Is this patient's presentation suggestive of undiagnosed Diabetes Insipidus (DI)?

Is This Undiagnosed Diabetes Insipidus?

No, this presentation does NOT meet diagnostic criteria for diabetes insipidus. The urine osmolality of 220 mOsm/kg is inappropriately low but not low enough to confirm DI, and the serum sodium of 143 mEq/L is normal, not elevated as expected in untreated DI.

Critical Diagnostic Gaps

The key problem is that your urine osmolality of 220 mOsm/kg falls into an indeterminate zone. True diabetes insipidus requires urine osmolality definitively <200 mOsm/kg in the setting of serum hyperosmolality 1, 2. Your value of 220 mOsm/kg is inappropriately dilute given the serum osmolality of 295 mOsm/kg, but many conditions can 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 1.

Your serum sodium of 143 mEq/L argues strongly against undiagnosed DI. Patients with untreated DI who cannot adequately compensate with fluid intake develop high-normal or elevated serum sodium (typically >145 mEq/L) 1, 2. The fact that your sodium is normal suggests either:

• You don't have DI, OR
• Your thirst mechanism is intact and you're drinking enough to compensate perfectly 1

What the Numbers Actually Show

Your laboratory values reveal:

• Serum osmolality 295 mOsm/kg: High-normal, not frankly elevated 2
• Urine osmolality 220 mOsm/kg: Inappropriately low for the serum osmolality, but not diagnostic 1, 2
• 24-hour urine volume 2L: This is actually normal for adults; DI requires >3 liters per 24 hours 1
• Serum sodium 143 mEq/L: Normal range 1, 2
• Potassium 3.2 mEq/L: Mild hypokalemia, which could suggest polyuria-induced losses if DI were present, but is not diagnostic 2

The 24-hour urine volume of 2 liters is the most important finding here—this is NOT polyuria. The diagnostic threshold for polyuria in adults is >3 liters per 24 hours 1. You're producing a normal amount of urine.

What You Actually Need to Do

First, rule out diabetes mellitus completely. Your glucose of 116 mg/dL after eating cookies 2 hours prior is not diagnostic, but you need a proper fasting glucose or HbA1c to definitively exclude diabetes mellitus, which causes polyuria through osmotic diuresis from glucosuria, not from ADH deficiency 1, 3.

If you truly suspect DI based on symptoms not captured in these labs, you need:

1. Simultaneous measurements of serum sodium, serum osmolality, and urine osmolality when you're experiencing symptoms 1, 2

2. Accurate 24-hour urine collection with these critical requirements 1:

   • Empty bladder completely at start and discard this urine
   • Collect ALL urine for exactly 24 hours
   • Maintain usual fluid intake based on thirst, don't artificially restrict or increase
   • Avoid electrolyte-containing solutions like Pedialyte during collection
   • Record total volume accurately

3. Water deprivation test followed by desmopressin administration if initial testing is equivocal—this remains the gold standard for diagnosis 4, 5, 6

4. Plasma copeptin measurement as an alternative diagnostic approach, with levels >21.4 pmol/L indicating nephrogenic DI and <21.4 pmol/L suggesting central DI or primary polydipsia 1, 2, 3

Critical Pitfalls to Avoid

Do not restrict water intake while investigating this. If you do have DI, water restriction is life-threatening and leads to severe hypernatremic dehydration 1. Patients with DI must have free access to fluids at all times 1, 2.

The characteristic triad of DI that you're missing:

• Polyuria (>3 liters/24 hours in adults) - You have 2L, which is normal 1
• Inappropriately dilute urine (osmolality <200 mOsm/kg) - You have 220, which is borderline 1, 2
• Normal-high or elevated serum sodium (>145 mEq/L with restricted water access) - You have 143, which is normal 1

Alternative Explanations

Given your normal 24-hour urine volume and normal serum sodium, consider:

• Incomplete urine collection leading to falsely low volume 1
• Primary polydipsia (excessive water intake), which would show lower serum sodium and osmolality due to water overload, though your values don't strongly support this 2, 6
• Partial nephrogenic DI or other renal concentrating defects that don't meet full DI criteria 1
• Normal variation with adequate compensatory water intake 1

Bottom line: Your presentation is inconsistent with classic diabetes insipidus. If symptoms persist, pursue formal water deprivation testing or copeptin measurement under endocrinology guidance 1, 2, 4.