Urine Osmolality in Diabetes Insipidus
In diabetes insipidus, urine osmolality is typically less than 200 mOsm/kg H₂O, regardless of the patient's hydration status or serum sodium level. 1, 2
Diagnostic Threshold
The pathognomonic finding for diabetes insipidus is inappropriately dilute urine with osmolality <200 mOsm/kg H₂O combined with high-normal or elevated serum sodium. 1, 2, 3
This threshold of <200 mOsm/kg H₂O represents maximally dilute urine that persists continuously because the collecting tubules cannot respond to or lack antidiuretic hormone (ADH). 2
The combination of urine osmolality <200 mOsm/kg H₂O with serum hyperosmolality forms the diagnostic triad that confirms the kidneys' complete inability to concentrate urine. 1, 4
Distinguishing from Partial Forms
Severe diabetes insipidus maintains urine osmolality below 250 mOsm/kg, while partial forms may show values between 250-750 mOsm/kg and require water deprivation testing for diagnosis. 5
Patients with acquired nephrogenic diabetes insipidus will demonstrate urinary osmolality less than 300 mOsm/kg H₂O despite water deprivation, and show little or no increase after vasopressin administration. 6
Values in the 200-300 mOsm/kg range do not represent true diabetes insipidus and may indicate partial dehydration, chronic kidney disease, or early renal disorders. 3
Clinical Context
The urine remains maximally dilute (osmolality <200 mOsm/kg H₂O) regardless of fluid consumption, distinguishing diabetes insipidus from conditions where urine concentration varies with hydration status. 2
This persistent dilute urine, when combined with high urine volumes (>3 liters/24 hours in adults), creates the clinical picture of polyuria and polydipsia. 5
Even a single measurement showing urine osmolality <200 mOsm/kg H₂O in the presence of elevated serum sodium is sufficient to suspect diabetes insipidus and warrant early genetic testing. 1
Common Pitfall
- Do not confuse diabetes insipidus with diabetes mellitus—the latter causes polyuria through osmotic diuresis from glucosuria and shows high urine osmolality from glucose, not the dilute urine (<200 mOsm/kg H₂O) seen in diabetes insipidus. 3