Fasting for Blood Tests: Water is Permitted
Patients can and should drink water while fasting for blood tests—complete NPO (nothing by mouth) is unnecessary and potentially harmful. The traditional "NPO after midnight" practice lacks evidence-based support for routine blood work and should be abandoned in favor of allowing clear fluid intake.
Clear Fluids Are Safe and Recommended
Water intake does not interfere with most blood tests and should be encouraged up to the time of blood draw 1.
For hormone-related blood studies (such as gastrin, VIP, and other neuroendocrine markers), an 8-hour fast from food is recommended, but this does not require restriction of water 1.
Prolonged fasting from fluids causes dehydration without improving test accuracy—studies demonstrate that a 12-hour fast from both food and water does not affect plasma or serum volume, yet creates unnecessary patient discomfort 2.
Specific Fasting Requirements by Test Type
Standard Blood Work
- No fluid restriction needed for routine laboratory tests including electrolytes, glucose, hematology (CBC), coagulation studies (INR, aPTT), creatinine, eGFR, and troponin 1.
- Water can be consumed freely without affecting these results 2.
Hormone and Metabolic Studies
- 8-hour food fast is generally recommended for hormone-related blood studies, but clear fluids (water) remain acceptable 1.
- Specific dietary restrictions apply to certain tests (e.g., 5-HIAA requires avoiding specific foods for 48 hours), but water restriction is not indicated 1.
Why Water Should Be Encouraged
Dehydration impairs venipuncture success and patient cooperation—maintaining hydration improves the technical ease of blood collection 3.
Patient satisfaction and well-being improve when clear fluids are permitted, without any increase in complications 1.
Urinary and blood biomarkers can detect hydration changes, but plasma and serum volumes remain stable despite 12-hour fluid restriction, confirming that water intake does not meaningfully alter blood test results 2.
Common Pitfalls to Avoid
Do not apply surgical NPO guidelines to blood tests—the 2-hour clear fluid restriction before anesthesia 1 does not apply to simple blood draws 3.
Avoid prolonged unnecessary fasting—traditional "NPO after midnight" orders are inappropriate for blood work and create barriers to patient care 4, 5, 3.
Distinguish between clear fluids and other liquids—water, pulp-free juice, and clear tea are acceptable; milk, coffee with cream, and other non-clear liquids should be avoided during the fasting period 1.
Practical Recommendations
For morning blood draws: Patients should fast from food for 8-12 hours (depending on the specific test) but may drink water freely up to the time of the blood draw 1.
For afternoon blood draws: The same principle applies—water is unrestricted, while food fasting duration is calculated from the last meal 1.
Special considerations: Patients with diabetes, renal failure, or specific medical conditions may require individualized hydration strategies, but water restriction is rarely indicated 1.
The evidence overwhelmingly supports permitting water intake during fasting for blood tests, as it improves patient comfort and procedural success without compromising test accuracy 1, 3, 2.