We all have come across some situation in our lives where we are required to undergo blood test – either as a part of a normal checkup or if we are unwell. You may notice, that your clinician advice you to visit the laboratory on an empty stomach. Your physician may advise you some hours of fasting before calling you in to draw blood.
Fasting before a test means that you are not supposed to eat or even drink water for about 8-12 hours before your blood test. You are also not allowed to smoke or chew gum.
Why do they ask you to fast? Which are the blood tests require fasting? What happens if I don’t fast? and can we take any medications before fasting blood test?– These are some questions clinicians always come across. This article will feed your curiosity regarding fasting for blood tests and will give you an insight into which blood tests require fasting and for how long.
Why Do Some Blood Tests Require Fasting?
Certain blood tests require you to fast for 8-12 hours. The reason for asking you to fast is because when eat or drink beverages, certain nutrients, substances or vitamins enter into the bloodstream and can be responsible for giving a wrong result. Fasting before these advised blood tests ensures more accurate results.
Certain blood tests require you to not even drink water. Otherwise, drinking normal water will keep you hydrated and prevent your veins from collapsing, so drawing blood becomes easier since your veins are visible.
Avoid food, alcohol, soda or aerated drinks, tea, coffee, chewing gums and even exercising while fasting.
You need to consult your physician if you can take your regular dose of medications (except for multivitamins and dietary supplements) during fasting or adjust the dose timings for the purpose of fasting. This is because certain drugs can alter test results.
What Happens If You Don’t Fast Before Blood Test?
Not fasting for a blood test will yield inaccurate results. As mentioned above, as the food is digested and metabolized, the nutrients which enter the bloodstream can interfere with test results.
If you happen to accidentally eat or drink something before a blood test, make sure to inform your clinician about it. In this case, both the clinician makes a note of it and test results are analyzed accordingly or you for better results, an appointment for the blood tests may need to be rescheduled.
Which Blood Tests Require Fasting?
Blood tests let your clinician understand your health status and how well your body is working. The following list of blood tests requires fasting:
9 – 12 hours fasting
| Sr. No. | Blood test | How long should you fast before test |
|---|---|---|
| 1 | Blood Glucose (Fasting) | |
| 2 | Lipid Profile | 12 hours fasting |
| 3 | Liver Function Test | 8 – 12 hours fasting |
| 4 | Renal Function Test | 8 – 12 hours fasting |
| 5 | Tests For Iron | 12 hours fasting |
| 6 | Thyroid Function Test | 12 hours fasting |
| 7 | Gamma Glutamyl Transferase | 8 – 10 hours fasting |
| 8 | Glucose Tolerance Test | 8 – 12 hours fasting |
1. Blood Glucose Fasting
Any food or beverage increase blood sugar levels. This test is especially important in diabetic individuals and also as a part of screening for regular health checkups. Fasting, therefore, gives a baseline estimate of your blood sugar levels after fasting.
2. Glucose Tolerance Test
In this test, a test for blood sugar is done on a fasting stomach. After that, a measured dose of glucose is given to be taken within 5 minutes. Blood sugar levels are done at a regular interval of 2 hours. Another variation for this is an oral glucose challenge test done usually in pregnant women which does not require fasting.
3. Gamma Glutamyl Transferase
This is an enzyme is found in all parts of the body, more abundantly in the liver. It is frequently used as a mirror for liver diseases. GGT is elevated after the consumption of alcohol and certain drugs like NSAIDs, aspirin, phenytoin, and barbiturates.
4. Lipid Profile
This blood test is done to analyze levels of cholesterol and triglycerides in your blood. Fasting before blood cholesterol levels will give accurate baseline readings. Again, for this test, if you end up eating something before the blood test, fatty substances from your food will enter into the bloodstream and may give you higher readings.
5. Liver Function Test
Your liver works as a factory inside your body. It is essential for the formation of enzymes, hormones, fats, proteins, and nutrients as well as for the metabolism of all of these. Getting accurate readings is therefore essential for a liver function test.
6. Thyroid Function Test
Release and circulation of the thyroid hormones depend on a lot of dietary factors as well. Nutrients from your food may interfere with test results if you happen to eat something before the blood test.
7. Renal Function Test
This test is an estimate of how well your kidneys are functioning in terms of blood levels of urea and creatinine. The main role of kidneys is to excrete waste from the body via urine. Eating before a blood test for RFT will cause the breakdown of food substance and an increase of excretory substances in the bloodstream.
Some laboratories also perform serum electrolytes as a part of the renal function test. Eating salty food or fruits will show a temporary rise in sodium and potassium; whereas the base counts may be much different when taken after fasting.
8. Iron Test
This test is particularly done in anemic subjects. The reason to be fasting before this blood test is that iron is one of those substances which are absorbed very quickly into the blood. Not fasting before a blood test for iron will show elevated levels of iron in the blood.
Another care to be taken is to not take any multivitamins or iron supplements at least 24 hours before a blood test for iron.
These are some of the common blood tests done as a part of the screening. Other blood tests done in special cases which require fasting are:
C-peptide test to detect insulin resistance, to know if you have type I or II diabetes8 – 12 hours fasting
| Sr. No. | Name of test | Indication | Duration of fasting |
|---|---|---|---|
| 1. | Lactose tolerance test | For patients who are unable to tolerate milk and dairy products | 8 – 12 hours fasting |
| 2. | |||
| 3. | Vitamin B12 and folate | To rule out nutritional and vitamin deficiency | 8 – 12 hours fasting |
| 4. | Test for gastrin | To assess levels of the hormone gastrin and severity of gastric illness. | 8 hours fasting |
Blood tests other than the ones mentioned above do not require fasting and can be done at any time of the day.
Schedule your blood test preferably in the early morning. Because your hours of sleep are also included in your fasting duration.
How Long Should I Fast Before Blood Test?
You are required to fast for up to 12 hours before going for a blood test. Drinking coffee, tea or even water can affect blood test results.
Avoid Over Fasting, this particularly applies to diabetic individuals and pregnant women. Over fasting can cause dehydration and hypoglycemia in diabetics. Over fasting can also sometimes alter test results.
Make sure to carry some snacks to your physician’s office so that you can eat immediately after blood is drawn. Blood glucose for postprandial levels requires you to eat and give a blood sample after an hour of a meal.
Hopefully, this article has given you a good idea about fasting for blood tests and which blood tests require fasting. These questions should additionally be clarified with your physician. This is because your physician will be able to guide you better.
Medically Reviewed By
Dr. Himanshi is a Homoeopathic consultant and currently working as a lecturer in Post-graduate faculty of Homeopathy, Parul University, Vadodara. Completed BHMS and MD in Homeopathy in January 2018 and also has a clinical experience of about 6 years. Personal interests include reading, spending time with family and traveling.
