Osmolarity (osmoles per liter of solution)

• Measure of osmoles of solute per liter of solution
• Changes with amount of solute, temperature, and pressure

Osmolality (osmoles per kilogram of water)

• Measure of osmoles of solute per kilogram of solvent
• Does not change with temperature or pressure

Serum Osmolality

• Measure of the different solutes in plasma
• Primarily determined by Na+ and corresponding anions (Cl- and bicarbonate), glucose, and urea
• Normal range is 275–295 mosm/kg (mmol/kg)
• Serum osmolality measured and also calculated
• Difference between measured and calculated serum osmolality is termed osmolal gap
• Elevated osmolal gap (>10) may suggest toxic alcohol ingestion

Increased SO

• Hyperglycemia (DKA, HHNK)
• Diabetes insipidus (central and nephrogenic)
• Hypernatremia due to dehydration
• Hypernatremia due to iatrogenic or accidental excessive sodium chloride (NaCl) or sodium bicarbonate (NaHCO) intake
• Alcohol ingestion
  

Decreased SO

• Hyponatremia with euvolemia (eg, psychogenic polydipsia, syndrome of inappropriate antidiuretic hormone (SIADH), hypothyroidism, adrenal insufficiency)
• Hyponatremia with hypervolemia (eg, cirrhosis, congestive heart failure, nephrotic syndrome)

Urine osmolality

• Measures the number of dissolved particles per unit of water in the urine
• More accurate measure of urine concentration than specific gravity
• Normal UO is 300-900 mOsm/kg
• Minimal UO is 40-80 mOsm/kg
• Maximal UO is 800-1,400 mOsm/kg

Increased UO

• Dehydration - UO may exceed plasma osmolality 3-4 fold
• SIADH
• Adrenal insufficiency
• Glycosuria
• Hypernatremia
• High-protein diet

Decreased UO

• Diabetes insipidus
• Excessive fluid intake
• Acute renal insufficiency
• Glomerulonephritis