How Long to Fast Before Blood Work, Test by Test

1. Check whether your blood work needs a fast at all

Start with the order itself. Many lab requisitions print "fasting" or "nonfasting" next to each test. If yours says nothing and the order includes glucose, call the office or the lab and ask.

Here is what labs usually ask for the common tests:

Test	Fasting usually needed?	Typical length
Fasting glucose	Yes	8 to 12 hours
Glucose tolerance test	Yes	At least 8 hours overnight, then 2 to 3 hours at the lab
Lipid panel (cholesterol)	Often no	9 to 12 hours if your doctor asks for it
Basic or comprehensive metabolic panel	Sometimes, since it includes glucose	8 to 12 hours if asked
Iron tests	Sometimes, usually as a morning draw	As your lab says
A1C, complete blood count, thyroid (TSH)	No	None

The cholesterol row surprises people. In 2016, the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine reviewed the data and recommended nonfasting lipid profiles as the routine choice. After ordinary meals, the largest average changes were small: triglycerides rose by about 26 mg/dL, total and LDL cholesterol fell by about 8 mg/dL, and HDL did not move. A 2024 review found that guidelines in the United States, Canada, Europe, Japan, Australia and several other countries now accept nonfasting lipid tests for some or all patients.

Doctors still order a fasting lipid panel in some cases, for example when an earlier nonfasting triglyceride result was very high (the European statement uses 440 mg/dL). If your order says fasting, fast.

2. Count back from your appointment time

Once you know the number of hours, work backward from the draw. For a 7:30 am appointment and a 12-hour fast, your last bite is at 7:30 pm the night before. For 8 hours, it's 11:30 pm. Book the earliest slot you can get, so most of the fast happens while you sleep.

The 8-hour mark is the convention labs have used for decades for fasting glucose. German researchers looked at 28,024 primary care patients without known diabetes and found that mean glucose differed only slightly between people who had fasted 8 hours or more and those who had fasted less. Your instructions still stand. The cut-offs used to read a fasting glucose come from fasting samples, so a short fast makes your number harder to interpret.

Eat your usual dinner the night before, since a test fast is meant to show your normal baseline. Many labs also ask you to skip alcohol for 24 hours. Going an hour or two past 12 hours is rarely a problem for a healthy adult. If your lab sheet sets an upper limit, keep to it.

3. Drink water and skip everything else

Plain water is allowed during a test fast, and it helps: phlebotomists generally find veins easier to reach when you are well hydrated.

Most labs ask you to skip the rest:

• Coffee and tea, even black. Our guide to coffee before blood work covers the details; the short rule is to skip it unless your lab says yes.
• Gum, mints and cough drops, which often contain sugar.
• Juice, soda, sports drinks and diet drinks, sweetened or flavored water included.
• Smoking or vaping on the morning of the test.

This is where a test fast is stricter than intermittent fasting. On a normal 16:8 morning you might drink black coffee at 9 am without a second thought. Before a fasting glucose test, the coffee waits until you walk out of the lab.

4. Take your usual medications unless told otherwise

Most prescription medicines should be taken as usual on the morning of the test, with water. Skipping a blood pressure pill to keep the fast clean can do more harm than good, and a tablet carries almost no calories. Our page on whether medication breaks a fast explains why pills and fasting get along.

Ask your doctor or pharmacist ahead of time about these:

• Insulin and sulfonylureas such as glipizide, glyburide or glimepiride. Taken at the usual dose without breakfast, they can push your blood sugar too low. Step 7 covers this.
• Pills your label says to take with food. Often the answer is to take them right after the draw, with your first meal.
• Thyroid medicine when thyroid levels are being checked. Some doctors want it taken after the draw.
• Supplements. Gummy vitamins and fish oil can wait until afterward. High-dose biotin, common in hair and nail products, can throw off some thyroid and hormone tests, so mention it when you book.

If you'll take anything with breakfast after the draw, put it in your bag the night before.

5. Tell the lab if you ate by mistake

It happens: a handful of almonds while packing lunch, or milk in the coffee on autopilot. Don't hide it. Tell the person drawing your blood what you had and when. They can draw anyway and mark the sample as nonfasting, or ask you to come back another morning. Either way, your doctor reads the result with the right context.

What happened	Glucose or glucose tolerance test	Cholesterol test only
Gum, a mint or a sip of juice	Tell the lab; they may rebook	Tell the lab; usually still drawn
Milk or sugar in your coffee	Tell the lab; often rebooked	Tell the lab; usually still drawn
A snack 4 to 6 hours before	Usually rebooked	Often drawn and marked nonfasting
A full breakfast	Rebook for another morning	Often drawn, unless your doctor wanted a fasting sample
Alcohol the night before	Tell the lab	Tell the lab; triglycerides may read higher

When one order covers several tests, the strictest one sets the rule. A glucose plus a cholesterol test follows the glucose instructions.

6. On 16:8, time the draw for the end of your fast

If you follow 16:8 fasting, you already do most of a test fast every day. Finish dinner at 8 pm, and a 9 am draw puts you at 13 hours, inside the range most labs ask for. Book a morning appointment and break the fast after the needle comes out.

The two kinds of fasting have different jobs. Intermittent fasting is a habit you shape around your life. A test fast is a standard condition, so your result can be compared with reference ranges built from people who prepared the same way, which is why the lab is stricter about drinks.

Longer daily fasts, like 18:6 or one meal a day, can put a morning draw at 18 hours or more. For a healthy person that is usually fine. In a small study from Croatia, 30 volunteers had blood drawn early in the morning and again in the early afternoon, still fasting and after their usual daily activities. Glucose, triglycerides, kidney markers and electrolytes moved a little, and none of the changes was large enough to change a medical decision. Everyone in the study was healthy, so mention your fasting hours to the lab if they run well past 12. If you use FastTrack, the timer already shows that number.

After a fast of two days or more, several blood values drift from your usual state, so book routine blood work for an ordinary week or tell your doctor what you did.

7. Call your doctor or lab first in these cases

A test fast is short and safe for most adults. Check with your doctor or the lab before you start if any of these apply to you:

• Diabetes treated with insulin or a sulfonylurea. Skipping breakfast on your usual dose can cause low blood sugar, sometimes while you are driving to the lab. One clinic in Missouri cut low readings among fasting patients on diabetes medication by about two thirds once it taught them to check their glucose and adjust doses before fasting tests. Ask for the earliest slot, a dose plan for the night before and the morning, and whether your test needs fasting at all. Bring a snack for afterward.
• Pregnancy. Glucose screening in pregnancy has its own instructions, and an empty stomach can make nausea worse. Follow what your OB or midwife gives you.
• Children and teens. A child's fasting time should come from the pediatrician, and many routine tests for kids need no fasting.
• A history of an eating disorder. A required fast can stir old patterns. Tell your doctor; a nonfasting test is often possible.
• Fainting at blood draws. Tell the phlebotomist before they start so you can lie down for it.

While you wait for the draw, eat or drink something with sugar if you feel shaky, sweaty, confused or close to fainting. With diabetes, treat a low the way your care team taught you. You can always rebook the test.

The short version

For a fasting glucose or a glucose tolerance test, plan 8 to 12 hours with nothing but water and book an early slot. Many routine cholesterol tests no longer need fasting, and an A1C, a blood count or a thyroid test never did. Take your usual pills unless told otherwise. Whatever your lab sheet says overrules any general rule, this one included.