Fasting for Copeptin and Laboratory Testing
For routine laboratory tests including copeptin measurement, you should fast from food for 6 hours and may drink water freely up to the time of blood draw—there is no need to restrict water intake. 1
Water Intake During Fasting
- Water should be consumed freely without restriction during the fasting period for laboratory tests, as it does not interfere with test results and actually facilitates blood draw 1
- The American Society of Anesthesiologists explicitly defines water as a clear liquid that can be consumed up to 2 hours before procedures, and for non-procedural lab work, water restriction is unnecessary 2
- Adequate hydration is recommended to maintain normal physiology and prevent complications such as dehydration-induced hypercoagulability 3
Food Fasting Requirements
- Fast from light meals (such as toast) for at least 6 hours before blood draw 2, 1
- Fried or fatty foods require 8 hours or more of fasting due to prolonged gastric emptying 2, 1
- Blood should ideally be drawn in the morning (before 10:00 AM) to account for diurnal fluctuations in biomarker levels 1
Copeptin-Specific Considerations
- For diagnostic copeptin testing in diabetes insipidus evaluation, the hypertonic saline infusion test (which measures stimulated copeptin) has replaced the traditional water deprivation test as the gold standard 4, 5
- Baseline copeptin levels can be measured without prior water restriction for most clinical purposes 5
- The water deprivation test, when performed for copeptin measurement in specialized circumstances, is conducted under medical supervision in an inpatient setting—not at home 4
Safety Considerations
- Fasting longer than 8-12 hours is unnecessary and potentially harmful, as it may cause hypoglycemia and metabolic stress 1, 6
- Extended water restriction (beyond what's needed for lab work) can lead to dangerous dehydration and complications including venous thromboembolism 3
- Patients with diabetes, liver disease, or delayed gastric emptying require individualized instructions and should consult their ordering physician 1, 6
Practical Instructions for Home Preparation
- Stop eating solid food 6 hours before your scheduled lab appointment 1
- Continue drinking plain water freely throughout the fasting period 1
- Black coffee (without milk, cream, or sugar) is acceptable as a clear liquid 1
- Avoid alcohol during the fasting period as it may interfere with results 1
- Take your scheduled lab appointment in the morning when possible 1
The key distinction is that "fasting" for lab work means food restriction only—water intake should continue normally and is actually encouraged for optimal sample collection.