Urine Osmolality Calculator

Measured osmolality by freezing point depression captures all solutes, including those not in the standard formula (e.g., mannitol, contrast agents, NH4+, other organic solutes). The calculated formula only estimates the contribution of Na, K, glucose, and urea. In clinical practice — especially in the ICU or in polyuria workup — measured Uosm is always preferred. The calculated value is useful when measured Uosm is unavailable.

Isosthenuria is urine osmolality approximately equal to plasma osmolality (280–300 mOsm/kg). It indicates loss of tubular concentrating and diluting ability. It is the hallmark of advanced tubular dysfunction seen in ATN, severe CKD, and other forms of intrinsic renal injury. Isosthenuric urine in an oliguric patient strongly suggests ATN rather than pre-renal disease.

Urine osmolality varies widely with water intake and physiological state. After overnight fasting and without excess water intake, it is typically 300–900 mOsm/kg in healthy adults. The minimum (maximum dilution) is ~50 mOsm/kg during a water load, and the maximum (maximum concentration) is 800–1400 mOsm/kg during dehydration. A random urine osmolality of >600 mOsm/kg generally indicates intact concentrating ability.

In the water deprivation test (used to diagnose diabetes insipidus), the patient is deprived of fluid until plasma osmolality rises above 295 mOsm/kg or serum Na exceeds 145 mEq/L. Urine osmolality is measured hourly. A maximum Uosm <300 mOsm/kg despite hypernatraemia confirms DI. DDAVP is then given: if Uosm rises >50% (typically >750 mOsm/kg), central DI is confirmed; if no response, nephrogenic DI is diagnosed.

No. SIADH is a clinical diagnosis requiring all of: (1) serum hypo-osmolality (<275 mOsm/kg), (2) inappropriately concentrated urine (Uosm >100 mOsm/kg, usually >plasma Osm), (3) clinical euvolaemia, (4) elevated urine sodium (>40 mEq/L), (5) normal renal, adrenal, and thyroid function, and (6) absence of diuretics. An elevated Uosm alone does not diagnose SIADH — it must be interpreted in the context of low plasma osmolality and euvolaemia.

Specific gravity measures the density of urine relative to water and is influenced disproportionately by large molecules (glucose, protein, contrast agents, dextran). A patient with heavy glycosuria may have a high specific gravity (1.030) but a normal or only mildly elevated osmolality. Osmolality measures only the number of particles, not their mass. For accurate assessment of tubular concentrating ability, osmolality is far superior to specific gravity.

The urine-to-plasma osmolality ratio (U/P Osm) is a simple clinical tool: a ratio >1.5 suggests appropriate concentration (pre-renal or physiological response); a ratio close to 1 suggests isosthenuria (ATN or severe CKD); a ratio <1 suggests dilute urine (water diuresis, DI, or recovery from ATN). It is used alongside FENa and clinical context in AKI evaluation.

In both central and nephrogenic DI, urine osmolality is inappropriately low (<300 mOsm/kg) in the face of high serum osmolality. The key distinction is the response to DDAVP: in central DI (ADH deficiency), DDAVP causes a >50% rise in Uosm (tubules are intact and respond to exogenous ADH). In nephrogenic DI (ADH resistance), Uosm does not rise significantly because the tubules are insensitive to ADH regardless of the source. Lithium and demeclocycline are classic drug causes of nephrogenic DI.