Fasting For Blood Test/Work, Why it Can Be Dangerous, When it’s Not Required

Posted by Medical Board on February 4, 2019 in Men Women Last updated on February 6, 2020 Fasting For Blood Test/Work, Why it Can Be Dangerous, When it’s Not Required

With the advent of the microscope came the emergence of quantitative diagnosis in the late 1800’s.  This allowed physicians to now identify organisms, bacteria, and other substances in the blood and tissue of patients.  By the turn of the century, laboratory medicine was well on its way, as numerous tests were developed for use in the detection of cholera, tuberculosis, typhoid, and diphtheria.  Today, more than 100 blood tests exist and are used to identify any number of disease pathologies and medical conditions.  Blood tests may also help predict the likelihood of developing certain diseases and help physicians determine whether specific treatments or medications are working.

Blood and Testing, the Facts

Blood consists of liquid known as plasma, which is made up of water, salts and protein.  The thicker blood material includes red blood cells, white blood cells, and platelets.  Blood is classified or “typed” as either, type A, type B, type AB, or type O.  Blood is also Rh-positive or Rh-negative.  In blood testing, a range of normal results is compared to the patient’s blood sample values.  The range is generally reflective of 95 percent of healthy individuals in a particular group of people that varies depending on age, gender, race, and other factors.[5]

Why are blood tests useful and what do they tell us?

Blood tests help physicians and other health practitioners determine whether particular disease pathology is present, and if organs are functioning correctly.  Blood tests are often administered to identify whether disease treatments are effective or need to be changed as well.

Blood tests may help determine if patients are at risk for certain diseases or medical conditions including whether blood is clotting effectively.[8]

Who gets blood tested?

Blood tests may be ordered for a number of people including:

  • Individuals with a history of certain medical conditions
  • Patients who exhibit symptoms of a particular disease pathology or medical condition
  • Individuals who are in treatment for certain medical conditions
  • Patients undergoing routine physical examinations
  • Children entering public school for the first time
  • Individuals who could be at risk for specific inherited medical conditions
  • Women who are pregnant
  • Those donating blood/organs/bone marrow
  • Persons who are preparing to undergo surgery or some other medical procedure

Federal government food program participants, including those enrolled in (Women, Infants, Children) WIC are required to take blood tests to identify nutritional risk factors.[4]

Some communities also offer blood tests for polyfluoroalkyl substances (PFAS).  These are global environmental contaminants used for the production of numerous products such as “Scotch guard” for carpets and furniture, Teflon coated cookware, and even pizza boxes in some places.  When blood tests are performed to detect PFAS levels, no fasting is required.[1]

Although there are many different blood tests administered to patients, most are one of four types involving, either a complete blood count (CBC), blood chemistry, blood enzymes, or blood testing to assess risk for heart disease. 

Fasting For Blood Test/Work—Why we do it 

Sometimes your healthcare provider will advise you to fast before taking a blood test.  This is because some food and drinks could affect the blood test, skewing the results.  When food is consumed, nutrients and other byproducts are absorbed into the bloodstream.  Some tests require blood samples to be free of outside substances.

What blood tests require fasting?

Many blood tests require fasting before the blood draw.  These include the following:

Glucose tests—Glucose tests measure the amount of sugar in the blood.  A glucose tolerance test requires fasting, then blood will be drawn at specific intervals over a period of time.

Lipid tests—These require fasting and test the amount of triglycerides (fat) found in the bloodstream, as well as cholesterol, a wax like substance made in the body.  These tests generally require fasting because food, drink, or other nutrients are believed to render the test results invalid.[6] 

Fasting Blood Test

Fasting can be difficult for individuals who lead very active lives, those who have diabetes, and for pregnant women or children.  When patients have difficulty fasting, forget to fast or “cheat” by eating a snack at some point before getting blood work, doctors must reorder tests, and patients must schedule additional visits to the doctor for follow up results.

Why do we fast for a cholesterol test?

When an individual is tested for cholesterol, the doctor most often orders a lipid panel.  Lipids consist of molecules that contain fat.  

A lipids panel includes tests that measure:

  • Total cholesterol concentration
  • Low-density lipoprotein
  • High-density lipoprotein
  • Triglyceride levels 

Historically, fasting before lipids testing has been done for two reasons.  Because certain foods can affect lipid levels, test results were thought to be more uniform if all patients refrained from eating before blood work.  Scientists and physicians also believed that the calculation of LDL-cholesterol (based on specific equations) would be skewed or distorted by eating before tests.  Some research suggests however, that measuring lipid levels while fasting actually creates an unclear picture for the doctor since most individuals are in a fed state, not fasting throughout the natural course of a day.[7]

For routine check ups to determine possible cardiovascular risk, many studies support cholesterol testing without fasting.

Dangers of Fasting Before Blood work

While fasting for cholesterol levels may still be a normal practice in most medical offices, studies have shown that fasting by individuals with diabetes, who are on diabetic medication, can promote hypoglycemia, or low blood sugar.  Because patients continue taking diabetes medication, then fast for cholesterol blood tests, this drives blood sugar down to dangerously low levels.  Fainting, confusion, and loss of consciousness have been reported in some patients before, during, or following a fasting blood test.  Other countries such as Canada support the non-fasting practice before cholesterol testing for this reason.  Much of Europe has also adopted the practice of non-fasting before lipids testing as well.[11]

Blood tests for sugar, what’s the difference?

