Which laboratory tests require fasting?

Laboratory Tests That Require Fasting

Most laboratory tests do not require fasting, but certain tests specifically need 8-12 hours of fasting for accurate results, including fasting plasma glucose, lipid panels when triglycerides are the focus, and oral glucose tolerance tests. 1

Tests That Require Fasting

Diabetes-Related Tests

• Fasting Plasma Glucose (FPG)

  • Requires 8 hours of fasting 2
  • Used for diabetes diagnosis when ≥126 mg/dL on two separate occasions 2
  • Fasting defined as no caloric intake for at least 8 hours

• Oral Glucose Tolerance Test (OGTT)

  • Requires 8-14 hours of fasting before the 75g glucose load 2, 1
  • Blood drawn fasting and 2 hours after glucose load
  • Diabetes diagnosed when 2-hour plasma glucose ≥200 mg/dL 2

Lipid-Related Tests

• Lipid Panels - fasting requirements depend on specific circumstances:

  • Required fasting (8-12 hours):

    • When triglycerides are the primary focus of measurement 1
    • When triglyceride levels are elevated (>400 mg/dL) 1
    • 4-8 weeks after initiating or adjusting lipid-lowering therapy 1
    • For adults with family history of premature cardiovascular disease or genetic hyperlipidemia 1

  • Fasting not required:

    • For routine cardiovascular risk screening 1, 3
    • For follow-up of patients on stable statin therapy 1
    • When measuring total cholesterol and HDL cholesterol 1

Tests That Do NOT Require Fasting

• HbA1C - no fasting required for diabetes screening or monitoring 1
• Random Blood Glucose - no fasting required 2
• Most routine blood tests - complete blood count, kidney function, liver function, etc.
• Standard lipid profiles for cardiovascular risk assessment can be done without fasting 1, 4, 3

Important Considerations During Fasting

• Water consumption: Patients should be advised to drink water during the fasting period 1
• Medication timing: Patients with diabetes should consult healthcare providers about medication timing when fasting 1
• Avoid smoking during the fasting period 1
• Avoid prolonged fasting (>14 hours) in patients with diabetes to prevent hypoglycemia 1

Impact of Non-Fasting vs. Fasting on Test Results

For lipid profiles, the maximum mean changes between non-fasting and fasting states are:

• Triglycerides: +26 mg/dL
• Total cholesterol: -8 mg/dL
• LDL cholesterol: -8 mg/dL
• HDL cholesterol: -4 mg/dL 1, 3

These differences are generally not clinically significant for most patients and don't affect cardiovascular risk classification 3.

Best Practices for Ordering Fasting Tests

• Clearly communicate to patients which tests require fasting and which do not
• Provide written instructions about fasting requirements when scheduling tests
• Consider scheduling fasting tests in the morning to minimize patient discomfort
• For patients with diabetes, consider the risk of hypoglycemia when requiring fasting

When possible, use non-fasting tests to improve patient compliance and safety, particularly for those with diabetes who risk hypoglycemia when fasting 1.