Which blood tests require fasting?

Blood Tests That Require Fasting

Most blood tests do not require fasting, with only lipid profiles and glucose tests requiring fasting in specific circumstances.

Tests That Require Fasting

Glucose Testing

• Fasting plasma glucose (FPG): Requires 8+ hours of fasting 1
• Oral glucose tolerance test (OGTT): Requires 8-14 hours of fasting before the test 1
• Hemoglobin A1c (HbA1c): Does NOT require fasting 1

Lipid Testing

• Complete lipid panel: Fasting is only required in specific circumstances 2:
  • When initial non-fasting triglycerides are ≥400 mg/dL
  • When evaluating patients with family history of premature cardiovascular disease
  • When diagnosing genetic hyperlipidemia
  • When assessing patients at risk for hypertriglyceridemia-induced pancreatitis
  • When monitoring lipid-lowering therapy (initial follow-up 4-8 weeks after starting)

Tests That Do NOT Require Fasting

• Complete blood count (CBC) with differential 1
• Chemistry panel 1
• HLA-B*5701 testing (for abacavir therapy) 1
• G6PD deficiency screening 1
• Routine lipid screening in most patients 2, 3

Impact of Fasting on Test Results

Lipid Testing

• Maximum changes after meals compared to fasting 2, 3:
  • Total cholesterol: -0.2 mmol/L (8 mg/dL)
  • LDL cholesterol: -0.2 mmol/L (8 mg/dL)
  • HDL cholesterol: -0.1 mmol/L (4 mg/dL)
  • Triglycerides: +0.3 mmol/L (26 mg/dL)

These changes are generally not clinically significant for most patients 3.

Special Considerations

Diabetes Patients

• Patients with diabetes should consult healthcare providers about medication timing when fasting 2
• Avoid fasting for prolonged periods (>14 hours) to prevent hypoglycemia 2

Lipid Testing Recommendations

• Non-fasting lipid profiles are acceptable for most clinical scenarios 2, 3
• The Friedewald formula for LDL calculation is valid when triglycerides are <400 mg/dL 2
• Direct LDL-C measurement is recommended when triglycerides are ≥400 mg/dL 2

Benefits of Non-Fasting Testing

• Improved patient compliance 4, 5
• Reduced laboratory congestion 2
• Better reflection of the body's typical metabolic state 2
• Decreased risk of hypoglycemia in patients with diabetes 2

Common Pitfalls to Avoid

1. Unnecessary fasting: Requiring fasting for tests that don't need it creates patient discomfort and reduces compliance 4, 6

2. Inadequate fasting time: When fasting is required, ensure patients understand the minimum time needed (typically 8 hours) 1

3. Misinterpreting non-fasting results: Small variations in lipid values due to fasting status rarely affect clinical decision-making 2, 3

4. Overlooking patient safety: Prolonged fasting can cause hypoglycemia in patients with diabetes 2