With a 12‑hour water fast and labs showing copeptin 4.6 pmol/L, urine osmolality 498 mOsm/kg, serum osmolality 301 mOsm/kg, uric acid 5.4 mg/dL, creatinine 85 µmol/L, eGFR 78 mL/min/1.73 m², potassium 4.2 mmol/L, and serum sodium 143 mmol/L, do I have diabetes insipidus or could these results be a false‑negative?

You Do Not Have Diabetes Insipidus

Your laboratory results definitively exclude diabetes insipidus—your urine osmolality of 498 mOsm/kg demonstrates excellent kidney concentrating ability, your copeptin level of 4.6 pmol/L indicates normal vasopressin function, and your serum sodium of 143 mmol/L is completely normal. 1, 2

Why These Results Rule Out Diabetes Insipidus

Urine Osmolality is the Key Finding

• Urine osmolality >300 mOsm/kg definitively rules out diabetes insipidus, and your value of 498 mOsm/kg is well above this threshold 2
• Diabetes insipidus is characterized by the pathognomonic triad of: (1) polyuria >3 liters/24 hours, (2) inappropriately dilute urine with osmolality <200 mOsm/kg, and (3) high-normal or elevated serum sodium 1, 3
• Your urine is appropriately concentrated at 498 mOsm/kg after a 12-hour fast, demonstrating that both your vasopressin secretion and kidney response are functioning normally 2

Copeptin Level Confirms Normal Vasopressin Function

• Your copeptin of 4.6 pmol/L is well within the normal range and is less than one-quarter of the 21.4 pmol/L threshold used to diagnose nephrogenic diabetes insipidus 1
• Baseline copeptin >21.4 pmol/L is diagnostic for nephrogenic diabetes insipidus in adults, while levels <21.4 pmol/L indicate either normal function, central diabetes insipidus, or primary polydipsia 1, 4, 5
• In the context of your appropriately concentrated urine (498 mOsm/kg), this low copeptin level confirms a normally functioning vasopressin system 1

Serum Osmolality and Sodium Are Normal

• Your serum osmolality of 301 mOsm/kg is normal (not elevated), and when combined with appropriately concentrated urine, this is completely inconsistent with diabetes insipidus 1, 2
• Your serum sodium of 143 mmol/L is normal—diabetes insipidus typically presents with serum sodium >145 mmol/L when water access is restricted 1, 3

Can These Labs Be False-Negative?

No, these results cannot be false-negative for diabetes insipidus. Here's why:

The Physiology Makes False-Negatives Impossible

• After a 12-hour water fast, patients with diabetes insipidus would be unable to concentrate their urine above 200-300 mOsm/kg due to either lack of vasopressin (central DI) or kidney resistance to vasopressin (nephrogenic DI) 3, 6
• Your ability to concentrate urine to 498 mOsm/kg proves that: (1) your brain is secreting adequate vasopressin, (2) your kidneys are responding appropriately to that vasopressin, and (3) the aquaporin-2 water channels in your collecting ducts are functioning normally 1, 2

Your Results Show the Opposite of Diabetes Insipidus

• In diabetes insipidus, the kidneys fail to concentrate urine despite dehydration, resulting in urine osmolality remaining <200 mOsm/kg even with elevated serum osmolality 1, 3
• Your kidneys did exactly what they should do during a 12-hour fast: they concentrated your urine appropriately to conserve water 2

What You Should Do Next

Rule Out Other Causes of Your Symptoms

If you're experiencing excessive thirst or urination, investigate alternative causes 2:

• Measure 24-hour urine volume to objectively quantify whether you truly have polyuria (>3 liters/24 hours in adults) 1, 3
• Check fasting glucose and HbA1c to rule out diabetes mellitus, which causes polyuria through osmotic diuresis from glucose in the urine (not from vasopressin deficiency) 7, 1
• Consider primary polydipsia (excessive habitual water drinking), which can cause similar symptoms but with appropriately dilute urine 6, 5
• Evaluate for medications that increase urination (diuretics, lithium, certain antidepressants) 1
• Check calcium and potassium levels, as hypercalcemia and hypokalemia can cause polyuria 2

Your Other Lab Values Are Reassuring

• Your creatinine of 85 µmol/L (approximately 0.96 mg/dL) and eGFR of 78 mL/min/1.73m² indicate normal kidney function 1
• Your potassium of 4.2 mmol/L and uric acid of 5.4 mg/dL are both normal 1

Critical Pitfall to Avoid

Do not proceed with a formal water deprivation test. When baseline testing shows urine osmolality >300 mOsm/kg with normal serum osmolality, a water deprivation test would be uncomfortable, potentially dangerous, and would add no diagnostic value 2. The water deprivation test is only indicated when baseline urine osmolality is in the indeterminate range (200-300 mOsm/kg) or when diabetes insipidus is still suspected despite initial testing 3, 5.