At your annual wellness check, your doctor ordered a comprehensive metabolic panel and instructed you to not eat before having your blood drawn. Unfortunately, before heading to the lab, you absentmindedly ate a piece of toast. Now what?
Believe it or not, this happens all the time. Plenty of people operate on autopilot until their morning coffee kicks in, at which point they realize they’ve broken their fast. To find out what you need to know, Bottom Line Health spoke to Joshua Hayden, PhD, section head of Clinical Biochemistry at Cleveland Clinic, Cleveland, Ohio.
Fasting for blood work
If you accidentally eat before a blood test, contact your healthcare provider’s office and explain what happened. There’s a good chance that your blood draw still can be conducted as scheduled, though it will depend on what test is being done and the specific reason why that test was ordered.
“Reference ranges” increasingly are available that allow clinical chemists to interpret blood test data for patients who didn’t fast pre-test. Example: A “random glucose” test can be used to measure the blood sugar levels of a patient who hasn’t fasted. It is crucial that the clinical chemist who interprets the blood test is aware that the patient didn’t fast before the blood draw, however. Make this clear both to your healthcare provider and to the lab where your blood is drawn.
Often the main downside of failing to fast is that the blood test might need to be repeated later—this time with fasting—if the initial test produces an unexpected result.
Common questions about fasting for bloodwork
What, if anything, can I eat and drink during a fast?
Avoid all food. Water is the only acceptable beverage.
What does the science say about how long to fast before blood work?
There’s surprisingly little data about the ideal fast length, in part because it can vary depending on the test being conducted as well as on the patient and his or her diet. Some patients have faster metabolisms than others, and some foods are metabolized faster than others.
Healthcare providers often err on the side of caution and recommend relatively long fasts of eight to 12 hours, even when shorter fasts could theoretically be sufficient. That’s why blood draws often are scheduled for the morning. Most of the fast time can occur while asleep. Warning: Do not dramatically exceed the fast length recommended by your healthcare provider. Over-fasting can affect blood test results, too. A few extra hours is unlikely to cause problems, but attempting a 24-hour fast could.
Can I take my medications, vitamins and supplements during a fast?
Some medications and supplements can interfere with certain test results, so discuss this with your healthcare provider. Make sure this healthcare provider is aware of any over-the-counter meds you take in addition to your prescription meds. You likely will be advised to continue taking crucial medications on schedule, but you might be asked to skip certain supplements during the fast. Example: Biotin (vitamin B7), which is included in many multivitamins, can cause clinically significant blood test errors.
Other than not eating, is there anything I should or shouldn’t do during a fast?
Don’t exercise. Vigorous or even moderate exercise can affect the results of blood tests.