Urine Osmolality of 220 mOsm/kg: Evaluation for Diabetes Insipidus
A urine osmolality of 220 mOsm/kg is highly suggestive of diabetes insipidus, but you cannot make this diagnosis based on urine osmolality alone—you must simultaneously measure your serum/plasma osmolality to confirm the diagnosis. 1
Diagnostic Criteria for Diabetes Insipidus
The diagnosis of diabetes insipidus requires both of the following criteria to be met simultaneously 1:
- Plasma osmolality >300 mOsm/kg (indicating dehydration/hyperosmolar state)
- Urine osmolality <300 mOsm/kg (indicating inappropriate failure to concentrate urine)
Your urine osmolality of 220 mOsm/kg meets the second criterion, but without knowing your serum osmolality, the diagnosis cannot be confirmed. 1
Why Both Values Matter
In diabetes insipidus, the kidneys produce maximally dilute urine continuously, with urine osmolality remaining <200 mOsm/kg regardless of fluid consumption, because the collecting tubules cannot respond to or lack ADH. 2 Your value of 220 mOsm/kg falls into this range and is concerning for DI.
However, the key pathognomonic finding is the combination of high plasma osmolality (>300 mOsm/kg) with inappropriately low urine osmolality (<300 mOsm/kg). 1 This demonstrates that despite your body being dehydrated (high plasma osmolality), your kidneys are failing to concentrate urine appropriately (low urine osmolality).
Next Steps You Need
You must obtain simultaneous measurements of: 1
- Serum sodium
- Plasma osmolality
- Urine osmolality (you already have this at 220 mOsm/kg)
If your plasma osmolality is >300 mOsm/kg with your urine osmolality of 220 mOsm/kg, this is diagnostic for diabetes insipidus. 1
Important Caveats
Before interpreting elevated serum osmolality as dehydration, you must verify that serum glucose and urea are within normal range, as these can falsely elevate osmolality without true dehydration. 3
Do not rely on simple clinical signs like skin turgor, mouth dryness, or urine color to assess for diabetes insipidus—these are unreliable and should NOT be used. 3, 1
Types of Diabetes Insipidus
If diabetes insipidus is confirmed, further testing distinguishes between 1, 2:
- Central DI: Low ADH production (responds to desmopressin)
- Nephrogenic DI: Kidney resistance to ADH (does not respond to desmopressin; baseline copeptin >21.4 pmol/L is diagnostic)
The critical point: Get your serum osmolality checked immediately. If it's >300 mOsm/kg with your urine osmolality of 220 mOsm/kg, you have diabetes insipidus and need urgent endocrine evaluation. 1