smoking before blood test
Smoking before a blood test can affect your test results

Some blood tests require you to fast for several hours beforehand, usually about 8-12 hours before the blood sample is collected. Of course, you shouldn’t eat or drink anything during that fasting period. But is that all?

Smoking can affect your blood test results. So if you have been asked to fast before your blood test, you should avoid smoking as well. Other things to avoid before the test include:

  • Chewing gum (even sugarless)
  • Exercising
  • Drinking alcohol (this should be avoided 24 hours before, because it can affect liver function test results)

Unless your doctor advises you to do otherwise, you should be able to take your usual prescription medications.

Why is it important to fast before a blood test?

Fasting is not mandatory for kidney function tests, testing vitamin levels in the blood, blood markers test, etc.

But for some blood tests, fasting beforehand helps improve the accuracy and reliability of the results. And accurate test results are essential for diagnosing various health conditions and monitoring the effectiveness of treatment.

When you eat and drink before blood tests, the nutrients you consume are broken down and absorbed into your bloodstream. This can increase the levels of certain substances in your blood, such as blood sugar, iron, cholesterol, triglycerides and enzymes, which affect the results of the test and could lead to an incorrect diagnosis and health complications. 

Sometimes, you may forget about the fast and accidentally eat or drink something. If this happens, you should talk to your doctor about rescheduling the blood test.

Which blood tests require fasting?

Below is a list of blood tests that require fasting, along with their fasting requirements

Table: Types of blood tests with fasting requirements
Type of blood test Uses Fasting requirements
Blood glucose test Measures blood glucose (sugar) levels and is used to diagnose diabetes or prediabetes. A fasting blood sugar level of 99 mg/dL or lower is normal, whereas 100 to 125 mg/dL indicates prediabetes and 126 mg/dL or higher indicates diabetes. 8 to 10 hours
Glucose tolerance test Assesses blood sugar levels after fasting and then after taking a glucose drink containing a fixed amount of glucose. If the blood sugar level at two hours is 140 mg/dL or lower, it is considered normal. However, sugar levels of 140 to 199 mg/dL indicate prediabetes and 200 mg/dL or higher indicate diabetes. 8 to 12 hours
Lipid test

Also called blood cholesterol test, lipid profile or lipid panel blood test, measures the quantities of different fats in the blood, such as:

  • High-density lipoprotein (HDL) cholesterol or good cholesterol
  • Low-density lipoprotein (LDL) cholesterol or bad cholesterol
  • Triglycerides
  • 9 to 14 hours
  • Alcohol should be avoided for 24 hours
Thyroid function test Measures the levels of thyroid hormone levels in the blood to assess the proper functioning of the thyroid.
  • 12 hours
  • Thyroid supplements must be avoided for 48 hours
Gamma-glutamyl transferase (GGT) test Measures the level of GGT enzyme in the blood. High levels may indicate liver disease, bile duct problems or alcohol abuse.
  • 8 to 10 hours
  • Alcohol, smoking and some prescription drugs should be avoided for 24 hours
Serum iron test Measures the level of iron in the blood and is used to diagnose iron deficiency anemia.
  • 8 to 12 hours
  • Iron supplements or multivitamin tablets that contain iron should not be taken for 24 hours
Liver function test Gathers information about the enzymes and nutrients in the liver to assess liver health. Some doctors prefer results in the fasting state because food may activate liver enzymes.
  • 8 to 12 hours 
  • Alcohol should be avoided for 24 hours
Lactose tolerance test Helps determine if people can tolerate and consume dairy products. 8 to 12 hours

QUESTION

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How to fast before a blood test

When fasting before a test, you should keep in mind the following: 

  • Fasting time: The amount of time needed varies depending on the test. For most tests, it is recommended that you don’t consume anything but water for 8, 12 or even 14 hours before the test. Scheduling fasting blood tests early in the morning is probably wise because most of your fasting time will be spent asleep.
  • Liquids (only water): Drinking water is permitted unless instructed otherwise by your doctor. Water helps keep more fluid in veins, making it easier to draw blood for the test. Drinking other liquids, such as alcohol, carbonated beverages, fruit juice, coffee, tea and soda can interfere with test results and should be avoided during the fast.
  • Medication: Unless your doctor has advised otherwise, most prescription medications can be taken while fasting. The notable exception is thyroxine and insulin, which can alter test results.
  • Pregnancy: Typically, it is safe for a pregnant woman to fast for 8-10 hours if she is in good health and not in a high-risk pregnancy.
  • Pre-fasting nutrition: You should try to eat a nutritious, balanced diet before starting the fast to prevent hunger and energy loss.
  • Things to avoid:

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Medically Reviewed on 5/26/2021
References
WebMD. Fasting for Blood Tests. https://www.webmd.com/cholesterol-management/fasting-blood-tests

Rehder K, Roth GM. Effect of Smoking on the Fasting Blood Sugar and Pressor Amines. Circulation. 1959;20:224-228. https://www.ahajournals.org/doi/pdf/10.1161/01.CIR.20.2.224

Medline Plus. Fasting for a Blood Test. https://medlineplus.gov/lab-tests/fasting-for-a-blood-test/