Based on my water‑fasting test results (copeptin 4.6 pg/mL, serum sodium 143 mmol/L, urine osmolality 498 mOsm/kg, serum osmolality 301 mOsm/kg) with normal blood pressure, potassium, calcium, renal function, and no nocturia, do I have diabetes insipidus?

You Do Not Have Diabetes Insipidus

Your water-fasting test results definitively exclude diabetes insipidus. Your urine osmolality of 498 mOsm/kg demonstrates excellent kidney concentrating ability—well above the 300 mOsm/kg threshold that rules out this diagnosis, and your copeptin level of 4.6 pg/mL is normal and far below the 21.4 pmol/L cutoff for nephrogenic diabetes insipidus. 1, 2

Why These Results Rule Out Diabetes Insipidus

Urine Concentrating Ability

• The American College of Physicians states that urine osmolality >300 mOsm/kg rules out diabetes insipidus, and your value of 498 mOsm/kg indicates completely normal kidney concentrating function. 1
• Diabetes insipidus is characterized by the pathognomonic triad of polyuria (>3 L/24 hours), inappropriately dilute urine (osmolality <200 mOsm/kg), and high-normal or elevated serum sodium—none of which you meet. 1, 3

Copeptin Level

• Your copeptin of 4.6 pg/mL falls within the normal reference range and is less than one-quarter of the 21.4 pmol/L threshold that would indicate nephrogenic diabetes insipidus. 1, 2
• A baseline copeptin >21.4 pmol/L without prior fluid deprivation identifies nephrogenic diabetes insipidus with 100% sensitivity and specificity, making your normal level highly reassuring. 2, 4

Serum Sodium and Osmolality

• Your serum sodium of 143 mmol/L is normal (not elevated), and your serum osmolality of 301 mOsm/kg is only mildly elevated—when combined with appropriately concentrated urine, this pattern is completely inconsistent with diabetes insipidus. 1
• True diabetes insipidus requires serum sodium >145 mmol/L (when water access is restricted) alongside urine that remains inappropriately dilute despite dehydration. 1, 3

Critical Diagnostic Pitfall to Avoid

Do not proceed with a formal water deprivation test. The American College of Physicians explicitly advises against performing this test when baseline urine osmolality already exceeds 300 mOsm/kg with normal serum osmolality, as it would be uncomfortable and potentially dangerous without adding any diagnostic value. 1

What Your Results Actually Show

Normal Kidney Function and ADH System

• Your ability to concentrate urine to 498 mOsm/kg after fasting indicates normal antidiuretic hormone (ADH) secretion and normal kidney response to ADH. 1
• All other parameters—blood pressure, potassium (normal at 4.4 mEq/L), calcium, and kidney function—are within normal limits, further excluding secondary causes of concentrating defects. 1

Absence of Nocturia

• Not waking at night to urinate is a strong clinical indicator against diabetes insipidus, as nocturnal polyuria with night waking is characteristic of organic polyuria from DI. 3

Alternative Explanations to Consider

If you are experiencing increased urinary frequency or thirst, the Endocrine Society and American Diabetes Association recommend investigating these alternative causes now that diabetes insipidus is excluded: 1

Primary Polydipsia (Excessive Fluid Intake)

• Habitual excessive water drinking—often driven by health beliefs or anxiety about dehydration—is a common cause of increased urination without true disease. 1
• Measure your total 24-hour fluid intake (including all beverages and high-water-content foods); intake exceeding 3 L/day suggests primary polydipsia. 1
• A fluid-restriction trial (approximately 1 L/24 hours) with proportional reduction in urine output would confirm this diagnosis. 1

Medication Review

• Diuretics (thiazides, loop diuretics) are the most frequent medication-related cause of increased urine volume and should be reviewed with your prescribing clinician. 1

Quantify Actual Urine Output

• Measure 24-hour urine volume to objectively determine if you have true polyuria (>3 L/24 hours in adults) versus simply increased voiding frequency with normal total output. 1, 3
• Frequent small-volume voids without nocturia may indicate overactive bladder rather than systemic polyuria. 5

Early Chronic Kidney Disease

• Your eGFR of 78 mL/min/1.73m² (CKD stage G2) can subtly impair concentrating ability without producing diabetes insipidus, though your urine osmolality of 498 mOsm/kg argues against this being clinically significant. 1
• Assess albumin-to-creatinine ratio to screen for early kidney injury and repeat creatinine in 3 months to verify stability. 1

Bottom Line

Your test results provide strong, objective evidence that you do not have diabetes insipidus. Your kidneys concentrate urine normally, your ADH system functions properly (evidenced by normal copeptin), and you lack the clinical hallmarks of this condition. No further testing for diabetes insipidus is warranted or recommended. 1, 2