Which laboratory tests require fasting?

Laboratory Tests Requiring Fasting

For diabetes screening and diagnosis, you need an 8-hour fast for fasting plasma glucose (FPG) and oral glucose tolerance tests (OGTT), but HbA1c requires no fasting at all. 1, 2

Tests That REQUIRE Fasting (8-Hour Minimum)

Glucose Metabolism Tests

• Fasting Plasma Glucose (FPG): Requires at least 8 hours with no caloric intake; used for diabetes diagnosis with threshold ≥126 mg/dL 1, 2
• Oral Glucose Tolerance Test (OGTT): Requires 8-14 hours fasting before the 75g glucose load, with 2-hour post-load measurement 1, 2
• Insulin, C-peptide, and proinsulin: Require fasting for proper interpretation, particularly when evaluating for insulinoma 3
• Glucose/insulin ratio: Requires fasting morning levels to assess insulin sensitivity 3

Gastrointestinal Hormone Tests

• Gastrin: Requires 8-hour fast for suspected gastrinoma workup, though PPIs should ideally be stopped 10-14 days before testing (with medical supervision) 3
• VIP (vasoactive intestinal peptide): Requires 8-hour fast when evaluating for VIPoma 3

Lipid Panel Considerations

• Traditional teaching required fasting, but current evidence shows fasting is NOT routinely required for standard lipid profiles 4
• Maximal mean changes after meals are clinically insignificant: triglycerides increase only +0.3 mmol/L (26 mg/dL), total cholesterol decreases -0.2 mmol/L (8 mg/dL), LDL decreases -0.2 mmol/L (8 mg/dL) 4
• Fasting is only recommended when non-fasting triglycerides exceed 5 mmol/L (440 mg/dL) 4
• HDL cholesterol, apolipoprotein A1, apolipoprotein B, and lipoprotein(a) are unaffected by fasting status 4

Tests That Do NOT Require Fasting

Diabetes Testing

• HbA1c: No fasting required; diagnostic threshold ≥6.5% for diabetes 1, 2
• Random plasma glucose: No fasting required, but needs classic symptoms (polyuria, polydipsia, weight loss) or hyperglycemic crisis with glucose ≥200 mg/dL 1, 2

Thyroid Function

• TSH and free T4: No fasting required, though samples should be collected at the same time of day for serial monitoring to minimize diurnal variation 3

Other Hormone Tests

• Prolactin, LH, FSH: No fasting required, though timing during menstrual cycle matters for women (day 3-6 recommended) 3
• ACTH and cortisol: Morning collection preferred (around 8 AM) for circadian rhythm, but fasting not required 3
• Growth hormone/IGF-1: No fasting requirement 3
• Chromogranin A, somatostatin, pancreatic polypeptide, calcitonin, PTH-related peptide: No fasting required 3
• Metanephrines, catecholamines: No fasting required for pheochromocytoma workup 3

Critical Timing Considerations (Not Fasting-Related)

Optimal Blood Draw Timing

• Morning collection before 10 AM is recommended for biomarker studies to address diurnal fluctuations, though this relates to circadian rhythm rather than fasting status 1
• Standardizing to morning fasting draws harmonizes sample collection across studies and addresses both timing and fasting variables 1

Special Dietary Restrictions (Not Fasting)

• 24-hour urine 5-HIAA testing: Requires 48-hour avoidance of specific foods (avocados, bananas, cantaloupe, eggplant, pineapples, plums, tomatoes, hickory nuts, plantains, kiwi, dates, grapefruit, honeydew, walnuts) and avoidance of coffee, alcohol, and smoking 3

Common Pitfalls to Avoid

• Do not confuse timing requirements with fasting requirements: LH/FSH need specific menstrual cycle timing (days 3-6) but no fasting 3
• Do not use point-of-care glucose meters for diagnosis: Only venous plasma from accredited laboratories is acceptable 2
• Do not interpret thyroid tests during acute illness: Euthyroid sick syndrome causes misleading results that should be repeated after metabolic stability 3
• Do not stop PPIs in suspected gastrinoma without supervision: Risk of severe complications outweighs testing accuracy 3
• Confirm all positive diabetes tests: Repeat testing on a separate day unless patient has unequivocal hyperglycemia (≥200 mg/dL with symptoms) 1, 2

Impact of Non-Compliance with Fasting

When fasting is required but not followed, expect these changes:

• Triglycerides increase by approximately 58% at 4 hours post-meal 5
• Red blood cell count, hemoglobin, hematocrit, and platelet count decrease 2 hours after meal consumption 6
• Lymphocyte count decreases and neutrophil count increases 1-2 hours after food intake 6
• Fat oxidation rapidly increases during fasting, peaking around 51 hours, while carbohydrate oxidation stabilizes at low levels after the second day 7