What is the diagnosis and management for a patient with normal kidney function, mild hyperglycemia, and normal uric acid and copeptin levels after a 12-hour water fast?

Laboratory Interpretation After 12-Hour Water Fast

Primary Assessment

These laboratory values indicate normal renal concentrating ability and do not suggest diabetes insipidus or SIADH. The copeptin level of 4.6 pmol/L (within normal range 0.0-5.9), combined with appropriately concentrated urine (osmolality 498 mOsm/kg) and normal serum sodium (143 mmol/L), demonstrates intact posterior pituitary function and appropriate renal response to water restriction.

Key Laboratory Findings Analysis

Renal Function and Concentrating Ability

• eGFR of 78 mL/min/1.73m² indicates mild reduction in kidney function (CKD stage 2), though serum creatinine of 0.86 mg/dL appears normal 1, 2
• The urine osmolality of 498 mOsm/kg after 12-hour water fast demonstrates preserved renal concentrating ability, which is appropriate and expected with normal posterior pituitary function
• The serum osmolality of 301 mOsm/kg is within normal range (280-295 mOsm/kg), indicating normal fluid balance

Copeptin Interpretation

• Copeptin of 4.6 pmol/L falls within the normal reference range and effectively rules out primary polydipsia (which would show copeptin <3.9 pmol/L) 3
• This copeptin level is not elevated enough to suggest hypovolemic states (which typically show copeptin >84 pmol/L) 3
• The normal copeptin with appropriate urine concentration excludes diabetes insipidus (central or nephrogenic)

Metabolic Parameters

• Glucose of 96 mg/dL is normal, not hyperglycemic - this is euglycemia, not requiring intervention 4
• Uric acid of 5.4 mg/dL is normal and does not suggest glycogen storage disease or other metabolic disorders 4
• Calcium of 9.8 mg/dL is normal
• BUN of 6 mg/dL is at the lower end of normal, with BUN/creatinine ratio of 7 (normal 10-20), which may reflect adequate hydration or lower protein intake

Electrolytes

• Serum sodium of 143 mmol/L is normal (135-145 mmol/L) and requires no intervention 4
• Potassium 4.2 mmol/L, chloride 103 mmol/L, and CO2 25 mmol/L are all within normal limits

Clinical Interpretation

No pathology is evident from these laboratory results. The water deprivation test demonstrates:

1. Normal posterior pituitary function - evidenced by copeptin within normal range
2. Normal renal concentrating ability - evidenced by urine osmolality >300 mOsm/kg after water restriction
3. Normal fluid balance - evidenced by normal serum sodium and osmolality
4. Mild reduction in GFR - though this may be age-related or represent early CKD stage 2, requiring monitoring but not acute intervention

Management Recommendations

No specific treatment is indicated based on these results. However:

• Monitor renal function periodically given eGFR of 78 mL/min/1.73m², as serum creatinine alone may underestimate renal impairment 1, 2
• Ensure adequate hydration in daily life (no water restriction needed, as sodium is normal) 4
• No intervention needed for glucose, electrolytes, or other parameters as all are within normal limits

Common Pitfalls to Avoid

• Do not rely solely on serum creatinine to assess renal function - eGFR provides more accurate assessment, particularly in patients with normal-appearing creatinine who may have reduced GFR 1, 2
• Do not misinterpret normal copeptin as abnormal - values within the reference range indicate normal vasopressin regulation 3
• Do not confuse a water deprivation test with clinical dehydration - this was a diagnostic test showing appropriate physiologic response