Do You Really Need to Fast Before Blood Work?

Whether you need to fast before blood work depends on which tests your clinician ordered. Fasting may be requested for tests such as fasting glucose and some cholesterol or triglyceride measurements, while many common tests do not require it. The instructions on your lab order take priority. Fasting can be unsafe for some people, especially those taking insulin or other diabetes medicines. This article explains which tests may need it, what you can drink, and what to do if you ate.

Do you need to fast before blood work? It depends on the test

There is no single rule for all blood tests. Some measurements change after a meal, so a sample drawn after eating can look different from one drawn after a period without food. Other measurements barely change with food, so fasting adds little. Your clinician decides which applies to the tests on your order.

The most reliable step is to read the instructions that came with your lab order or patient portal message. If they are missing or unclear, call the office or the lab before the appointment. Asking beforehand can save you a second trip.

MedlinePlus, the National Library of Medicine's consumer health site, explains in its Blood Tests overview that some tests require preparation, such as not eating or drinking for a period beforehand, and that your provider will tell you if so.

Why food can change a result

After you eat, your body absorbs sugars and fats, and the levels of several substances in your blood can rise. Blood sugar is the clearest example. A meal can raise it, which is why a test meant to look at your baseline sugar is often done after an overnight period without food. Triglycerides, a type of fat in the blood, can also be higher after a meal. Several factors contribute to how much any one person's numbers shift, including what you ate, how much, and your own metabolism.

That is the reason for fasting: it aims to reduce the effect of your last meal so that the number reflects your baseline. It is not a test of willpower, and a slightly different result after eating does not by itself establish any condition.

Tests that may require fasting

Your clinician may ask you to fast for tests like these. Whether fasting is needed for you can differ from the list.

• Fasting blood glucose. This measures sugar in your blood after a period without food. MedlinePlus describes how blood sugar testing is used in its Diabetes topic page. Clinicians combine your history, symptoms and more than one result rather than relying on a single reading.
• Lipid panel. This looks at cholesterol and triglycerides. Some clinicians ask for fasting, while others accept a sample taken without fasting. See MedlinePlus on Cholesterol for background. Our article Understanding Your Lipid Panel covers what each number is.
• Glucose tolerance tests. These involve fasting, drinking a sugary solution, and having blood drawn at set times. They are used in several situations, including during pregnancy, and the lab gives specific instructions. MedlinePlus has general information on Pregnancy care. Follow the directions from your obstetric clinician.
• Some other panels. Certain metabolic panels, iron studies or vitamin levels may come with a fasting request, depending on the lab and the clinician's preference.

Tests that often do not need fasting

Many routine tests are commonly done without fasting. Examples can include a complete blood count, thyroid-stimulating hormone, hemoglobin A1C (a measure that reflects average blood sugar over past weeks to months), and many tests for infection or inflammation. Practices differ, and a test that usually does not need fasting may be bundled with one that does. If your order has several tests, follow the strictest instruction on it.

Some clinicians now accept a nonfasting lipid sample for routine screening, but others still prefer fasting, for example when triglycerides were high before. Your clinician knows your history, so ask which they want rather than deciding yourself.

What you can drink and take while fasting

• Water. Plain water is generally allowed and often encouraged. Being well hydrated can make the blood draw easier because veins are easier to find. Confirm with your lab if you have a special test.
• Coffee and tea. Plain black coffee or tea may be acceptable for some tests and not for others. Cream, sugar and sweeteners can interfere. Ask the lab.
• Gum, mints and cough drops. Many contain sugar, and some labs ask you to avoid them. Check the instructions.
• Alcohol. It can affect some results, including triglycerides and liver tests. Your lab instructions may tell you how long to avoid it.
• Exercise and smoking. Hard exercise or smoking shortly before a draw can affect some values. Ask whether they matter for your tests.

