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Expert Tips On Fasting For Blood Tests

By Dr. Nikita Toshi +2 more

It is a common question whether or not to fast before a blood test. Whether you have to fast for 8 hours or 12 hours or less or more is based on the type of blood test you have been prescribed by your doctor. The specifics about preparing for your stated test will be notified to you by your healthcare provider. But it is common knowledge that is consuming food before most blood tests that mention fasting samples is a big NO! Even what you drink can seriously impact the outcome of the test. In case you are thirsty, you are allowed to drink a clear liquid like water, but other drinks like tea, coffee, coconut water or milk are strictly prohibited.

 Even small things like smoking, chewing gum (sugarless or not) or even working out can rev up digestion and influence the test results. In this blog, let us find out a bit more about fasting before blood tests.

Fasting blood glucose estimation and lipid profile tests are commonly recommended tests which should be done in the fasting state. Overnight fasting therefore gives the most accurate test results.

Dr. Ashish Bajaj, M.B.B.S., M.D.

What tests may require fasting

The lab professional or doctor can advise you on fasting before a test you have booked. Make sure to ask about this if not notified. Some common blood tests that require fasting are listed below:

  • Lipid profile: Evaluates cholesterol level and other blood fats such as triglycerides. Test results showing high levels indicate the risk of developing heart conditions or a stroke. You may not be asked to fast for this test if you are younger than twenty-five years of age or if you only need a partial lipid profile or if the doctor specifically asks for a non-fasting result.

Recommended fasting time for the test: 9-12 hours.

  • Fasting blood glucose: It measures the quantity of sugar or glucose present within the blood sample and checks whether you are prediabetic or diabetic.

Recommended fasting time for the test: Minimum 8 hours.

  • Basic/Comprehensive metabolic panel: This kind of test evaluates blood sugar, fluid and electrolyte balance and kidney function. The comprehensive test also takes into account liver function.

Recommended fasting for the test: 10-12 hours.

  • Iron tests: Check whether the level of iron in your system is too high or low.

Recommended fasting time for the test: 12 hours.

  • Vitamin B12 test: Evaluates the amount of vitamin B12 in the blood. Helps to diagnose a particular anaemia type and other issues. You need to notify your doctor about all the medications you are currently on, as some can interfere with the test results.

Recommended fasting time for the test: 6-8 hours.

  • Gamma-Glutamyl transferase (GGT): The test is prescribed to show the GGT enzyme levels in the system. A higher reading indicates liver disorder, alcohol abuse or bile duct issues. You may be asked by the healthcare professional to avoid drinking alcohol and taking certain prescription medications a day ahead of the test to prevent them from altering the GGT levels.

Recommended fasting time for the test: Minimum 8 hours.

Important thing and misconception is that fasting will reduce blood sugar report , it doesn’t. HbA1c shows average blood sugar of 3 months.

Dr. M.G. Kartheeka, MBBS, MD

Also Read: CRP Blood Test Cancer: Understanding its Significance in Diagnosis and Treatment

Why is it important to fast before giving a blood sample?

Usually, your doctor or the lab professional will ask you to fast before a test, meaning that they will advise you to refrain from eating or drinking anything besides water for several hours (typically 8+ hours). This is because whatever you consume, solids or liquids are absorbed into the bloodstream, which affects the results of certain blood test types. This is why the patient is asked not to consume anything before the test starts the night before.

Tips to remember when fasting for a blood test

  • Samples are collected after 10-12 hours of overnight fasting.
  • The hours you spend sleeping are considered part of fasting, so schedule the test at the earliest.
  • Only a clear fluid like water can be consumed prior to the test. Water hydrates the body and makes the vein plump.
  • Drink several glasses of water prior to the test to make it easier to draw the blood
  • Inform your doctor about routine medications that you may be taking. Do not skip any medicine unless advised by your doctor. Diabetics on oral or injectable medications should consult the doctor before continuing medications prior to the test.
  • Even drinking black coffee or black tea without sugar can interfere with blood test reports.

Tips in case your child is fasting:

Ø Schedule tests as early as possible.

Ø Pack a snack for your child to have immediately after the test.

Ø If your child has already consumed something prior to the test, best to reschedule the test.

Tips in case you are pregnant:

Ø Inform the doctor that you are pregnant when they are prescribing a blood test.

Ø When you are pregnant, fasting is safe if you are not having a high-risk pregnancy.

Ø If the weather is sweltering, the doctor may ask you to drink lots of water and remain indoors.

Ø In case you are experiencing heartburn at the time of pregnancy, inform your doctor immediately.

Other tips to keep in mind:

Ø Don’t consume high-fat foods the night before.

Ø Do not drink alcohol.

Ø Do not smoke.

Ø Do not engage in a strenuous exercise until the test is over.

Also Read: Does Black Coffee Reduce Weight: Research-Based Insights into Coffee’s Role in Weight Loss


Getting ready for a blood test is not something to worry about. Like any routine test, it involves following a few simple guidelines. Some tests would require you to fast for 8-12 hours and some might demand shorter fasting periods. Everything will be informed to you by either your doctor or your healthcare professional at the lab prior to the test. While being prescribed a test by a doctor, ensure to notify them if you have any pre-existing condition or you are on any ongoing medications or herbal supplements to ensure that your test results are as accurate as possible. 

Also Read: Signs to Stop Water Fasting: Boundary Between Health and Harm

Disclaimer: The information provided here is for educational/awareness purposes only and is not intended to be a substitute for medical treatment by a healthcare professional and should not be relied upon to diagnose or treat any medical condition. The reader should consult a registered medical practitioner to determine the appropriateness of the information and before consuming any medication. PharmEasy does not provide any guarantee or warranty (express or implied) regarding the accuracy, adequacy, completeness, legality, reliability or usefulness of the information; and disclaims any liability arising thereof.

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