Fasting before blood work: the plain-English rules
Some blood tests need you to skip food for a while first. Here is how long, what you can still drink, and which common tests actually require it.
The short version
- How long: 8 to 12 hours. Your doctor’s order says which. If it just says “fasting,” 12 hours is the safe choice.
- Water: Yes, as much as you like. It makes the blood draw easier.
- Coffee and tea: No, even black. Caffeine can change some results.
- Your pills: Take them as usual with water, unless your doctor said not to.
- Easiest way to do it: book the earliest morning appointment and skip breakfast. You sleep through most of the fast.
What “fasting” means for a blood test
Fasting means nothing to eat or drink except plain water for a set number of hours before your blood is drawn. Food and drinks raise the sugar and fat in your blood for hours afterward. If your test measures those, eating first gives a false reading.
The National Library of Medicine puts it this way: “You usually need to fast for 8 to 12 hours before a test. Your provider will tell you exactly how long to fast.”
What you can and can’t have
| Item | Allowed? | Why |
|---|---|---|
| Plain water | Yes | Encouraged. Well-hydrated veins are easier to find. |
| Flavored or sparkling water | No | May contain sugar or sweeteners. |
| Coffee or tea, even black | No | Caffeine can stimulate digestion and alter some readings. |
| Juice, soda, milk, alcohol | No | All contain sugar or fat that shows up in your blood. |
| Chewing gum | No | Even sugar-free gum starts digestion. |
| Smoking or nicotine | No | Can affect several results. |
| Exercise | Skip it | A workout before the draw can change blood sugar and other values. |
| Prescription medicines | Usually yes | Take them with water unless your doctor said otherwise. Ask if you’re not sure. |
| Vitamins and supplements | Ask | High-dose biotin (vitamin B7) can interfere with thyroid and hormone tests. Tell the lab what you take. |
Which common tests need fasting
These answers match what each of our test pages says. The order from your doctor wins if it says something different, so read it, and if it doesn’t say, call and ask before the day of your appointment. Showing up unsure can mean getting sent home to come back another day.
Yes, fast
| Diabetes screening (fasting glucose) | Yes, 8 hours | The word "fasting" is in the test name. Water only. |
| Basic metabolic panel (BMP) | Often yes | It includes glucose. 10 to 12 hours if your doctor asks for fasting. |
| Comprehensive metabolic panel (CMP) | Often yes | Same reason as the BMP. |
It depends, so ask
| Lipid panel (cholesterol) | Usually not anymore | Current heart guidelines allow non-fasting lipid tests. Some doctors still want it fasting. Follow the order. |
| Cholesterol screening | Usually not anymore | Same test as the lipid panel. Ask when you book. |
| Folate | Ask | Some labs prefer fasting, and it’s often drawn with tests that require it. |
| Ferritin / iron studies | Ask | Your doctor may ask for a 12-hour fast (National Library of Medicine). |
| Homocysteine | Ask | Practices differ. |
| Testosterone | Morning, often fasting | Drawn between 7 and 10 a.m., when levels are highest. Your doctor may also ask you to fast. |
No fasting needed
| Hemoglobin A1c | No | Measures a 3-month average, so a meal today doesn’t change it. |
| CBC (complete blood count) | No | Unless it’s drawn with a test that does. |
| TSH (thyroid) | No | |
| Free T4 (thyroid) | No | |
| Vitamin D | Typically no | |
| Vitamin B12 | No | Some doctors ask for it anyway. Follow the order. |
| Magnesium | No | |
| PSA (prostate) | No | |
| hs-CRP | No | |
| ANA | No | |
| Sed rate | No | |
| PT/INR | No | Take your blood thinner as usual unless told otherwise. |
| Hepatitis C, HIV | No | |
| Urinalysis | No | A urine sample, not blood. |
| Fecal occult blood test | No | A stool sample. Follow the kit’s food instructions instead. |
One catch: if a no-fasting test is drawn at the same visit as a fasting test, you fast for the visit. The lab draws everything at once.
If you slipped
- You had black coffee or a sip of something. Call your doctor’s office before you go. For some tests it doesn’t matter. For others they’ll want you to reschedule.
- You ate. Reschedule. Labcorp’s advice is that after food or a sugary drink, “it’s generally best to reschedule.” A test done on a bad sample can come back abnormal, and then you get retested anyway, which can mean paying twice.
- Don’t hide it. Tell the person drawing your blood. They note it on the order so the results are read correctly.
After the draw
You can eat right away. Bring a snack. If you take medicines with food, plan to take them as soon as the draw is done.
One more thing to check before you go
Fasting is about getting an accurate result. The other thing to sort out before the lab is whether Medicare will pay. Many tests are covered only for certain diagnoses or only so often. If Medicare says no, the lab can bill you its own price, which is often far more than the test costs to buy directly.
What to do before you go, step by step →
How often Medicare pays for common lab tests →
Look up your test →
Sources
- MedlinePlus (National Library of Medicine): Fasting for a Blood Test
- Quest Diagnostics: Fasting for lab tests
- Labcorp: Fasting for Blood Work
Checked September 23, 2026. This page is general information about preparing for a test. Your doctor’s instructions for your test come first.
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