If you’re not feeling well and your doctor isn’t sure why, he or she may order lab tests to help better understand what’s going on inside your body. One test they may order is a fasting lab test. Before this blood test, you may be asked to stop eating or drinking for a few hours before.
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.
When would I need a fasting lab test?
If you have one or more of these symptoms, your doctor may order a fasting lab test.
- Excessive urination and thirst
- Swollen feet and ankles
- A high blood pressure reading
- A yellowish tint to your skin and/or eyes
- Fatigue and an irregular heartbeat
- Pale skin and leg cramps
- Non-specific symptoms that are difficult to diagnose
If you’re having symptoms that just don’t feel right, call or visit your doctor so they can request the appropriate tests.
If I need a fasting blood test, am I allowed to eat or drink anything?
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.
Why do I have to fast?
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:
- Anemia (iron blood test)
- Diabetes (blood glucose test)
- Electrolyte disorder (metabolic test)
- High cholesterol (blood cholesterol test or lipid profile)
- Kidney function (renal function panel)
- Liver disease (gamma-glutamyl transferase test)
- Vitamin B12 levels
What happens if I don’t fast before a blood 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.
When can I eat or drink after my lab test?
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.
When will I get the test results?
Most fasting lab results are available for your doctor in about 24 hours. However, some tests may take longer to complete. The lab will provide a report to your doctor, and your doctor will use the report to help with next steps in your diagnosis and treatment.
Learn more about the lab services we offer at Bon Secours.