Why fast for blood test?
Whether someone needs to fast or not before a blood test depends on the type of blood test they need to undergo. Some blood tests require fasting to provide accurate results, while others do not.
The types of blood tests that require fasting are:
A fasting blood glucose test can help diagnose diabetes.
Diabetes is a condition that can lead to excessive amounts of sugar in the blood. A fasting blood glucose test measures levels of sugar in the blood to see whether they are healthy.
It is important that an individual has not had anything to eat or drink other than water for 8–10 hours before a fasting blood glucose test. A person will typically fast overnight and do the test early in the morning.
Fasting helps ensure that the blood test records an accurate measure of fasting blood sugar levels. The results help a doctor diagnose or rule out diabetes.
Cholesterol is a fatty substance in the blood. High cholesterol can lead to an increased risk of certain health conditions.
Blood cholesterol tests, also known as lipid profiles, assess the quantities of fats in the blood. The different fats tested for include:
The amounts of these fats will increase if a person has recently eaten food. That is why people are asked not to eat for 9–12 hours before the test, which helps give an accurate profile of the amounts of these fats in the blood.
Research has suggested that fasting may not be necessary before all cholesterol and triglyceride tests. However, people who are having these tests should refrain from drinking alcohol for 24 hours before the test. It is always best for individuals to check with a doctor to see whether these new guidelines apply to them.
A gamma-glutamyl transferase (GGT) test helps diagnose liver disease. GGT is an enzyme in the liver that helps it work effectively.
A person may need to avoid all eating and drinking for 8 hours before the test, as well as avoid drinking alcohol and taking some prescription medications.
An iron blood test measures the levels of the mineral iron in the blood. This test helps identify conditions that are caused by a lack of iron in the blood, such as anemia.
Iron is contained in some types of food and is absorbed very quickly from food into the blood. Therefore, if a person eats food before the iron blood test, the results may show inflated levels of iron.
To ensure accurate results, a person should fast from midnight the night before the test until after the test is done.
People may be asked to fast for:
When you eat and drink normally, those foods and beverages are absorbed into your bloodstream. That could affect the results of certain types of blood tests.
It's also recommended that you avoid alcohol for 24 hours before your test, as well as any strenuous exercise.
Food, most drinks, and strenuous exercise can all lead to inaccurate blood test results. If the results of the blood test are unclear, the process will have to be completed again to get numbers that reflect your true state of health.
In most cases, water (still or sparkling) may be consumed before a fasting blood test. If you’re currently taking any medications, check with your doctor to see if you can take your regular dosage without affecting the results.
Fasting actually affects the results of very few blood tests.
For example, measurements of kidney, liver, and thyroid function, as well as blood counts, are not influenced by fasting.
Fasting is required before commonly ordered tests for:
You should fast for up to 8 hours before having your blood glucose measured to check for diabetes or to see how well treatments are working. An alternative test for glucose level that does not require fasting measures a substance called haemoglobin A1c, which reflects average blood sugar over the previous 3 months. However, a blood glucose test is often considered more accurate and appropriate in specific circumstances.
Doctors measure triglycerides after fasting, since the concentration of these fatty particles remains elevated in the bloodstream for hours after a meal. The healthy range of triglycerides is less than 1.7 mmol/L after a fast of 10 to 12 hours.
An accurate triglyceride test result is also important because doctors use this number to calculate your level of LDL (bad) cholesterol. If you eat or drink before a standard lipid panel blood test, it skews both the triglyceride and LDL numbers. In contrast, eating does not significantly affect measurements of total cholesterol and HDL (good) cholesterol.
You should fast for 8 hours before having an iron level blood test. This is to ensure that levels of iron detected in your blood are not skewed by food or drink intake.
If you’re having a blood test for ferritin levels – a measure of how much iron is stored in the body – you don’t usually need to fast before.
There is a range of things that you can do to make sure your fasting blood test goes as smoothly as possible. These include:
It’s important to drink lots of water when fasting to stay hydrated. Water doesn’t affect the results of a blood test and is perfectly fine to drink when asked to fast. Other liquids, such as fruit juice, coffee, teas, and sodas can adversely affect the results of a blood test as they impact digestion, so stick to just still or sparkling water.
If you’re well hydrated, it also helps your doctor or nurse find a vein to draw your blood, making your test go smoothly.
Whether you’re asked to fast for 8, 12, or even 24 hours, it’s a good idea to work out what is the latest time you can eat or drink before the test. For example, if a person is asked to fast for 12 hours before a blood test at 10 a.m., they should not eat anything after 10 p.m. the night before.
It’s a good idea to schedule your blood test as early in the day as possible. This means much of your fasting time will be spent asleep, making it much easier. It’s also preferred to have a blood test for anaemia early in the day, as iron levels tend to fluctuate throughout the day.
Unless your doctor has told you otherwise, make sure you continue to take any medication during your fast.
We do not routinely recommend that diabetics fast for their blood tests because they are at higher risk of experiencing negative effects of fasting.
It’s usually okay for pregnant women to fast. However, it’s important to do it safely so we recommend getting advice from a doctor.
Before beginning your fast, be sure to eat a nutritious, balanced diet full of lean proteins, fruits, vegetables, and plenty of whole grains. This can help to stave off hunger and energy loss during the fast.
