Ruling Out Diabetes Insipidus
To rule out diabetes insipidus, you need simultaneous measurements of serum sodium, serum osmolality, and urine osmolality—if your urine osmolality is ≥200 mOsm/kg with normal serum sodium (135-145 mEq/L), you do not have DI. 1, 2
Initial Screening Tests
The diagnostic triad that confirms or excludes DI requires three simultaneous measurements 1:
- Serum sodium: Should be normal (135-145 mEq/L) or high-normal if you have DI 1
- Serum osmolality: Would be high-normal or elevated (>295 mOsm/kg) in DI 2
- Urine osmolality: The key discriminator—must be <200 mOsm/kg to diagnose DI 1, 2
If your urine osmolality is >200 mOsm/kg, especially if it's in the 200-300 mOsm/kg range or higher, you do not have diabetes insipidus. 1 Many other conditions (partial dehydration, chronic kidney disease, early renal disorders) can cause urine osmolality in the 200-300 range without representing true DI 1.
Additional Required Measurements
Beyond the basic triad, you need 1:
- 24-hour urine volume: DI requires >3 liters per 24 hours in adults (or >2.5 L despite attempts to reduce fluid intake) 1
- Blood glucose: Must check this first to distinguish DI from diabetes mellitus, which causes polyuria through glucose-induced osmotic diuresis rather than ADH deficiency 1
A normal potassium level (like 4.4 mEq/L) does not rule out or confirm DI, as potassium is not a diagnostic criterion for this condition. 1
When Basic Tests Are Equivocal
If initial testing shows borderline results (urine osmolality 200-300 mOsm/kg), the gold standard confirmatory test is 1, 3, 4:
- Water deprivation test followed by desmopressin administration: This remains the definitive diagnostic test when initial measurements are inconclusive 1
- Plasma copeptin measurement: A newer alternative that can distinguish between central DI (copeptin <21.4 pmol/L), nephrogenic DI (copeptin >21.4 pmol/L), and primary polydipsia 1, 2, 3
Critical Pitfall to Avoid
Do not perform 24-hour urine collection during acute illness, fever, urinary tract infections, or uncontrolled hyperglycemia, as these conditions transiently increase urine output and will give false results. 1 Wait until these conditions resolve before testing.
What Normal Results Look Like
If you do NOT have DI, you should see 1, 2:
- Urine osmolality ≥200 mOsm/kg (typically 300-900 mOsm/kg in healthy individuals)
- Serum sodium 135-145 mEq/L
- Serum osmolality 275-295 mOsm/kg
- 24-hour urine volume <2.5-3 liters
The combination of inappropriately dilute urine (osmolality <200 mOsm/kg) with high-normal or elevated serum sodium is pathognomonic for DI—if you don't have this combination, you don't have DI. 1, 2