Some physicians order an A1C test rather than a traditional fasting blood sugar test.  

A1C Test—This measures the amount of hemoglobin (a complex protein in red blood cells) that contains attached glucose.  The test does not require fasting and can be a useful test to help diagnose a diabetic, or prediabetic condition over time (usually a period of months).  

Fasting Before Blood Test

Tests that determine levels of blood sugar do require fasting.  These tests will be used to help diagnose very serious medical conditions including, prediabetes conditions, or type 1, or type 2 diabetes.

Fasting Blood Sugar Test—This checks sugar levels after a significant fast, usually the fast is overnight.  

Glucose Tolerance Test—A glucose tolerance test measures initial blood sugar levels after fasting, then at various time intervals after consuming a sugary liquid.  The test is used to determine how quickly glucose clears from the blood and is administered to help identify a number of conditions including diabetes, insulin resistance, carbohydrate disorder, reactive hypoglycemia and acromegaly, a rare pituitary disease.[2]  

Other tests that require fasting include:

Metabolic profile—A metabolic profile measures how well the body manages metabolism and the various functions of the body that use energy.  This test does require fasting and will analyze kidney and liver function, blood sugar, and cholesterol and calcium in the body.  Protein levels, chloride levels, sodium, and potassium are measured with a metabolic profile as well.

Sometimes a renal function panel, to test kidney function, or a lipid panel to measure cholesterol is ordered separately by a physician.  These generally require fasting before the blood is drawn as well.[9]

Can you drink water before blood test?

Yes, in fact it is recommended that you drink plenty of water before blood testing.  This makes it easier for the phlebotomist to find veins because water hydrates these vessels.  It is also much easier to draw blood from a hydrated vein.  It is important to follow your doctor’s instructions completely before a blood test.  Consuming other liquids including coffee, tea, or soda may interfere with the accuracy of test results.[6]

How long to fast before blood work?

Fasting times may vary based on the type of blood test you are taking.  The majority of tests require a fast of at least 8-12 hours before testing.  While this is a general guideline, the doctor will provide you with specific instructions regarding fasting before the actual blood test.  

Can I take required medications before a fasting blood test?

In general, if your doctor does not specify otherwise, it is okay to take your usual medication.  Some nutrient supplements could interfere with test results however, so it is important to clarify with your doctor whether or not these can be taken before blood is drawn.

Once the fasting blood test is over can I eat and drink immediately?

Yes, once blood is drawn you can consume food and beverages normally.  If possible, schedule your fasting blood test first thing in the morning so you do not have to fast as long.  Many people bring a snack to eat directly after testing.

Are there certain blood tests that do not require fasting?

Yes, many blood tests do not require fasting before blood is drawn because test results will not be influenced by food or drink.  These include:

  1. Blood tests for thyroid function
  2. Tests for kidney function
  3. Liver function tests[10]

How do I know if the blood testing facility I visit is safe and clean?

The federal government, including the Food and Drug Administration (FDA), the Centers for Medicare and Medicaid Services (CMS), and the Centers for Disease Control (CDC) ensure blood testing facilities and labs (even if within a physician’s/dentist’s office) adhere to Clinical Laboratory Improvement Amendments (CLIA.)  These ensure blood-testing facilities have the proper certificates of compliance, adherence, and registration and maintain specific health and safety standards.[3]

Blood testing can be an important diagnostic tool for doctors and other medical professionals to help identify the presence of disease or a possible medical condition.  It is critical however, to follow any pretest instructions to the letter.  If you fail to do so, you could be misdiagnosed and treated improperly for a condition you may or may not have.  Always inform your doctor if you misinterpret directions, or make a mistake in preparing for a blood test.   

References (2019).PFAS Blood Testing. [online] Available at: [Accessed 4 Feb. 2019]. (2019). Getting Tested | Basics | Diabetes | CDC. [online] Available at: [Accessed 4 Feb. 2019]. (2019). [online] Available at: [Accessed 4 Feb. 2019]. (2019). Bloodwork Rule | Food and Nutrition Service. [online] Available at: [Accessed 4 Feb. 2019]. (2019). Blood | MedlinePlus. [online] Available at: [Accessed 4 Feb. 2019]. (2019). Fasting for a Blood Test: MedlinePlus Lab Test Information. [online] Available at: [Accessed 4 Feb. 2019].

7Naomi D. L. Fisher, M. (2019). Farewell to the fasting cholesterol test? – Harvard Health Blog. [online] Harvard Health Blog. Available at: [Accessed 4 Feb. 2019]. (2019). Blood Tests | National Heart, Lung, and Blood Institute (NHLBI). [online] Available at: [Accessed 4 Feb. 2019].

9panel, C. (2019). Comprehensive metabolic panel: MedlinePlus Medical Encyclopedia. [online] Available at: [Accessed 4 Feb. 2019].

10Publishing, H. (2019). Ask the doctor: What blood tests require fasting? – Harvard Health. [online] Harvard Health. Available at: [Accessed 4 Feb. 2019].

11University, M. (2019). Fasting for lab tests isn’t good for patients with diabetes. [online] MSUToday. Available at: [Accessed 4 Feb. 2019].

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