Lab Notation Error and Diabetes Insipidus Diagnosis
A lab notation error stating "no fasting" when you actually water fasted does not affect your diabetes insipidus evaluation, because fasting status is irrelevant to diabetes insipidus testing—only plasma and urine osmolality matter for this diagnosis. 1
Why Fasting Status is Irrelevant to Diabetes Insipidus
The confusion here stems from terminology: "diabetes insipidus" and "diabetes mellitus" are completely different conditions despite sharing the word "diabetes." 2, 3
- Diabetes mellitus (the common form of diabetes) requires fasting for certain diagnostic tests like fasting plasma glucose, where "fasting is defined as no caloric intake for at least 8 hours" 4
- Diabetes insipidus is a water balance disorder characterized by the inability to concentrate urine, diagnosed through urine osmolality, plasma osmolality, and copeptin/vasopressin levels—none of which require fasting 1, 2
Understanding Your Diabetes Insipidus Evaluation
If your urine osmolality is normal (above 750 mOsmol/kg), you do not have diabetes insipidus. 2
The diagnostic criteria for diabetes insipidus include:
- Severe DI: Urine osmolality remains below 250 mOsmol/kg with serum sodium greater than 145 mmol/L 2
- Partial DI: Urine osmolality between 250-750 mOsmol/kg requires water deprivation testing 2
- Normal: Urine osmolality above 750 mOsmol/kg effectively rules out diabetes insipidus 2
The Water Deprivation Test Context
If you underwent a water deprivation test (which is the standard functional test for diabetes insipidus), the "water fasting" you performed was actually the intended protocol—not a mistake. 1, 2
- The water deprivation test requires withholding fluids to assess the kidney's ability to concentrate urine in response to dehydration 1, 3
- This is fundamentally different from the "fasting" required for diabetes mellitus testing, which only restricts caloric intake but allows water 4
Common Pitfall to Avoid
Do not confuse laboratory requirements for diabetes mellitus testing with diabetes insipidus testing. 1, 2 The lab notation about "no fasting" was likely a clerical error or referred to a different test entirely (such as glucose or lipid testing), but has zero bearing on your diabetes insipidus evaluation. 5
Pre-analytical errors like incorrect documentation account for 46-68% of total laboratory errors, but when the actual specimen collection and testing were performed correctly, these notation errors don't invalidate the results. 5
Bottom Line on Your Results
Your normal urine osmolality rules out diabetes insipidus regardless of any fasting notation error. 1, 2 The lab notation about fasting status applies only to tests for diabetes mellitus (blood sugar disorders), not diabetes insipidus (water balance disorders). If there were any concern about your results, the appropriate next step would be measuring baseline plasma copeptin levels (diagnostic if >21.4 pmol/L for nephrogenic DI) or proceeding to formal water deprivation testing with desmopressin administration. 1