Which Blood Test Do I Need To Fast For?
Learn how blood tests work, including which blood tests need you to fast, and how to properly fast for the most accurate results.
Blood tests are a routine part of healthcare for many people. These tests play integral roles in the diagnosis of certain conditions and diseases, including diabetes, issues with kidney function, unbalanced thyroid hormones, and even allergies.
Certain blood tests require more preparation than others to get the most accurate results. One common aspect of preparing for a blood test is an overnight fast, which involves not eating any food or drinking anything except for water prior to the test. These fasts help the medical professionals who analyze your test results get a clear look at your blood at a baseline level unaffected by certain food or drinks that can raise glucose, affect hormones, and more.
If you’re curious about blood tests and how fasting plays a role in getting accurate test results, this post focuses on how blood tests work, the primary types of tests, and which tests require fasting.
How Do Blood Tests Work?
During a blood test, a Healthcare Provider collects a sample of your blood using a technique called venipuncture. While there are other ways to draw blood, venipuncture is the most common.
The venipuncture process starts with the provider preparing a vein for the insertion of a needle by first choosing a vein to draw from and then cleaning the site with alcohol to reduce the risk of infection. After the injection site is sterile, the needle is injected and blood is drawn.
The entire process usually takes less than two minutes for most tests.
Alternatives to the Standard Blood Test
For patients who are nervous about venipuncture, patients who want to take a blood test at home or are newborn babies, there are several other blood test methods that doctors may recommend; these include the finger prick test and the heel stick test.
A finger prick test involves puncturing a small needle just into the tip of your finger to collect enough blood for analysis. One of the most common uses for the finger prick method is at-home allergy testing.
The heel stick test draws blood from the bottom of the heel with a small needle. This procedure is primarily used for babies.
What Are the Most Common Blood Tests?
Getting a blood test is often part of regular preventative screening or to test for health conditions — these are the most common blood tests you may encounter.
The Complete Blood Count (CBC) Test
This type of blood test measures how many red blood cells and white blood cells you have in your body, as well as your levels of hemoglobin. CBC tests can help your doctor determine if you are suffering from anemia, and provide insight as to how active the immune system currently is within the body. They are some of the most common blood tests during standard annual checkups.
The Basic Metabolic Panel Test
Tests in the basic metabolic panel group include glucose tests, calcium tests, and electrolyte tests. These are just some of the primary substances in the human bloodstream, and your doctor might check them to determine blood sugar levels or check for deficiencies in certain nutrients.
A fasting blood glucose test is one of the types of blood tests that requires you to skip food and certain liquids for a certain period of time. Your doctor may ask you to fast for this type of test in order to accurately determine the amount of glucose in your blood. A fasting blood glucose test typically measures your blood sugar levels after you fast overnight.
Fasting blood glucose tests are often used to diagnose a patient with prediabetes or diabetes. A normal range for fasting blood sugar is at 99 mg/dL or lower. A fasting blood sugar level of 100 to 125 mg/dl is within the prediabetes range. Levels higher than 125 mg/dl are within the diabetes range.
The Blood Enzyme Test
Blood enzyme tests primarily measure the levels of substances like creatine, kinase, and troponin in your blood. These tests are often used to determine whether you’ve had a heart attack or other heart problems in the past. This is because heart attacks and other similar heart issues can cause changes to your blood enzyme levels.
Heart Health Tests
In addition to basic metabolic panel tests, blood enzyme tests, and CBC tests, you might also get blood drawn to check your cholesterol and triglyceride levels. Your lab results from these tests can help your doctor determine whether you have heart disease or are at risk of developing a heart condition in the future.
Nutrient Deficiency Tests
If you suspect that your body is lacking a certain vitamin or mineral because of symptoms you may be experiencing, blood testing is one of the best ways to know for sure.
Your doctor can analyze blood test results to check your levels of nutrients like vitamin B12, calcium, iron, vitamin D, and more. Deficiencies in these nutrients can cause health problems in the long run, so identifying them early and making the necessary changes to your diet and lifestyle is essential.
Which Blood Tests Require Fasting?
Many blood tests don't require a period of fasting for accurate results. However, most tests related to cholesterol, blood sugar, and liver health have a required fasting period.
These tests include:
- Fasting blood glucose tests: You need to fast for these tests to ensure that your results aren't affected by what you eat and drink right before the test (for example, sugary soda can spike blood sugar and show high blood sugar levels even if you generally have blood sugar that’s within range).
- Tests that check your levels of triglycerides, which are involved in checking for health conditions like heart disease. Fasting for these tests is not always required, so make sure to ask your provider if you need to fast for your test beforehand.
- Low-density lipoprotein (LDL) cholesterol and high-density lipoprotein (HDL) cholesterol tests: These tests measure your "good cholesterol" and "bad cholesterol" levels. Each type of cholesterol can tell you different information about your health. If high cholesterol levels show up on your test results, your doctor may recommend making changes to your diet and lifestyle.
- Any other basic metabolic panel tests or a comprehensive metabolic test: Since these tests measure levels of certain nutrients and chemicals in your blood, you usually need to fast before testing starts.
- Renal function panel tests that check for liver disease and function
- Iron blood tests also require fasting, as certain foods can affect how your iron levels appear on your lab test results.
Why Do You Need To Fast for Certain Blood Tests?
The main reason why certain blood tests require a period of fasting is to stop your meals and snacks from interfering with your test results. There are several ways that eating and drinking normally could make your blood tests less accurate, but it primarily comes down to changes in your blood sugar.
What Happens If You Don’t Fast?
If you’re required by your doctor to fast for a blood test and end up making a mistake, it’s best to mention the slip-up when you arrive for your test. If you completely forgot to fast, your doctor will probably ask to reschedule your blood test to a later date.
However, small mistakes while fasting may be able to be taken into account when analyzing blood test results, so it may be possible to undergo testing after accidentally breaking your fast with your usual morning latte, though you should expect to be rescheduled. Regardless, always let your medical team know if you have broken the fast so they can help make the right call on next steps.
What Can You Do Before a Fasting Blood Test?
If you’re about to undergo a blood test and are asked to fast by your doctor, the only thing you can consume is water. Staying hydrated before a test can help to make the fasting period more bearable, and it can also make it easier for the provider to draw blood from a vein.
What To Avoid Before a Fasting Blood Test
When you get a blood test that requires overnight fasting, make sure to stick to the following guidelines for the most accurate results.
- Avoid alcohol, which is a carb that can affect blood sugar and liver readings
- Skip fruit juice and other caloric drinks, as well as caffeinated drinks like coffee, soda, and tea, which can dehydrate you and make for a harder draw
Luckily, the fasting time for a blood glucose test is relatively short, which means you aren't likely to feel intense hunger before the test starts. If you have any worries about the fasting process, make sure to bring them up with your doctor before undergoing testing.
Fasting before a blood test can be inconvenient, but it’s an important part of the testing process that shouldn’t be skipped. Taking steps like staying hydrated and asking your doctor any questions you may have can help to make the fasting and testing process easier.
And, of course, you can go back to eating and drinking normally right after your test is over.
If you have any questions about fasting or your actual blood test, reach out to your Provider for additional assistance.
What You Need to Know About Blood Testing | MedlinePlus Medical Test
Complete blood count (CBC) | Mayo Clinic
Fasting Blood Sugar: Screening Test for Diabetes | Cleveland Clinic