severe >320 associated with altered mental status. Low osmolality (<275): Hyponatremia (SIADH, psychogenic polydipsia, diuretics), overhydration, adrenal insufficiency. Evaluate with urine osmolality - concentrated urine (>100) in low serum osm suggests SIADH; dilute urine (<100) suggests primary polydipsia or water intoxication.
How is osmolality used in hyponatremia workup?
Step 1: Confirm hypotonic hyponatremia (low serum osm <275). Step 2: Check volume status (hypovolemic, euvolemic, hypervolemic). Step 3: Measure urine osmolality and sodium. Urine osm >100 + urine Na >40 = SIADH. Urine osm >100 + urine Na <20 = volume depletion. Urine osm <100 = primary polydipsia. Serum osm normal/high with low Na = pseudohyponatremia (hyperglycemia, hyperlipidemia). Osmolality essential for proper diagnosis and treatment.