💧 Hypotonic Urine [Neurosurgery Wiki]

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===== 💧 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 |

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==== 🧠 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)

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==== 🔬 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

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==== ✅ Practical Tip ====

A **urine specific gravity < 1.005** in the presence of **elevated plasma osmolality** or **hypernatremia** strongly supports a diagnosis of **AVP-D**.

<WRAP important>
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.
</WRAP>