Medicines and supplements on a fasting morning

Do not change the timing or dose of a prescription on your own. Some medicines are taken with food, and skipping or delaying them can cause problems. Others may be fine to take with water. Ask the prescriber or pharmacist which applies to you, and tell the lab which medicines you take. MedlinePlus has general advice on safe medicine use on its Drug Safety page.

Supplements can matter too. Biotin, a common ingredient in hair and nail products, can interfere with certain lab tests. Our article Can Biotin Supplements Throw Off Your Lab Results? covers this. Tell your clinician about every supplement you use so they can decide whether to pause any before the draw.

If you take insulin or other diabetes medicines

Fasting deserves extra planning if you have diabetes. Insulin and some other diabetes medicines can lower blood sugar. Taking them without food, or skipping food while taking them, may cause low blood sugar (hypoglycemia). MedlinePlus describes symptoms and treatment on its Hypoglycemia page.

Before a fasting test, ask the prescriber how to handle your medicines and whether your appointment can be scheduled early in the day so the fast is short. Do not decide on your own to skip or change a dose. Keep your usual monitoring routine as your clinician recommended.

Signs of low blood sugar can include shakiness, sweating, fast heartbeat, hunger, irritability, headache and trouble concentrating. A glucose reading below 70 mg/dL is generally treated as low. If you have these symptoms during a fast, treat them right away rather than waiting for the test. If you are awake and can swallow, many treatment plans call for about 15 grams of fast-acting sugar, such as juice or glucose tablets, and a recheck about 15 minutes later. If you have a meter, check your glucose. This ends the fast, so tell the lab and your clinician afterward.

Severe low blood sugar can cause confusion, seizures or loss of consciousness. That is an emergency. Do not put food or drink in the mouth of someone who is confused, unconscious or cannot swallow safely, because it can enter the lungs. If glucagon has been prescribed and is available, a bystander can give it as directed, and should call 911.

Vomiting that keeps you from holding down fluids also deserves attention when you have diabetes. Together with abdominal pain, fast or deep breathing, fruity-smelling breath, marked drowsiness or confusion, or signs of severe dehydration, it can suggest diabetic ketoacidosis or serious dehydration, which need emergency care. This is a particular concern with insulin or an SGLT2 inhibitor.

Other people who should ask before fasting

• Pregnant people. Dizziness and nausea can be more likely, and some tests have specific timing. Ask your obstetric clinician.
• Children. Younger children may not tolerate a long fast. Ask the pediatrician what is appropriate for your child's age and the test. Do not guess at amounts of food or drink.
• Older adults. Dehydration and low blood pressure can make fainting more likely. Water usually helps, but ask about medicines.
• People with a history of fainting at blood draws. Tell the phlebotomist so they can have you lie down.
• People who have had an eating disorder or who are underweight. Prolonged fasting may be a concern. Tell your clinician.

What if you ate or drank by mistake?

It happens often. Tell the phlebotomist or lab staff what you had and roughly when. Depending on the test, they may draw the sample anyway and note it, or suggest rescheduling. Your clinician can also interpret a result with the context that you had eaten. Do not skip the visit out of embarrassment, and do not hide it, because an accurate note helps your clinician read the numbers correctly.

Practical tips for a fasting draw

• Schedule the appointment early in the morning if you can, so the fast overlaps with sleep.
• Drink water unless told otherwise.
• Bring a snack to eat right after the draw, along with your usual medicines if your clinician has said to take them afterward.
• Wear a short-sleeved or loose top that rolls up easily.
• Tell the phlebotomist if you feel lightheaded before, during or after. Sitting or lying down can help.
• Avoid driving if you feel faint afterward until you feel steady.

Understanding the results afterward

A single result is one piece of information. Clinicians combine your history, an examination and, when needed, repeat testing. A fasting glucose that is higher than expected, for example, usually leads to follow-up rather than an immediate conclusion. Our articles High Fasting Blood Glucose: What It Can Indicate and Fasting Blood Glucose of 110: What It Means go through that topic. If anything on your report worries you, ask your clinician to explain it.

When to Seek Medical Care