Water During Fasting Blood Sugar (FBS) Preparation
Yes, water is allowed during fasting blood sugar preparation—patients should drink water to thirst to avoid dehydration while maintaining the required fast from caloric intake.
Clear Fluid Guidelines for Fasting
The distinction between fasting from calories versus fasting from all intake is critical for FBS preparation:
Water is classified as a clear fluid and does not contain calories, sugar, or nutrients that would interfere with fasting blood glucose measurements 1.
Multiple anesthesia guidelines explicitly permit clear fluids (including water) up to 2 hours before procedures requiring fasting, demonstrating that water intake does not compromise the fasting state 1.
For pediatric patients undergoing FDG-PET/CT scans requiring fasting, plain water is specifically allowed while sugar-containing liquids are prohibited, establishing the principle that water does not break a metabolic fast 1.
Physiological Rationale
Allowing water during FBS preparation provides important benefits:
Prolonged fasting without water leads to dehydration, which can cause volume contraction and potentially affect test accuracy 1.
Patients should be advised to drink water to thirst in a pattern consistent with their usual fluid consumption to maintain euvolemia during testing 1.
Dehydration from complete fluid restriction can cause discomfort and is medically unnecessary when only metabolic fasting is required 1.
Practical Implementation
For FBS testing specifically:
Instruct patients to fast from all food and caloric beverages (juice, milk, soda, coffee with cream/sugar) for 8-12 hours before the test 2.
Water intake should be permitted and encouraged throughout the fasting period to maintain hydration 1.
Patients should avoid only beverages containing sugar, calories, or other nutrients that could elevate blood glucose 1.
Common Pitfall to Avoid
The most common error is instructing patients to avoid all intake including water when preparing for FBS. This outdated "nil per os" approach causes unnecessary patient discomfort, dehydration risk, and provides no benefit for glucose measurement accuracy. Water does not contain glucose or stimulate insulin release, making its restriction during FBS preparation medically unjustified 1.