Show
Medically Reviewed by Nayana Ambardekar, MD on August 13, 2022 It's the morning of your bloodwork and your doctor said to fast before the test. But your stomach is growling and you have serious caffeine withdrawal hours before you roll up your sleeve. A bite of toast and a few gulps of coffee won't really make a difference, right? Not so fast. Your results could come back wrong if you give in to temptation. Fasting means you don't eat or drink anything but water usually for 8 to 12 hours beforehand. If your appointment is at 8 a.m. and you're told to fast for 8 hours, only water is OK after midnight. If it's a 12-hour fast, avoid food and drink after 8 p.m. the night before. You also shouldn't smoke, chew gum (even sugarless), or exercise. These things can rev up your digestion, and that can affect your results. Take your prescription medications unless your doctor tells you to skip them. But ask your doctor before you take any over-the-counter drugs. Blood tests help doctors check for certain health problems and find out how well your body is working. Doctors also use them to figure out how well treatments are working. You don't need to fast before all blood tests. Your doctor will tell you if you need to. These tests typically require fasting: Nutrients in food and drinks go into your bloodstream and can change things measured by the tests, skewing your results. For instance, if you eat or drink before a fasting blood glucose test, your blood sugar probably will be higher than if you hadn't had anything. When you're fasting, doctors get a baseline result so tests can be compared to give a true picture of your sugar levels over time. If you make a mistake and eat or drink anything besides water, tell the person taking your blood. Your doctor will want to know so they can interpret your tests correctly. For the best results, they may ask you to reschedule. As soon as your blood is taken, your fast is over. You might want to bring a snack and a drink with you so you can eat as soon as possible after the test. URL of this page: https://medlineplus.gov/lab-tests/fasting-for-a-blood-test/
If your health care provider has told you to fast before a blood test, it means you should not eat or drink anything, except water, for several hours before your test. When you eat and drink normally, those foods and beverages are absorbed into your bloodstream. That could affect the results of certain types of blood tests.
The most common types of tests that require fasting include:
Glucose tests are used to diagnose diabetes.
You usually need to fast for 8–12 hours before a test. Most tests that require fasting are scheduled for early in the morning. That way, most of your fasting time will be overnight.
No. Juice, coffee, soda, and other beverages can get in your bloodstream and affect your results. In addition, you should not: These activities can also affect your results. But you can drink water. It's actually good to drink water before a blood test. It helps keep more fluid in your veins, which can make it easier to draw blood.
Ask your health care provider. Most of the time it's OK to take your usual medicines, but you may need to avoid certain medicines, especially if they need to be taken with food.
Tell your health care provider before your test. He or she can reschedule the test for another time when you are able to complete your fast.
As soon as your test is over. You may want to bring a snack with you, so you can eat right away.
Be sure to talk to your health care provider if you have any questions or concerns about fasting. You should talk to your provider before taking any lab test. Most tests don't require fasting or other special preparations. For others, you may need to avoid certain foods, medicines, or activities. Taking the right steps before testing helps ensure your results will be accurate.
Learn how to cite this page Regular blood testing is one of the most important ways to keep track of your overall physical well-being. Getting tested at routine intervals can allow you to see the way your body changes over time and empower you to make informed decisions about your health. What does a blood test show?Some blood tests can help your doctor determine how different organs in your body are working. Examples of organs whose malfunctions can be visible in a blood test include your thyroid, liver, or kidneys. Your doctor can also use blood tests to search for markers of diseases and health conditions such as:
Even if a person does not have heart disease, a blood test can show whether they may be at risk of developing the condition. Other blood tests can indicate whether the medications you’re taking are working properly, or assess how well your blood is clotting. Let’s take a closer look at some common blood tests. 1. Complete blood countA routine complete blood count (CBC) checks for levels of 10 different components of every major cell in your blood: white blood cells, red blood cells, and platelets. Important components this test measures include red blood cell count, hemoglobin, and hematocrit. Here’s the typical range of results, although every laboratory may have its own range that varies slightly: Abnormal levels of these components may indicate:
Based on your results, your doctor will order follow-up tests to confirm abnormal levels and a possible diagnosis. 2. Basic metabolic panelA basic metabolic panel (BMP) usually checks for levels of eight compounds in the blood:
This test may require you to fast for at least 8 hours before your blood is drawn, depending on the instructions of your doctor and what the test is measuring. See our chart for normal results. Abnormal results may indicate:
Your doctor will perform follow-up tests to confirm a diagnosis. 3. Comprehensive metabolic panelA comprehensive metabolic panel (CMP) includes all the measurements of a BMP as well as additional proteins and substances related to liver function, such as: The same conclusions can be drawn from a CMP as from a BMP for the same substances that a BMP covers. Other abnormal levels can also indicate underlying conditions, such as: 4. Lipid panelThis test checks levels of two types of cholesterol: HDL is “good” because it removes harmful substances from your blood and helps the liver break them down into waste. LDL is “bad” because it can cause plaque to develop in your arteries, increasing your risk of heart disease. You may need to fast for at least 8 hours before this test. Here are the ranges for each type: Normal levels can also vary by age. 5. Thyroid panelA thyroid panel, or thyroid function test, checks how well your thyroid is producing and reacting to certain hormones, such as: Your thyroid is a tiny gland in your neck. It helps regulate bodily functions like your mood, energy level, and overall metabolism. Here are normal results:
Abnormal levels of these hormones can indicate numerous conditions, such as: 6. Cardiac biomarkersEnzymes are proteins that help your body accomplish certain chemical processes, such as breaking down food and clotting blood. They’re used throughout your body for many vital functions. Abnormal enzyme levels can indicate many conditions. Common enzymes tested include:
Here are the normal ranges for the enzyme listed above:
7. Sexually transmitted infection testsMany sexually transmitted infections (STIs) can be diagnosed using a blood sample. These tests are often combined with urine samples or swabs of infected tissue for more accurate diagnoses. The following STIs can be diagnosed with blood tests:
Blood tests aren’t always accurate right after contracting an infection. For an HIV infection, for example, you may need to wait at least a month before a blood test can detect the virus. 8. Coagulation panelCoagulation tests measure how well your blood clots and how long it takes for your blood to clot. Examples include the prothrombin time (PT) test and fibrinogen activity test. Clotting is a crucial process that helps you stop bleeding after a cut or wound. But a clot in a vein or artery can be deadly since it can block blood flow to your brain, heart, or lungs. This can cause a heart attack or stroke. Coagulation test results vary based on your health and any underlying conditions that may affect clotting. Results from this test can be used to diagnose: 9. DHEA-sulfate serum testThe dehydroepiandrosterone (DHEA) hormone comes from your adrenal glands. This test measures whether it’s too high or too low. In men, DHEA helps develop traits like body hair growth, so low levels are considered abnormal. In women, high levels can cause typically male traits, like excess body hair, to develop, so low levels are normal. Low levels may be caused by:
High levels in men or women can result from: 10. C-reactive protein testC-reactive protein (CRP) is made by your liver when tissues in your body are inflamed. High CRP levels indicate inflammation from a variety of causes, including:
The higher the level, the higher the risk of heart disease:
If you are showing concerning symptoms of any illness or long-term condition, visit your doctor for a checkup. They will conduct a physical examination and let you know what kinds of blood tests you’ll need to undergo. If you want to request a routine blood test, the most common ones to consider are: Some other tests you may want include: How often you should get a physical exam may depend on your age, according to guidelines from a variety of clinical organizations:
Your doctor will decide whether you need any blood tests during your physical exam. In general, recommendations call for at minimum a lipid test starting at age 20 and every 5 years after that for people with a low risk of heart disease. For people at a higher risk of heart disease, more frequent lipid testing may be necessary. In addition, you should get a glucose blood test if your blood pressure is continuously above 135/80. People over age 50 should get a fecal occult blood test to screen for colorectal cancer annually. But some doctors may still suggest routine blood work during your yearly physical exam, and in some cases, you may want to get additional testing. For example, when:
Talk with your doctor first if you want to get certain tests done, or have them done more often than once a year. Who orders my blood tests?Your doctor typically orders blood tests for you during a physical, checkup, or an appointment intended to screen for a specific condition. It’s possible to order your own blood tests without a doctor through laboratories like LabCorp and Quest Diagnostics, but health insurance may not cover these tests. While such blood tests may more accessible and convenient, it may be harder to interpret the results without a medical professional. Some blood testing facilities may also not give you accurate results. One infamous case of this is Theranos. The California biotechnology firm shut down in 2018 when an investigation uncovered lies and fraud around the accuracy of its private blood-testing technology. Currently, litigation is underway against the founder and chief executive of the company, Elizabeth Holmes. Where can I get blood work done?There are different locations that offer laboratory services that include blood work. Most hospitals contain a laboratory that you can visit to get tests done. Some laboratories will have walk-in options. Others may require an appointment. Additional locations for blood testing may include:
Everything you eat and drink contains vitamins, proteins, and other nutrients that can cause the related levels in your blood to temporarily spike or drop. Fasting for 8 to 12 hours helps ensure that blood test results are free from these variables, making your test results as accurate as possible. Some common tests that may require fasting include:
Results may take anywhere from a few hours to a few days to become available. Here’s an overview of how long some common tests may take:
Timing can depend on the specific lab where you get tested, and how many tests you get done at once. If you order multiple tests, you may not get the complete results until all of the tests are completed. Sometimes a lab will only release results to your doctor, who reviews them and then releases them to you. While every laboratory or test-providing company may structure their result reports differently, they all must include the same components as mandated by federal legislation. Some of that may be administrative content, such as the name of the person who did the blood test, the date the test was done, and the name of the doctor who ordered the test. When it comes to understanding the results, you can look for the following:
A nurse or technician usually performs a blood test at a laboratory or in a doctor’s office. The procedure takes just a few minutes. To perform a blood test, the nurse or technician:
The risks of routine blood tests are very low but can include:
Blood tests can offer a good snapshot of your overall health. They’re also a good way to catch illness or disease early, and to see how well your body responds to treatments for various conditions. Many people get routine blood tests done at least once a year. Talk with your doctor to learn whether there are any other tests you may need to ensure your optimal health. Last medically reviewed on November 16, 2021 |