Urination During Saline Administration in Undiagnosed Diabetes Insipidus
No, urinating twice during one hour of saline administration is not unusual for undiagnosed diabetes insipidus (DI) and would actually be expected given the pathophysiology of the condition.
Understanding the Physiological Response
In patients with DI, the kidneys cannot concentrate urine due to either inadequate vasopressin production (central DI) or renal resistance to vasopressin (nephrogenic DI) 1, 2. When saline is administered intravenously, the following occurs:
- Fluid loading increases urine output: The kidneys in DI patients cannot reabsorb free water, so any fluid administered—including isotonic saline—will be rapidly excreted as dilute urine 1
- Polyuria persists despite hydration: The hallmark of DI is excretion of large volumes of hypotonic urine regardless of hydration status 3, 2, 4
- Saline provides volume but not concentrating ability: Isotonic saline expands intravascular volume but does not correct the underlying defect in water reabsorption 5
Why This Pattern Fits DI
The scenario described—minimal oral intake for 3-4 hours followed by urination twice during one hour of IV saline—is entirely consistent with undiagnosed DI:
- Patients with DI typically produce 3-20 liters of urine per day, with urine osmolality remaining inappropriately low (typically <300 mOsm/kg) 2, 4
- The brief period of fluid deprivation (3-4 hours without drinking) would not suppress urine output in DI as it would in normal individuals 1
- Administration of isotonic saline provides additional fluid load that the kidneys immediately excrete as dilute urine 5
- Urinating twice in one hour during IV hydration represents a normal physiological response for someone with DI receiving fluid replacement 1
Clinical Context and Caveats
Important considerations when evaluating this presentation:
- Volume of urine matters: If the patient produced large volumes (>500 mL total) during that hour, this strengthens suspicion for DI 1, 2
- Urine osmolality is diagnostic: The key finding would be inappropriately dilute urine (osmolality <300 mOsm/kg) despite plasma hyperosmolality or hypernatremia 3, 2, 4
- Serum sodium levels: Check if the patient had hypernatremia on presentation, which would occur with DI if fluid intake was restricted 1, 5
The pattern described does not rule out DI—in fact, it supports it. A normal individual who was mildly dehydrated from 3-4 hours without fluids would concentrate their urine and produce minimal output during rehydration, not urinate twice in one hour 1, 2.
Diagnostic Next Steps
To confirm or exclude DI in this clinical scenario:
- Measure paired serum and urine osmolality when the patient is producing large urine volumes 3, 2, 4
- Check serum sodium concentration 5, 6
- Consider formal water deprivation test or copeptin measurement if initial labs suggest DI 3, 2, 4
- Obtain detailed history about chronic polyuria and polydipsia symptoms 1, 2
The key pitfall is dismissing polyuria during IV hydration as "normal diuresis" when it may actually represent the pathognomonic inability to concentrate urine seen in DI 1, 2.