Fasting before a blood test
Only some screening blood tests need fasting, and the instructions from your provider or lab are what count. When fasting is asked, it usually means 8 to 12 hours with nothing but plain water. Do not fast if nobody asked you to, since that can also change some results. (MedlinePlus, 2025)
Which tests need it
| Test | Fasting? | More |
|---|---|---|
| Fasting plasma glucose, glucose tolerance test | Yes, at least 8 hours, nothing but sips of water. The ADA defines fasting as no calories for at least 8 hours (NIDDK, 2022) (ADA, 2026) | A1C and blood sugar results |
| A1C | No, you can eat and drink before it (NIDDK, 2022) | A1C and blood sugar results |
| Cholesterol (lipid panel) | Sources differ: the AHA/ACC guideline allows fasting or nonfasting for screening; MedlinePlus says you may be asked to fast 9 to 12 hours and NHLBI 8 to 12 hours (AHA/ACC, 2018) (MedlinePlus, 2026) (NHLBI, 2024) | Cholesterol test results |
| Basic metabolic panel | Usually (MedlinePlus, 2025) | Metabolic panel results |
| Kidney (eGFR) and liver tests | Sometimes; for eGFR you may be asked to fast or avoid meat for several hours (MedlinePlus, 2025) (MedlinePlus GFR, 2025) | Kidney test results |
| Thyroid (TSH) | Not for TSH itself; only if other tests on the same draw need it (MedlinePlus TSH, 2024) | Thyroid test results |
| HIV screening | No special preparation (MedlinePlus HIV, 2024) | HIV test results |
| PSA | No fasting is listed; avoid ejaculation for 24 hours before, since it can raise PSA (MedlinePlus PSA, 2025) | Prostate cancer screening |
How to fast
- How long: usually 8 to 12 hours; your provider tells you exactly how long. Some tests also ask you to skip alcohol for longer. (MedlinePlus, 2025)
- What breaks a fast: any food or drink other than plain water, including juice, coffee and soda. Also do not chew gum, smoke or exercise while fasting. (MedlinePlus, 2025)
- Water: plain water is fine and can make the blood draw easier; skip flavored or lemon-flavored sparkling water. (MedlinePlus, 2025)
- Your usual medicines: ask your provider whether to take them. Do not stop a medicine unless your provider tells you to, and mention vitamins and supplements. (MedlinePlus, 2025)
- If you ate by mistake: tell your provider; the test may need to be rescheduled. (MedlinePlus, 2025)
- After the draw: you can eat right away. Feeling light-headed is common after a fasting test, so a snack can help. (MedlinePlus, 2025)
Questions people ask
How long do I fast before a blood test?
Usually 8 to 12 hours, but your provider tells you exactly how long for the test they ordered. MedlinePlus suggests booking early in the morning so most of the fast happens while you sleep. (MedlinePlus, 2025)
Can I drink water while fasting for a blood test?
Yes, plain water. MedlinePlus says juice, coffee, soda and other drinks can affect results, and to avoid flavored or lemon-flavored sparkling water. Staying hydrated can make the blood draw easier. (MedlinePlus, 2025)
Do I need to fast for a cholesterol test?
Not always. The 2018 AHA/ACC cholesterol guideline says screening can be done fasting or nonfasting. MedlinePlus says you may be asked to fast for 9 to 12 hours, and NHLBI says 8 to 12 hours. Follow what your provider tells you. (AHA/ACC, 2018) (MedlinePlus, 2026) (NHLBI, 2024)
Do I need to fast for an A1C test?
No. You can eat and drink before an A1C. A fasting plasma glucose test and a glucose tolerance test need at least 8 hours with nothing to eat or drink except sips of water. (NIDDK, 2022) (ADA, 2026)
Should I take my medicines before a fasting blood test?
Ask your provider. MedlinePlus says some tests need you to pause a medicine, but you should not stop any medicine unless your provider tells you to, and you should mention vitamins and supplements too. (MedlinePlus, 2025)