Is this diabetes insipidus given the patient's laboratory results, including normal glucose, hypernatremia (elevated serum sodium), low urine osmolality, and polyuria (high 24-hour urine volume)?

Diagnostic Assessment: This is NOT Diabetes Insipidus

Based on the laboratory values provided, this patient does NOT have diabetes insipidus. The urine osmolality of 220 mOsm/kg is above the diagnostic threshold for DI, the serum sodium is normal at 143 mmol/L, and the 24-hour urine volume of 1300 mL is within normal limits 1, 2.

Critical Diagnostic Criteria for Diabetes Insipidus

Diabetes insipidus requires ALL of the following features to be present simultaneously:

• Urine osmolality definitively <200 mOsm/kg H₂O in the setting of serum hyperosmolality 1, 3
• High-normal or elevated serum sodium (typically >145 mmol/L) combined with inappropriately dilute urine 1, 2
• Polyuria >2.5-3 liters per 24 hours in adults 1, 3

Why This Patient Does NOT Meet Criteria

Urine Osmolality is Too High

• Your patient's urine osmolality of 220 mOsm/kg falls in the indeterminate range and does not meet the diagnostic threshold 3
• True diabetes insipidus produces maximally dilute urine with osmolality <200 mOsm/kg H₂O continuously, regardless of fluid consumption 1
• Many conditions cause urine osmolality in the 200-300 mOsm/kg range without representing true DI, including partial dehydration, chronic kidney disease, or early stages of various renal disorders 3

Serum Sodium is Normal

• The serum sodium of 143 mmol/L is completely normal and does not indicate the hypernatremia expected in untreated DI 1, 2
• Diabetes insipidus characteristically presents with high-normal or elevated serum sodium as the hallmark finding 1, 2

Urine Volume is Normal

• The 24-hour urine volume of 1300 mL is well within normal limits (normal range approximately 800-2000 mL/day) 1, 3
• True DI requires polyuria >2.5-3 liters per 24 hours in adults to meet diagnostic criteria 1, 3

Serum Osmolality is Normal

• The serum osmolality of 295 mOsm/kg is normal (normal range 275-295 mOsm/kg) 2
• DI typically presents with serum hyperosmolality driving the compensatory polydipsia 1, 2

Additional Supporting Evidence Against DI

• BUN of 9 mg/dL is low-normal, suggesting adequate hydration rather than the volume depletion that would occur in untreated DI 2
• Normal glucose appropriately rules out diabetes mellitus as the cause of any urinary symptoms 3
• The eGFR of 77 mL/min/1.73m² indicates mild chronic kidney disease (CKD stage 2), which could explain the urine osmolality in the 200-300 range 3

Clinical Pitfall to Avoid

Do not confuse borderline or indeterminate laboratory values with a definitive diagnosis of diabetes insipidus. The diagnosis requires the complete triad of markedly dilute urine (<200 mOsm/kg), hypernatremia or high-normal sodium, and significant polyuria (>2.5-3 L/day) occurring simultaneously 1, 3, 2. This patient meets none of these criteria definitively.

If DI Were Suspected: Proper Diagnostic Approach

Should clinical suspicion arise in the future with different laboratory findings, the proper workup would include:

• Water deprivation test followed by desmopressin administration remains the gold standard for diagnosis 3, 4, 5
• Plasma copeptin measurement can distinguish between central DI (copeptin <21.4 pmol/L) and nephrogenic DI (copeptin >21.4 pmol/L) 3, 5
• Ensure proper 24-hour urine collection technique: empty and discard the first void, then collect all urine for exactly 24 hours, maintaining usual fluid intake based on thirst 3