Why Fasting Before a Blood Test Matters and When It Doesn't
- Category: Educational, Primary Care, Information, Laboratory
- Posted On:
If you have ever been told to fast before a blood test, you may have wondered: Do I really need to fast? What can I drink? How long do I have to go without eating?
The answer depends on the blood tests being performed.
Some lab tests can be affected by what you eat and drink before your appointment, while many others do not require fasting at all. Knowing the difference can help you prepare correctly and avoid having to repeat a test.
As you schedule preventive appointments, it is a good time to make sure you know what to expect before your next blood draw.
RELATED BLOG: How Your Primary Care Doctor Helps Protect Your Heart
Why does fasting before a blood test matter?
Eating and drinking can temporarily change the levels of certain substances in your blood.
For example, a recent meal can affect triglycerides, which are a type of fat found in your blood. It can also affect some cholesterol measurements and blood sugar levels.
“Following fasting instructions helps make sure your results are as accurate and consistent as possible,” said Katie Hoff, DHA, MLS(AMT), Manager, Lab Outreach and Outpatient Services at Norman Regional Health System.
If your doctor tells you to fast before your blood test, it is important to follow those instructions. Eating before a test that requires fasting could affect the results and may mean you will have to repeat the test.
Which blood tests require fasting?
Whether you need to fast depends on the tests your physician orders and what they are trying to evaluate.
Fasting may be recommended for certain tests involving:
- Triglycerides: A type of fat in the blood that can increase after eating.
- LDL cholesterol: Often called “bad” cholesterol, LDL can be affected by whether you have recently eaten.
- VLDL cholesterol: Another type of cholesterol that can be influenced by recent food intake.
- Glucose: Blood sugar measurements can be affected by eating, depending on the type of glucose test being performed.
- Some metabolic testing: Certain metabolic panels may be ordered with fasting instructions depending on what your doctor is evaluating.
Not every cholesterol or blood sugar test requires fasting, however. Your physician will determine whether fasting is needed based on the specific tests ordered and the information they need.
Do you need to fast before every blood test?
No.
Many common blood tests are not significantly affected by whether you have recently eaten.
For example, fasting generally is not required for tests such as:
- A1c, which measures your average blood sugar levels over the past two to three months
- eGFR, which helps estimate how well your kidneys are filtering blood
- hsCRP, a test that measures a marker associated with inflammation
- Lp(a), a type of cholesterol-related particle used to help assess cardiovascular risk
- HDL cholesterol, often called “good” cholesterol
- Total cholesterol
Some tests can be performed whether you have eaten or not, while others may have different preparation requirements depending on why they are being ordered.
That is why it is important not to assume that you need to fast, or that you do not need to fast, based solely on the name of the test.
Always follow the specific preparation instructions provided by your doctor or laboratory.
RELATED BLOG: New Blood Test to Detect Concussions, Norman Regional First in Oklahoma to Offer
What does fasting before a blood test actually mean?
In a clinical setting, fasting generally means not eating food or drinking beverages other than water for a specified amount of time before your blood draw.
Ask your doctor or call the laboratory well in advance to make sure you know how long you need to fast. The length of time can vary depending on the tests being performed.
During your fasting period, water is usually allowed and encouraged unless you have been specifically instructed to restrict fluids.
Avoid beverages such as:
- Coffee
- Tea
- Soda
- Juice
- Energy drinks
- Sports drinks
- Alcohol
Even beverages that do not seem like food can contain sugar, calories or other ingredients that may affect certain test results.
If you are unsure whether a beverage, medication or supplement is allowed before your blood test, it is better to ask before your appointment rather than guessing.
When you call Norman Regional Laboratory Services, a real person answers – no automated call trees or long hold times. Our staff is available 24/7 to assist with scheduling, test results, and any questions you may have. Call 405-307-1100.
How can you make fasting before a blood test easier?
Fasting can sound more difficult than it actually is, especially when you are able to schedule your blood draw early in the morning.
Here are a few ways to make it easier:
Schedule an early-morning appointment
An early appointment means much of your fasting period can happen while you are asleep. You can have dinner the night before, follow your fasting instructions overnight and complete your blood draw in the morning.
Drink water
Staying hydrated can make the blood draw easier by helping your veins remain easier to access. Drink plenty of water unless you’ve been instructed to limit fluids.
Bring a snack
Pack something to eat for after your blood draw, especially if you have a longer drive or other plans afterward.
Follow your instructions exactly
If you were given specific instructions about fasting, medications or other preparation, follow those instructions. If something is unclear, contact your doctor or the laboratory before your appointment.
What do blood tests look for?
Blood tests can provide information about many different aspects of your health.
Depending on the tests ordered, laboratory testing may help evaluate:
- Blood sugar
- Cholesterol and triglycerides
- Kidney function
- Inflammation
- Blood cell counts
- Liver function
- Other markers related to your health
Blood tests may be part of routine preventive care, or they may be ordered when you have symptoms, are monitoring an existing condition, are checking how a treatment is working or have risk factors for certain diseases.
The tests you need and how often you need them depend on your individual health history and needs. Your healthcare team can help determine which testing is right for you.
RELATED BLOG: A Women’s Health Check-Up List for Every Age
What happens if a blood test result is abnormal?
An abnormal lab result does not automatically mean that something is wrong.
Laboratory results are typically compared with established reference ranges, which are ranges used to help interpret test results. A result outside that range may be worth discussing with your physician, but it needs to be considered in context.
Your doctor may look at your:
- Medical history
- Symptoms
- Medications
- Previous lab results
- Other test results
- Overall health
“Looking at the overall clinical picture is often more useful than focusing on a single number,” Hoff said.
Depending on the result, your doctor may recommend repeating the test, ordering additional laboratory testing, making lifestyle changes, adjusting medications or performing additional evaluation.
For example, if certain cholesterol or triglyceride levels are unexpectedly elevated after a non-fasting blood draw, your doctor may recommend repeating the test while fasting.
Do you need a doctor to order a blood test?
Not necessarily.
Many laboratory tests are ordered by a physician or other advanced practitioner, but Norman Regional also offers direct-access testing through MDSave.
With direct-access testing, patients can choose and purchase eligible laboratory tests through MDSave and see the cash-pay price upfront. After purchasing a test, patients can take their voucher to a Norman Regional lab patient service center to have their blood collected and the testing performed.
Find a Norman Regional Lab Near You
Before your next blood draw, know your fasting instructions
Fasting before a blood test is not a requirement for every type of laboratory testing. Some tests can be affected by recent food or drink, while many common tests do not require fasting.
The most important thing is to know the preparation instructions for the specific tests you are having done.
If fasting is required, an early-morning appointment, plenty of water and a snack waiting for afterward can make the process easier.
And if you are not sure whether you need to fast, do not guess. Check with your healthcare team or laboratory before your appointment so you can get the most useful results from your blood test.
