Before your blood test
Does coffee or caffeine affect blood test results?
Short answer
Black coffee before a fasting test is a gray area: it has almost no calories, but caffeine raises cortisol and catecholamines, can nudge glucose and free fatty acids, and is a mild diuretic that concentrates the sample. Labs and most clinicians say water only for a strict fast; for cortisol, catecholamine or glucose-tolerance testing, skip caffeine entirely that morning. Anything with milk, sugar or creamer breaks a fast outright.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Cortisol (morning) | raises | Peaks 30–60 minutes after intake; effect fades within a few hours | No caffeine the morning of a cortisol draw [source] |
| Glucose | can raise | Hours; larger effect in people with insulin resistance | Water only before fasting glucose or an A1c-plus-glucose panel [source] |
| Triglycerides / free fatty acids | can raise | A few hours | Skip coffee before a lipid panel if a strict fast was requested [source] |
| Sodium, hematocrit, creatinine (mild concentration) | can raise | Diuretic effect over the morning | Drink water; caffeine is not a substitute for hydration [source] |
| Catecholamines / metanephrines | raises | Same day | Avoid caffeine for 24 hours before these tests as most labs instruct [source] |
Why is black coffee still discouraged before a fasting test?
Coffee itself carries only a few calories, so it does not break a fast the way food does. The issue is caffeine: it triggers a stress-hormone response, mobilizes fat stores and increases urine output, each of which can move a result by a small amount. For most routine panels that shift is inside normal day-to-day variation; for tests that hinge on a threshold — fasting glucose at 100 or 126 mg/dL, a triglyceride cutoff — a small nudge can change the category.
Milk, cream, sugar and flavored syrups add calories and fat and do break a fast. If you cannot skip coffee, black and small is the least disruptive option, and tell the lab or clinician you had it.
When should you avoid caffeine completely?
Cortisol, ACTH stimulation tests, plasma or urine metanephrines and catecholamines, and glucose tolerance tests are the ones where caffeine measurably distorts the result. Most labs' instructions for those tests say no caffeine for 12–24 hours; follow the specific instruction you were given.
Energy drinks and pre-workout powders add sugar, B vitamins and sometimes biotin or creatine on top of caffeine — skip those the morning of any draw.
Common questions
Can I drink black coffee before a fasting blood test?
Safest answer: no — water only. If you do, keep it black and mention it; for routine lipids and metabolic panels the effect is small, for glucose, cortisol and catecholamines it is not.
Does tea count?
Plain tea has less caffeine but the same issue in kind. Herbal, caffeine-free tea without sweetener is closer to water; still, water is the standard.
How long before a cortisol test should I stop caffeine?
From the evening before. Cortisol is usually drawn between 7 and 9 a.m.; a coffee at 6 can lift the value into a range that prompts unnecessary follow-up.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.