Recommended Fasting Duration Before Blood Work
For most routine blood work, you should fast for at least 8 hours before your blood draw, with the blood collection ideally performed in the morning before 10:00 AM. 1
Standard Fasting Requirements
Core Recommendation
- 8-hour fasting period is the standard requirement for most diagnostic blood tests including fasting glucose, lipid panels, and metabolic panels 1
- Blood should be drawn in the morning (before 10:00 AM) to account for diurnal fluctuations in biomarker levels 1
- The total time from blood draw to sample freezing should be ≤2 hours for optimal sample quality 1
What You Can Consume During Fasting
- Plain water is freely permitted without restriction during the entire fasting period 2
- Black coffee (without milk, cream, or sugar) may be consumed up to 2 hours before the blood draw 2
- All other foods, beverages, and alcohol must be avoided 2
Test-Specific Fasting Requirements
Diabetes Screening and Diagnosis
- 8-hour fast required for fasting blood glucose measurement (diagnostic threshold ≥126 mg/dL on two separate occasions) 1
- Oral glucose tolerance test also requires an 8-hour fast before the 75-g glucose load 1
- A1C testing does not require fasting 1
Lipid Panels
- 8-hour fasting is standard for triglyceride measurement 1, 3
- Research demonstrates that 8-hour fasting produces equivalent triglyceride results to 12-hour fasting (no significant difference, p=0.493), while 6-hour fasting shows significantly elevated levels (p=0.003) 3
Hormone Testing
- 8-hour fast recommended for most hormone panels including gastrin, VIP, and neuroendocrine tumor markers 1
- Specific dietary restrictions apply to certain tests (e.g., 48-hour avoidance of specific foods before urine 5-HIAA testing) 1
Iron Studies
- Morning collection is mandatory with 24-hour avoidance of iron-containing supplements and foods when evaluating conditions like restless legs syndrome 4
Important Caveats
Avoid Prolonged Fasting
- Do not fast longer than 8 hours unless specifically instructed, as fasting beyond this duration may cause hypoglycemia, particularly in children and patients with diabetes 1
- Fasting >12 hours can induce metabolic stress and significantly alter blood values 1
Special Populations Requiring Modified Instructions
- Patients with diabetes, gastroesophageal reflux disease, or delayed gastric emptying require individualized fasting protocols and should consult their ordering physician 2
- These patients may be at higher risk for complications from standard fasting periods 2
Complete Blood Count (CBC)
- While CBC can be performed without fasting, fasting blood draws improve consistency and repeatability of results 5
- Food consumption decreases red blood cell count, hemoglobin, hematocrit, and platelet count within 2 hours, and alters white blood cell differential counts 5
Practical Implementation
The 8-hour fasting window means:
- If your blood draw is scheduled for 8:00 AM, your last meal should be no later than midnight 1
- Continue drinking plain water throughout the fasting period 2
- Take your morning blood draw appointment before 10:00 AM when possible 1
- Avoid fried or fatty foods for the meal before fasting begins, as these require longer gastric emptying times 2