Urine Sodium and Diabetes Insipidus
Your urine sodium of 220 mmol/L (within the reference range of 150-1150 mmol/L) does NOT diagnose or rule out diabetes insipidus (DI). Urine sodium concentration is not a diagnostic criterion for DI and provides no meaningful information about this condition.
Why Urine Sodium is Irrelevant to DI Diagnosis
The diagnosis of diabetes insipidus depends entirely on urine osmolality, not urine sodium. 1, 2
- DI is diagnosed when plasma osmolality is >300 mOsm/kg with inappropriately low urine osmolality (<300 mOsm/kg), indicating the kidney's failure to concentrate urine despite adequate physiologic stimulus 1, 2
- Urine sodium concentration measures electrolyte content, not the kidney's ability to concentrate or dilute urine, which is the fundamental defect in DI 3
What Actually Diagnoses DI
The diagnostic criteria require simultaneous measurement of:
- Plasma/serum osmolality: Must be >300 mOsm/kg (or serum sodium >145 mmol/L with normal glucose and urea) 2
- Urine osmolality: Must be inappropriately low (<300 mOsm/kg for any DI, <500 mOsm/kg specifically for nephrogenic DI) 1, 2
- Urine-to-plasma osmolality ratio: A low ratio confirms impaired concentrating ability 2
Polydipsia with inappropriately dilute urine (urine osmolality <300 mOsm/kg) in the setting of normal-to-high serum osmolality is pathognomonic for diabetes insipidus. 1
Clinical Context Required
You cannot diagnose DI from a single urine sodium value. The essential clinical features that must be present include:
- Polyuria: Excretion of copious volumes (>3 liters/24 hours in adults) of dilute urine 3, 4
- Polydipsia: Excessive thirst and fluid intake 5
- Persistence during water deprivation: The polyuria continues even when fluid intake is restricted 4
- Nocturnal symptoms: Night waking to urinate is a good sign of organic disease rather than behavioral polydipsia 4
Critical Pitfall to Avoid
Do not confuse urine sodium with urine osmolality—they measure completely different things. 2 Urine sodium reflects electrolyte excretion and dietary intake, while urine osmolality reflects the kidney's concentrating ability, which is what fails in DI. Your urine sodium being within normal range tells you nothing about whether you have DI.
Next Steps if DI is Suspected
If you have symptoms of polyuria and polydipsia, the appropriate workup includes:
- Simultaneous measurement of serum sodium, plasma osmolality, and urine osmolality 2
- 24-hour urine volume quantification 4
- Baseline plasma copeptin measurement (>21.4 pmol/L is diagnostic for nephrogenic DI in adults) 1, 2
- Genetic testing if nephrogenic DI is suspected, especially with family history 1
- Pituitary MRI if central DI is suspected 4
Water deprivation testing should be avoided when possible due to risks of hypernatremic dehydration, particularly in confirmed or suspected nephrogenic DI 6