As well as food and drink, there are some other things to avoid when fasting for a blood test. These include:
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 also refrain from drinking alcohol.
Smoking can also affect blood test results. If a person has been asked to fast before a blood test, they should avoid smoking. If you vape, it’s fine to continue doing this during your fast.
Chewing gum, even sugar-free gum, should be avoided when fasting for a blood test. This is because it can speed up digestion, which can affect results.
Exercise can also speed up digestion and affect results, so people should avoid it for the recommended fasting period.
At Nuffield Health we offer a wide range of blood tests including ones that require fasting.
Our range of in-depth Health Assessments include a variety of common blood tests that will give you a clear picture of your overall health and fitness.
- blood glucose test.
- liver function test.
- cholesterol test.
- triglyceride level test.
- high-density lipoprotein (HDL) level test.
- low-density lipoprotein (LDL) level test.
- basic metabolic panel.
- renal function panel.
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Blood tests are an important medical tool that offer a wide range of applications. They can evaluate current health, identify potential infections, and help to support diagnosis when used with other tests. However, depending on the blood test, your healthcare provider may ask that you go through a fasting period prior to getting blood drawn. Wondering, "Why fast before a blood test?" Read on to find out more.
Fasting refers to avoiding all food and drinks, except for water, for a certain amount of time. When your healthcare provider asks you to fast, you cannot eat or drink anything other than water for several hours prior to getting your blood drawn .
Although your digestive system might seem entirely separate from your blood, the two are intrinsically connected. Everything you eat and drink gets broken down into its component parts. The vitamins and nutrients then get absorbed into the bloodstream via the digestive system. This could then affect certain types of blood tests .
The good news is that fasting for blood tests is becoming rarer. Most routine blood level tests, including thyroid tests, CBC tests, and tests for kidney and liver are not affected by fasting . Additionally, some at-home blood tests from Everlywell, such as Nutritional & Digestive Health tests, do not require fasting but only a quick finger prick for sample collection.
Glucose tests tend to be the most common tests that require fasting prior to getting blood drawn. They measure blood sugar levels, which can naturally fluctuate based on what you have eaten and how recently you’ve eaten. For a glucose tolerance test, you may need to fast for up to eight hours before the test. Once you arrive at the site, you will get your fasting blood tested, after which you will drink a liquid containing glucose. An hour later, you will get your blood drawn again. This may continue two hours later and three hours later for a more accurate result .
Lipid profile tests may also require fasting. Lipid profile tests measure cholesterol and a type of fat found in the body called “triglycerides.” Cholesterol comes in two forms, HDL and LDL. Generally, high levels of LDL cholesterol (considered the “bad” cholesterol) combined with high triglycerides can increase the risk of heart disease and other disorders .
If you are asked to fast prior to a blood test, your healthcare provider will typically give you more specific instructions. In general, you may not eat or drink anything other than water for eight to 12 hours before the test. That includes drinking alcohol, coffee, juice, and/or other beverages as they can all potentially alter test results. Your healthcare provider may also recommend that you avoid smoking, exercise, and chewing gum during your fast .
Thankfully, most tests that require fasting are scheduled in the morning, meaning that most of the fasting period happens overnight, while you are asleep. Once the blood has been drawn, you can return to eating and drinking normally, so you may want to bring a snack with you to eat right after your test is over .
Fasting is generally not common for most blood tests, but your healthcare provider may ask you to fast for certain blood tests. Make sure you follow instructions and ask for any clarification prior to your test.
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While not all lab tests require it, some tests require fasting for the most accurate results. Most of the time, this means you’ll be asked to stop having any food or liquid between eight to 24 hours before your lab test or bloodwork, depending on the test.
If you have one or more of these symptoms, your doctor may order a fasting lab test.
If you’re having symptoms that just don’t feel right, call or visit your doctor so they can request the appropriate tests.
Some tests let you have a few sips of water. However, other tests don’t even allow water. You should avoid coffee and gum before a fasting lab test because both substances affect your digestive system and can change the results of your test. You should also avoid alcohol, smoking and exercise.
If you take medications, continue taking them as normal unless your doctor says otherwise. If you take vitamin/mineral supplements, ask your doctor whether you should continue to take those.
Food and drinks can affect your results because sugar, fats, enzymes, minerals and cholesterol all absorb into your blood stream. Affected results could then lead to an inaccurate diagnosis.
These are some conditions you may be tested for that can require a fasting lab test:
If you don’t fast before a test that requires it, the results may not be accurate. If you forget and eat or drink something, call your doctor or lab and ask if the test can still be done. They can then tell you if you need to reschedule your test.
It’s important to follow fasting instructions before a lab test because a wrong or inaccurate test can lead to more health problems or missed diagnoses. Be honest with your doctor or lab technician if you mistakenly ate or drank.
Don’t wait until the day of your blood test to ask if you should fast as some tests require you to fast for 12 hours or more. Thus, you’ll want to know if your test requires fasting with enough time to follow the fasting instructions.
Once you’ve had your blood drawn, you can eat and/or drink anything immediately. In fact, it’s a good idea to bring a snack or drink to your appointment.
You may also want to schedule your blood test for the early morning to minimize the length of time you’ll go without food.