In some cases, fasting before a blood test is important because nutrients from food — including carbohydrates, fats, minerals, proteins, and vitamins — are absorbed into your bloodstream and can affect the results.
Your healthcare provider may have asked you to fast before a blood test, but it’s likely you’ll have questions like how long you need to fast, whether there are certain foods you might be able to eat, and if you’re allowed to drink anything. You could also have added concerns about pregnancy, what fasting means for any medication you might already be on and what you to do if it’s your child who needs to have a fasting blood test.
In this article, we’ll answer all these questions and more, so you’re less likely to have to go back to your healthcare provider for a repeat test.
How long should I fast before a blood test?
How long you need to fast before a blood test depends on what you’re being tested for — and you should always check this with your healthcare provider — but for most tests, it’s eight hours, with some requiring you to fast for up to 12 hours.
If you have to fast before a blood test, try to schedule your test for the morning, as the hours you spend sleeping will count towards your fasting period and make it easier for you to go without food. Continue reading to find out more about fasting before blood tests. Which blood tests require you to fast beforehand?
As stated above, you don’t need to fast before all blood tests, but some of the ones you’re likely to have to fast for are:
- Basic or comprehensive metabolic panel
- Fasting blood glucose test
- Cholesterol test or lipid profile
- Gamma-glutamyl transferase (CGT) test
- High-density lipoprotein (HDL) level test
- Lipoprotein panel
- Liver function test
- Low-density lipoprotein (LDL) level test
- Other nutritional tests
- Renal function panel
- Triglyceride level test
There are some blood tests, like a fecal occult blood test, which don’t require fasting, but you are limited with what foods you are allowed to eat. Also, if you’re not on statin medications for cholesterol, you might not have to fast. The hemoglobin A1c test for diabetes doesn’t require fasting either, as it assesses blood sugar control over a number of months.
Note that the type of fasting required for blood work isn’t the same as the NPO (nothing by mouth) after midnight order that healthcare providers give before procedures. If your healthcare provider doesn’t mention fasting before a blood test, always ask whether you need to — just to be sure.
What happens if you don’t fast before a blood test?
Eating before a fasting blood test can give inaccurate results. Red meats and broccoli in particular can cause a false-positive result. Here are some examples of what could happen if you don’t fast before certain blood tests:
- Blood glucose test — Eating beforehand will raise your blood sugar
- Cholesterol panel — Eating beforehand can raise the triglyceride levels and LDL levels (bad cholesterol)
If you do accidentally eat something during the fasting window, it’s important to tell your healthcare provider. It’s likely they’ll have to reschedule your test, but in some cases, they might be able to still do the test and make a note of the food intake so the results take this into consideration.
Can I drink before a fasting blood test?
In most cases, it’s recommended that you drink water before a blood test, as dehydration can affect cholesterol tests, for example. Dehydration causes your veins to flatten and become less visible, which makes venipuncture more difficult. To stay hydrated, drink the recommended amount of water for your activity level and weight.
It’s fine to drink either tap or bottled water, but leave out the lemon. Club soda and seltzer should also be avoided, as should other carbonated beverages and juices. High quantities of sugar can interfere with a wide range of blood test results.
In some cases, you may be allowed to drink black coffee or tea without milk or sugar, however, this depends on the test. Coffee and tea contain caffeine and soluble plant matter which could skew your results. Also, coffee is a diuretic, so you may become dehydrated.
For some blood tests — particularly those that assess liver health or triglyceride levels — you may have to avoid alcohol for a full 24 hours and if you’re a smoker, check whether you should also refrain from smoking during the fasting period.
Again, it’s vital that you follow the advice given by your healthcare provider, as some blood tests require you to have a totally empty stomach, as you would for some surgical procedures.
Can I take medication before a fasting blood test?
Unless you’re specifically told not to, you should take any prescribed medications during your fasting period — but clarify this with your healthcare provider, to be on the safe side.
Vitamins and supplements can affect certain tests, so it’s likely you’ll have to hold off on taking these before having blood work.
Do I have to fast before a blood test if I’m pregnant?
Some blood tests require you to fast beforehand, even if you’re pregnant. For example, if you need to have the glucose challenge test to screen for gestational diabetes, you’ll only be allowed to consume a particular sugary beverage that contains glucose.
If you’re in good health and your pregnancy isn’t considered high-risk, it’s usually safe to fast while pregnant, but your healthcare provider may advise you to drink plenty of water and stay indoors, especially in hot or humid weather.
Always make sure your healthcare provider knows about your pregnancy before your blood test, and tell them immediately if you’re experiencing uncomfortable symptoms, such as heartburn.
What if my child has to fast before a blood test?
If your child’s paediatrician requires them to fast before a blood test, there are some things you can do to make this easier:
- Schedule their blood test for as early in the day as possible
- Distract them with games, toys or cartoons
- Bring a snack to the blood test appointment so they can eat as soon as it’s done
If they do eat something while you’re not looking, it’s best to reschedule the test to prevent an inaccurate reading.
Where can I get a blood test?
You can get a blood test at a range of healthcare facilities across the country, from hospitals to doctors’ surgeries. If you’re unable to see your regular doctor for a blood test, you can use an urgent care search tool to find your nearest urgent care center, where you can also have a blood test done.
Fasting before a blood test is important because the nutrients from food, which are absorbed into your bloodstream, can affect the results. How long you need to fast before a blood test depends on what you’re being tested for, but for most tests, it’s eight to 12 hours.
In most cases, it’s recommended that you drink water before a blood test as dehydration causes your veins to flatten and become less visible, which makes it harder to draw blood. Unless you’re specifically told not to, you should take any prescribed medications during your fasting period. If you’re in good health and your pregnancy isn’t considered high-risk, it’s usually safe to fast while pregnant.
If your child needs to have a blood test, you can make it easier by scheduling their blood test for as early in the day as possible, distracting them with games, toys or cartoons and bringing a snack to the appointment so they can eat as soon as it’s done. Note that this article is for information only and you should always talk to your healthcare provider if you are unsure about anything to do with your blood test.