Lab Interpretation After Informal Water Fasting
Your lab results definitively exclude diabetes insipidus (DI), and no further testing for this condition is warranted. 1
Why Diabetes Insipidus is Ruled Out
Your laboratory values demonstrate normal kidney concentrating ability and exclude both central and nephrogenic DI:
Urine osmolality of 498 mOsm/kg is well above the 300 mOsm/kg threshold that rules out DI and indicates completely normal kidney concentrating function. 1
Serum osmolality of 301 mOsm/kg is only minimally elevated and, when paired with appropriately concentrated urine, is incompatible with any form of DI. 1
Copeptin level of 4.6 pmol/L is normal (reference range typically 1-14 pmol/L) and far below the 21.4 pmol/L threshold that would suggest nephrogenic DI in adults. 2, 3, 1
Serum sodium of 143 mEq/L is normal, whereas DI characteristically presents with normal-high or elevated sodium as part of its pathognomonic triad (polyuria, dilute urine, elevated sodium). 1
Understanding Your Results in Context
The informal water fasting likely explains some of your laboratory findings:
Your low-normal BUN/creatinine ratio of 7 can occur with high fluid intake during fasting, which is a benign finding given your other normal results. 1
Water-only fasting for 8+ hours causes predictable metabolic changes including dehydration, altered electrolyte handling, and changes in urine concentration, but these normalize with refeeding. 4
Your glucose of 96 mg/dL is normal and excludes diabetes mellitus, which can cause polyuria through osmotic diuresis from glucosuria—a completely different mechanism than DI. 1
Critical Clinical Pitfall to Avoid
Do not proceed with a water deprivation test. 1 When baseline testing shows urine osmolality >300 mOsm/kg with normal serum osmolality, formal water deprivation testing would be uncomfortable, potentially dangerous, and adds no diagnostic value. 1
What These Results Actually Mean
Your kidneys are functioning normally:
Normal creatinine (0.86 mg/dL) and eGFR (78 mL/min/1.73m²) confirm adequate kidney function. 1
The ability to concentrate urine to 498 mOsm/kg demonstrates intact vasopressin secretion (ruling out central DI) and intact renal response to vasopressin (ruling out nephrogenic DI). 1, 5
If Symptoms Persist
If you continue to experience polyuria or polydipsia symptoms despite these normal results, investigate alternative causes: 1
- Primary polydipsia (excessive water intake with normal vasopressin function)
- Medications that affect fluid balance
- Metabolic disturbances such as hypercalcemia or hypokalemia
- Early chronic kidney disease (though your current kidney function is normal)
Measure 24-hour urine volume to objectively quantify whether true polyuria (>3 liters/24 hours in adults) is actually present. 1