Do I have undiagnosed Diabetes Insipidus (DI) despite normal copeptin levels and a reported 12-hour water fast?

Your Labs Do Not Indicate Diabetes Insipidus

Based on your laboratory results, you do not have diabetes insipidus. Your copeptin level of 4.6 pmol/L, combined with your appropriately concentrated urine (osmolality 498 mOsm/kg) and normal serum sodium (143 mmol/L), definitively rules out both central and nephrogenic diabetes insipidus 1, 2.

Why Your Labs Are Normal

Copeptin Interpretation

• Your copeptin of 4.6 pmol/L is well below the diagnostic threshold for nephrogenic diabetes insipidus, which requires baseline levels above 21.4 pmol/L 1, 2
• In nephrogenic DI, copeptin levels are typically markedly elevated (often >21-65 pmol/L) because the kidneys cannot respond to vasopressin, causing compensatory increases in its secretion 3, 4
• Your level indicates normal vasopressin regulation 5, 6

Urine Concentration Ability

• Your urine osmolality of 498 mOsm/kg demonstrates excellent kidney concentrating ability 1
• This is incompatible with any form of diabetes insipidus, where urine remains inappropriately dilute (typically <300 mOsm/kg) despite elevated serum osmolality 7, 8
• The fact that your kidneys concentrated urine to nearly 500 mOsm/kg after a 12-hour fast proves normal vasopressin secretion AND normal kidney response 6

Serum Osmolality and Sodium

• Your serum osmolality of 301 mOsm/L and sodium of 143 mmol/L are both normal 1
• Diabetes insipidus typically presents with hypernatremia (>145 mmol/L) and elevated serum osmolality due to water loss 7, 3

Regarding the "Non-Fasting" Lab Note

The laboratory likely flagged your results as "non-fasting" because:

• Standard fasting protocols for metabolic panels require 8-12 hours without ANY intake, including water 1
• You performed a "water fast" (no food but continued water intake), which is different from a true fast 1
• Your glucose of 96 mg/dL is completely normal and does not suggest diabetes mellitus (diagnostic threshold is ≥126 mg/dL fasting) 1
• The lab cannot verify fasting status and must document this for proper interpretation of certain tests (like lipids or glucose tolerance testing) 1

Additional Reassuring Findings

• BUN/Creatinine ratio of 7 is normal (normal range 10-20), indicating adequate hydration 1
• eGFR of 78 mL/min is within normal range for most adults 1
• Electrolytes (potassium 4.2, chloride 103, CO2 25) are all normal, ruling out other causes of polyuria like Bartter syndrome or renal tubular acidosis 1
• Calcium 9.8 mg/dL is normal, excluding hypercalcemia as a cause of polyuria 1

Clinical Context

If you were experiencing true polyuria-polydipsia syndrome (the hallmark of diabetes insipidus), you would have:

• Urine output >3 liters per day with persistent thirst 7, 8
• Inappropriately dilute urine (<300 mOsm/kg) despite dehydration 6, 8
• Either very low copeptin (<2 pmol/L in central DI) or very high copeptin (>21.4 pmol/L in nephrogenic DI) 1, 2, 6
• Hypernatremia and elevated serum osmolality 7, 3

Your results show none of these abnormalities 1, 2.