FAQs On Fasting Before Blood Tests

FAQs_Doc Speaks
(Last Updated On: September 27, 2017)

Questions encountered frequently with respect to precautions and fasting before blood tests are given below for ready reference:

  1. Do I need to fast before all blood tests?

The need to fast or not before a blood test depends on the type of blood test a patient is being subjected to as some blood tests require fasting to be effective, while others don’t.

2. Why fasting is required?

When people eat food and drink alcohol, the food and liquid gets broken down in their stomach and absorbed into the bloodstream, which can affect the levels of certain substances in the blood. Eating and drinking can change the level of substances in the blood, including sugar and iron levels.

3. Which are the blood tests for which I need to fast ?

 The Blood tests that you need to fast for include:

  • blood glucose test
  • cholesterol test
  • liver function test
  • Iron blood test
  • lipoprotein panel
  1. Fasting blood glucose test

A fasting blood glucose test can diagnose diabetes as there is increased level of sugar in the blood. The patient has to observe fasting for 8 to 10 hours before a fasting blood glucose test is performed (or overnight fasting). Fasting helps ensure that the blood test records an accurate measure of fasting blood sugar levels.

  1. Blood cholesterol tests

High level of cholesterol, a fatty substance in the blood can lead to an increased risk of certain health conditions. Lipid profiles assess the quantities of different fats in the blood which include:

  • HDL cholesterol, also known as “good cholesterol”
  • LDL cholesterol, also known as “bad cholesterol”
  • triglycerides

The amounts of these fats will increase if a person has recently eaten food. So, people are asked not to eat for 9 to 12 hours before the test, which helps to give an accurate profile of the amounts of these fats in the blood.

They should refrain from drinking alcohol for 24 hours before the test.

  1. Liver function test (Gamma-glutamyl transferase test)

A gamma-glutamyl transferase (GGT) test. GGT is an enzyme in the liver that helps it to work effectively and measurement of its level helps to diagnose liver diseases.Drinking alcohol and smoking can affect GGT levels. People having this test are asked not to consume alcohol or smoke for 24 hours before the test.

4. Iron blood test

Iron is contained in some types of food and is absorbed very quickly from food into the blood. So, if a person eats food before the iron blood test, the results may show inflated levels of iron. In addition to this, Iron supplements and multivitamins should be avoided when fasting for a blood test, as they can affect the accuracy of the test results. If a person regularly takes these supplements, they should not take them for 24 hours before the iron blood test.

  1. Vitamin B12 test:

This test is performed to know the levels of the vitamin B12 in a person’s blood for which patients are asked to fast for 6 to 8 hours.

6. Should I need to fast a diabetes detection test like haemoglobin A1c?

This alternative test for glucose level does not require fasting as it measures a substance called hemoglobin A1c, which reflects average blood sugar over the previous three months.

7. How long should I fast before a blood test?

The amount of time you need to fast for will vary according to the test although for most of tests you should not consume anything but water for eight hours leading up to the test. For some tests, a 12-hour fast may be needed. Therefore ideally tests should be scheduled as early in the morning as possible on an empty stomach.

8. Is it okay to drink water before a blood test? 

It’s fine to drink water before a blood test, unless you’re instructed otherwise by your doctor. This is different from some surgical procedures, which may require you to have a totally empty stomach.However carbonated beverages, flavored or otherwise, shouldn’t be consumed during a fast.

9. Can I take coffee if I am fasting before a blood test?

Even if you drink it black, coffee can interfere with blood test results. That’s because it contains caffeine and soluble plant matter, which might interfere with your test results. Coffee is also a diuretic, which by increasing dieresis can have a dehydrating effect.

10. Can I drink alcohol if I am fasting before a blood test?

Some blood tests, such as those that assess liver health or triglyceride levels, may require you to not drink any alcohol for a full 24 hours. Alcohol can also affect blood sugar and fat levels, giving inaccurate results to blood tests that require fasting. If a person is being asked to fast before a blood test, they should refrain from drinking alcohol as well.

11. Can I take smoke if I am fasting before a blood test?

Smoking can also affect blood test results. If a person has been asked to fast before a blood test, they should avoid smoking.

12. Can I do my regular exercise if I am fasting before a blood test?

Exercise can also speed up digestion and may affect results, therefore patients should better avoid it for the recommended fasting period.

13. What are the guidelines for fast before a blood test for children?

Children may need blood tests that require them to fast beforehand just like adults. It is always wise to pack a snack for them to devour as soon as the test is done.

14. What about fasting for a blood test during pregnancy?

There are several blood tests may be needed during pregnancy to assess any potential health concerns that patient or baby might experience during pregnancy or after birth. The principles of fasting before tests are same as for a general patient.

15. What happens if I don’t fast before a blood test? Should I still do the test? 

If the patient doesn’t fast before a test that requires it, the results may not be accurate. If you forget and eat or drink something, be honest and tell the technician when you have your blood drawn. They may make a note so that the results are reviewed with the food intake as a variable and in case fasting is absolutely necessary for meaningful results, they may reschedule your blood test.

16. Should I take my regular medications before the test? 

Take your regular medications during fasting unless you are specifically told to take them only with food.


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Dr. Udaya Kumar Maiya is one of India's leading oncologists. He is a Director at the Bangalore Hospital as well as a Senior Consultant at Apollo Hospital, Bangalore, and the Bhagwan Mahavir Jain Hospital. Dr. Maiya focuses on Radiotherapy and Oncology and has published a large number of scientific articles on these topics and teaches at the postgraduate level in Bangalore.



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