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Important Facts about Fasting for Blood Tests

Important Facts about Fasting for Blood Tests

Max Lab

May 18, 2022

Many blood tests require fasting to ensure the most accurate results. The proteins, fats, minerals, vitamins, carbohydrates, and other nutrients available in all foods and drinks can affect glucose levels in the blood, impacting the test results. However, fasting depends on the type of lab tests patients have been advised to undergo. The doctors or healthcare professionals arranging the test notify them about how to prepare for the test. While one can eat or drink as normal before some tests, fasting blood glucose and other such tests require people not to eat or drink anything, except water, for specific hours before the test. The doctor may also ask the person to avoid smoking, alcohol, or certain medications for 8 to 12 hours. Here is everything you need to know about fasting requirements, different types of fasting blood tests, etc. 

Which Blood Tests Require Fasting?

The doctor instructs people not to have food or drink anything for a few hours before the test. However, not all blood tests need fasting. Some specific tests need fasting for 8 to 12 hours before the appointment. The fasting blood tests list includes:

  • Fasting Blood Glucose

This test is used to detect or manage diabetes by evaluating the level of glucose (sugar) in the blood. Fasting before a blood glucose test is vital to get accurate results, helping the doctor interpret better. One needs to fast for at least 8 hours before this test.

  • Basic or Comprehensive Metabolic Panel

A comprehensive metabolic panel (CMP) test evaluates various substances in the blood, including glucose level, electrolytes, Albumin, calcium, blood urea nitrogen, protein, and more. This test needs fasting for 10 to 12 hours.

  • Lipid Profile

A lipid profile test checks the number of fat molecules known as lipids in the blood. This fasting blood test includes HDL cholesterol, LDL cholesterol, total cholesterol, and triglycerides. While fasting is not required in all situations, it is vital to ask the healthcare provider beforehand whether to fast or not before the test. More often than not, people have to fast for 10 to 12 hours of fasting before the lipid panel blood test.

  • Gamma-Glutamyl Transferase (GGT)

Gamma-glutamyl transferase (GGT) is an enzyme found in the body, especially in the liver. A GGT test is done to detect liver diseases, bile duct issues, or any other alcohol-caused problems. The test requires fasting for at least 8 hours beforehand. The doctor also asks the patient to avoid smoking and alcohol for 24 hours before the test.

  • Vitamin B12 Test

Another fasting blood test is the Vitamin B12 test, which is used to check the number of vitamin B-12 in the blood to measure the body's overall vitamin B-12 level. This test helps diagnose certain health issues caused by the deficiency of Vitamin B12. Fasting time for this test is 6 to 8 hours or as per the doctors' prescription.

  • Iron Tests

Iron is a mineral vital for making red blood cells in the blood. The iron test is used to diagnose several health conditions due to a lack of iron. Fasting time for this test is 12 hours to ensure the correct result.

Can I Drink Water Before a Fasting Blood Test?

In most blood tests, people are advised not to consume anything except water. Drinking plain water is considered good before the test as it helps to add more fluids to the veins, making it easier for the health professional to draw blood. However, one must avoid having any lemon-flavoured drinks, juice, carbonated waters, and tea or coffee until the test is completed.

When Can I Eat or Drink Again?

One can eat or drink immediately once the test is done. It is wise to book an early morning appointment for a blood test to reduce the duration of staying without food and drinks. People should consult their doctors about how many hours they need fasting for blood sugar and other tests. The amount of fasting time depends on the test.

What to Do If One Eats or Drinks by Mistake?

If a person skips the fasting and eats or drinks a beverage before the test, they should let their healthcare provider know about the same. The doctor may ask the person to go ahead and get the test done depending on the test type, or s/he will ask to reschedule the appointment. 

Fasting before a blood test is crucial otherwise the test results may be inaccurate, leading to misinterpretation of the issue or further health complications. One should consult their doctors for better guidance on whether they need to fast before a particular test or not, blood test fasting time, and other queries about a lab test.


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