Can you drink water before a blood test?
The short answer
Yes. You can drink plain water before almost any blood test, including fasting tests, and it does not break your fast. Just keep it to plain water with nothing added.
What to do
- Drink plain water as you normally would, including the morning of your draw.
- Keep it plain: no lemon, sweetener, flavor drops or electrolyte mixes.
- Don't force extra water; normal amounts are enough.
- Use water to take your usual medications unless told otherwise.
- Follow any special instruction on your test page if it says otherwise.
Which results it changes
| Marker | Effect | How long |
|---|---|---|
| Glucose[2] | No significant change after drinking 500 mL of water | Measured 1 hour after drinking |
| Triglycerides[2] | No significant change after drinking 500 mL of water | Measured 1 hour after drinking |
| Total, HDL and LDL cholesterol[2] | Small shift, not judged clinically meaningful | Measured 1 hour after drinking 500 mL |
| Hemoglobin and hematocrit[2][3] | Small shift after 500 mL of water; unchanged after 12 hours without water | 1 hour after drinking; overnight without water |
| Lymphocytes[2] | The only result that changed by a clinically meaningful amount after 500 mL of water | Measured 1 hour after drinking |
| Creatinine and urea[2] | Small shift, not judged clinically meaningful | Measured 1 hour after drinking 500 mL |
Water does not break a fast
Plain water has no calories, so it does not count as eating. Fasting instructions for JustLabs tests, such as the Lipid Panel, Fasting Insulin and Iron and TIBC, say water is allowed.
This matches lab-medicine guidance. A European working group on blood sampling suggests a 12-hour fast with water allowed as desired on the morning of the draw. Labs do differ on the amount: in one review, allowed water ranged from half a glass to as much as you like.
What the research shows
In a 2026 study, healthy adults had blood drawn before and one hour after drinking 500 mL of water (about 17 ounces). Glucose and triglycerides did not change significantly. Several values, such as hemoglobin, cholesterol and creatinine, moved slightly, but only the lymphocyte count changed by an amount the authors judged clinically meaningful. Their conclusion: water before a draw has minimal clinically relevant effects.
Skipping water overnight did not seem to matter much either. In another study, 12 hours without food or water did not change plasma volume, hematocrit or hemoglobin in healthy adults.
Why hydration still matters
Larger shifts in body water are a different story. Lab researchers describe hydration status as a major pre-analytical variable, because water moving in and out of the bloodstream can change how concentrated some results look. Even posture before the draw can shift fluid.
The practical takeaway is simple: arrive as you normally are. Drink water as usual, don't skip it to be 'extra fasted', and don't chug large amounts right before the visit.
What counts as water
For a fasting test, only plain water counts. Flavored water, water with lemon, sweetened drinks, sports and electrolyte drinks, juice, coffee and tea do not. If a test doesn't need fasting, you can eat and drink normally. Many tests, such as TSH, Hemoglobin A1c and Vitamin D, need no fasting at all.
A simple plan for a fasting draw
Eat a normal dinner, then switch to plain water for the rest of the evening. Have a glass of water when you wake up, along with any medications you normally take in the morning. Book an early visit online through JustLabs, or walk in during the center's hours.
Bring a photo ID and your lab order, which is in your JustLabs dashboard and email. Most visits take 10 to 15 minutes, so you can eat and have your coffee soon after. Results usually arrive in your JustLabs account in 1 to 2 business days.
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
- Zhai ZZ, Yu XJ, Chen RY, Lv XY, Wang YH, Bai L, Li L, Zeng QL. Effect of water intake on routine laboratory test parameters in healthy adults: a cross-sectional study. Front Med (Lausanne). 2026;13:1884690. doi: 10.3389/fmed.2026.1884690. PMID: 42487942. PubMed
- Morgan JE, Dunning ON, Tocci ND, Mauney EL, Yazell AS, Rogatzki MJ. Plasma and serum volume remain unchanged following a 12-h fast from food and drink despite changes in blood and urinary hydration markers. Eur J Clin Nutr. 2025 Mar;79(2):161-167. doi: 10.1038/s41430-024-01526-5. PMID: 39420215. PubMed
- Ritchie RF, Ledue TB, Craig WY. Patient hydration: a major source of laboratory uncertainty. Clin Chem Lab Med. 2007;45(2):158-66. doi: 10.1515/CCLM.2007.052. PMID: 17311501. 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.