💧 Hypotonic Urine
🧪 Definition
Hypotonic urine refers to urine that is less concentrated than plasma, typically indicating a failure to conserve water.
| Parameter | Threshold | Interpretation |
|---|---|---|
| Urine osmolality | < 300 mOsm/kg | Hypotonic |
| Urine specific gravity (SG) | < 1.005 g/mL | Suggests AVP deficiency or water diuresis |
🧠 Clinical Relevance
In the postoperative neurosurgical setting (e.g. after pituitary surgery), hypotonic urine may signal Arginine Vasopressin Deficiency (AVP-D), also known as central diabetes insipidus.
Key signs:
- Polyuria (often >250 mL/h)
- Low urine SG or osmolality
- Rising serum sodium (>145 mmol/L)
🔬 Monitoring Tools
* Urine dipsticks (e.g., Combur-10) allow bedside estimation of SG * Refractometers provide more accurate SG measurement * Plasma osmolality and serum sodium should be monitored in parallel
✅ Practical Tip
A urine specific gravity < 1.005 in the presence of elevated plasma osmolality or hypernatremia strongly supports a diagnosis of AVP-D.
In patients self-monitoring with test strips, using a cut-off of 1.015 g/mL can improve sensitivity and safely rule out hypotonic urine.