Can you drink coffee before a blood test?
The short answer
If your test needs fasting, skip coffee, even black coffee, and drink only plain water until after your draw. If your test doesn't need fasting, your usual cup of coffee is generally fine.
What to do
- Check whether your test needs fasting.
- For fasting tests, drink only plain water until after your draw.
- Never add milk, cream, sugar or syrups before a fasting test.
- For tests that don't need fasting, have your usual coffee.
- If you're having a cortisol test, consider waiting until after the draw.
- Book an early visit so you can have coffee soon after.
Which results it changes
| Marker | Effect | How long |
|---|---|---|
| Insulin[3] | Higher after black coffee than after water in fasted volunteers | Seen over 2 hours after a glucose drink taken 30 minutes after coffee |
| Glucose[3] | Higher at 60 minutes after dark-roast coffee; overall glucose response not different | Within 2 hours after a glucose drink |
| Insulin sensitivity[4] | Lower after caffeinated coffee taken with a carbohydrate meal | Still seen at a second carbohydrate load 3 hours later |
| Cortisol[5] | Rises after caffeine; smaller but not eliminated in daily caffeine users | Raised for hours after a dose |
Why black coffee is a gray area
Black coffee has almost no calories, so some labs allow it during a fast. But coffee is not just water. Caffeine and other compounds in coffee act on the body, and researchers list coffee among the factors that change how the body handles food.
Labs don't agree. A review of lab fasting instructions found they generally asked patients to skip coffee and tea on the morning of the draw, while some test instructions allow black coffee. Because the rules differ, JustLabs takes the conservative path: plain water only for fasting tests.
Coffee, glucose and insulin
The clearest evidence is for blood sugar control. In a small trial, fasted volunteers drank 300 mL of black coffee or water and then took a glucose drink 30 minutes later. Both a light and a dark roast raised insulin and lowered insulin sensitivity compared with water. Glucose was also higher at 60 minutes after the dark roast, although the overall glucose response didn't differ.
In another trial, caffeinated coffee taken with a carbohydrate meal lowered insulin sensitivity during a second carbohydrate load three hours later. These studies didn't measure a standard fasting blood test, but they show coffee can shift glucose and insulin. That matters most for Fasting Glucose, Fasting Insulin and C-Peptide.
Coffee and cortisol
Caffeine raises cortisol. In a controlled trial, people who regularly took in caffeine had a smaller cortisol response than people who had abstained, but the response was not eliminated, and cortisol stayed higher for hours after an afternoon dose. The Cortisol test doesn't require fasting, but if you're testing cortisol, it's reasonable to have your coffee after the draw.
Coffee and cholesterol tests
We found little direct research on how one cup of black coffee changes a cholesterol or triglyceride result the same morning. What is clear is that milk, cream, sugar and flavored syrups add fat and sugar, so they break a fast. If you're unsure, water is the safe choice: it never affects a fasting test.
If the instructions for one of your tests say black coffee is fine, that's an option for that test. If your order includes other fasting tests, such as fasting glucose or insulin, follow the strictest rule and stick to water.
Tests that don't need fasting
Many tests need no fasting, including TSH, Hemoglobin A1c, Vitamin D, Ferritin, the CBC, Apolipoprotein B and Lipoprotein(a). For these, you can drink your usual coffee. If you're getting fasting and non-fasting tests in the same visit, the fasting rules apply.
Frequently Asked Questions
More on preparing
Sources
- Grankvist K, Sigthorsson G, Kristensen GB, Pelanti J, Nybo M. Status on fasting definition for blood sampling in the Nordic countries - time for a harmonized definition. Scand J Clin Lab Invest. 2018 Nov-Dec;78(7-8):591-594. doi: 10.1080/00365513.2018.1528503. PMID: 30338721. PubMed
- Simundic AM, Cornes M, Grankvist K, Lippi G, Nybo M. Standardization of collection requirements for fasting samples: for the Working Group on Preanalytical Phase (WG-PA) of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM). Clin Chim Acta. 2014 May 15;432:33-7. doi: 10.1016/j.cca.2013.11.008. PMID: 24269503. PubMed
- Rakvaag E, Dragsted LO. Acute effects of light and dark roasted coffee on glucose tolerance: a randomized, controlled crossover trial in healthy volunteers. Eur J Nutr. 2016 Oct;55(7):2221-30. doi: 10.1007/s00394-015-1032-9. PMID: 26342706. PubMed
- Moisey LL, Robinson LE, Graham TE. Consumption of caffeinated coffee and a high carbohydrate meal affects postprandial metabolism of a subsequent oral glucose tolerance test in young, healthy males. Br J Nutr. 2010 Mar;103(6):833-41. doi: 10.1017/S0007114509992406. PMID: 19889241. PubMed
- Lovallo WR, Whitsett TL, al'Absi M, Sung BH, Vincent AS, Wilson MF. Caffeine stimulation of cortisol secretion across the waking hours in relation to caffeine intake levels. Psychosom Med. 2005 Sep-Oct;67(5):734-9. doi: 10.1097/01.psy.0000181270.20036.06. PMID: 16204431. PubMed
Content reviewed for accuracy by the JustLabs editorial team.
Educational information, not medical advice. Don’t stop or change a prescribed medicine without talking to your prescriber. If your lab order gives different instructions, follow your lab order.