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Estimate urine osmolality and assess renal concentrating ability
If measured, calculates urine osmolal gap
Urine Osm = 2(Na + K) + Urea/2.8 + Glucose/18
Prerenal: concentrated urine (>500). ATN: isosthenuric (~300). Helps differentiate cause of AKI.
SIADH: concentrated urine despite low serum Osm. Primary polydipsia: dilute urine.
Persistently dilute urine despite dehydration. Central DI responds to desmopressin; nephrogenic does not.
Estimates NH4+ excretion in NAGMA. Gap >150 = appropriate NH4+ excretion (GI cause). Gap <150 = impaired (RTA).
Osmolality is measured per kg of solvent (water), while osmolarity is per liter of solution. In clinical practice, they are nearly equivalent and often used interchangeably.
Measured is more accurate. Calculation is useful when direct measurement is not available. The osmolal gap (measured - calculated) can indicate unmeasured solutes like NH4+.
Healthy kidneys can concentrate urine to ~1200 mOsm/kg (4x plasma). Values >800 mOsm/kg generally indicate intact concentrating ability.
ADH (vasopressin) increases water reabsorption in collecting ducts, concentrating urine. Without ADH, urine is dilute. SIADH causes inappropriate concentration.
Ratio >1 = concentrated, <1 = dilute. In prerenal AKI, ratio is typically >1.5. In ATN, it approaches 1 (isosthenuric).
Overnight fluid restriction naturally concentrates urine. First morning void reflects maximal concentration and is most useful for assessing concentrating ability.
In hyponatremia: Urine Osm >100 with low serum Osm suggests SIADH or other cause of impaired dilution. Urine Osm <100 suggests primary polydipsia (appropriate dilution).