Urine Osmolality in Undiagnosed Diabetes Insipidus
In undiagnosed diabetes insipidus, urine osmolality is characteristically less than 200 mOsm/kg H₂O, often accompanied by high-normal or elevated serum sodium. 1
Diagnostic Threshold
The hallmark finding is urine osmolality <200 mOsm/kg H₂O in the presence of polyuria (>2.5-3 L per 24 hours) and inappropriately dilute urine despite elevated serum osmolality. 1, 2
This combination of dilute urine with high-normal or elevated serum sodium is pathognomonic for diabetes insipidus and distinguishes it from other causes of polyuria. 1
Some sources note that urine osmolality may be as low as <110 mOsm/kg H₂O in severe cases, particularly in nephrogenic diabetes insipidus. 3
Clinical Context
The classic triad consists of polyuria, polydipsia, and inappropriately dilute urine (osmolality <200 mOsm/kg H₂O), which when present together with high-normal or elevated serum sodium confirms the diagnosis. 1, 2
Patients with intact thirst mechanisms and free access to water typically maintain normal serum sodium at steady state because their osmosensors drive adequate fluid replacement, but the urine remains persistently dilute. 2
A critical pitfall: urine osmolality in the 200-300 mOsm/kg range does not represent true diabetes insipidus and may indicate partial dehydration, chronic kidney disease, or other renal disorders. 2
Diagnostic Confirmation
The diagnosis requires simultaneous measurement of serum sodium, serum osmolality, and urine osmolality to demonstrate the characteristic pattern of dilute urine despite normal or elevated serum osmolality. 1, 2
After water deprivation testing, patients with diabetes insipidus will have urine osmolality <300 mOsm/kg H₂O (and typically <200 mOsm/kg H₂O) despite significant water restriction. 4
Plasma copeptin levels can then distinguish subtypes: >21.4 pmol/L suggests nephrogenic diabetes insipidus, while <21.4 pmol/L indicates central diabetes insipidus or primary polydipsia. 1, 5