Coffee Before Fasting Laboratory Tests
Black coffee is acceptable before fasting laboratory tests, as it does not clinically significantly alter lipid panels or glucose measurements, though coffee with milk or cream should be avoided as these additives are not considered clear liquids.
Clear Liquid Definition and Coffee
The American Society of Anesthesiologists explicitly defines black coffee as a clear liquid that may be consumed up to 2 hours before procedures requiring fasting 1, 2. This classification is critical because it distinguishes black coffee from beverages containing milk or cream, which require extended fasting periods of 6 or more hours 1.
Impact on Lipid Testing
Research demonstrates that black coffee consumption has minimal clinical impact on fasting lipid measurements:
A single cup of black coffee produces statistically but not clinically significant changes in total cholesterol (increased by 3.1 mg/dL) and HDL cholesterol (increased by 1.6 mg/dL), with no significant effect on LDL cholesterol 3.
Black coffee does not affect fasting or postprandial triglycerides when consumed 30 minutes before blood draw in healthy individuals 4.
However, coffee with milk (café latte) significantly increases fasting blood glucose by approximately 12% (from 87.2 to 97.3 mg/dL), though it does not significantly alter LDL cholesterol or non-HDL cholesterol 5.
Impact on Glucose Testing
For glucose measurements, the evidence shows:
Black coffee does not modify glycemic or insulinemic responses when consumed with carbohydrates 6.
Black coffee consumed 30 minutes before testing does not affect baseline fasting glucose or postprandial glucose measurements in abbreviated fat tolerance tests 4.
Coffee with milk or sugar causes clinically significant glucose elevation and should be avoided before glucose testing 5.
Practical Clinical Recommendations
For routine fasting lipid panels:
- Black coffee without any additives is acceptable and will not meaningfully alter results 3, 4.
- Patients should avoid coffee with milk, cream, sugar, or non-dairy creamer, as these are not clear liquids 1.
For fasting glucose testing:
- Black coffee is acceptable and does not affect results 4, 6.
- Any coffee with additives (milk, cream, sugar) must be avoided as it can increase glucose by >10% 5.
For metabolic panels or comprehensive testing:
- Instruct patients that only black coffee is permitted, emphasizing "no milk, cream, sugar, or any additives" 1.
Important Caveats
Do not confuse general fasting guidelines with specific medication instructions. For example, SGLT2 inhibitors like empagliflozin require discontinuation 3 days before surgery, which is unrelated to food/beverage fasting 7. Similarly, patients undergoing procedures requiring contrast should maintain adequate hydration regardless of coffee consumption 1.
Caffeinated beverages are generally discouraged before certain specialized tests (such as PET/CT imaging) because even "sugar-free" varieties may contain trace carbohydrates 1. However, for routine laboratory fasting tests, black coffee specifically is acceptable 1.