Ever booked a lab test and then panicked the night before, wondering,
“Wait… was I supposed to fast?”
You’re not alone. Fasting before a blood draw is one of those little medical to-dos that’s easy to overlook — but it can make a big difference in your results.
Why Fasting Matters
When you eat, your body starts processing nutrients — sugars, fats, proteins — and those get absorbed into your bloodstream. That’s normally great, but not so great if your doctor wants a clear, unfiltered look at what’s going on inside.
Fasting gives your blood a clean baseline. No breakfast, no juice, no sneaky mint — just you, your water bottle, and a little patience.
According to the Centers for Disease Control and Prevention (CDC), fasting before certain blood tests, like glucose tests, ensures accurate results by preventing recent food intake from affecting your blood sugar levels
If you’re doing a home blood draw through a service like myOnsite Healthcare, you’ll usually get a heads-up the day before so there’s no confusion.
How Long Do You Really Need to Fast?
Here’s a quick cheat sheet:
| Test Type | Fasting Required? | How Long? |
| Fasting Blood Glucose | Yes | 8 hours |
| Lipid Panel (Cholesterol) | Often yes | 9–12 hours |
| Basic Metabolic Panel (BMP) | Usually yes | 8–12 hours |
| Iron, GGT, or Vitamin B12 | Depends on the test | Often 8–12 hours |
| CBC, A1C, Thyroid Panel | No fasting needed | Eat as usual |
Not sure what you’re getting tested for? Ask your doctor — or check your lab order.
For more detailed information on fasting requirements for various blood tests, you can refer to MedlinePlus, a resource provided by the U.S. National Library of Medicine.
What’s Allowed and Not During a Fast?
You CAN Have:
Plain water (drink plenty — it actually makes the blood draw easier!)
Avoid These:
- Food
- Coffee or tea (even black)
- Juice
- Gum or mints
- Smoking
- Working out
Pro Tip: Even black coffee can interfere. Caffeine stimulates your body and may skew test results.
Can I Take My Medications While Fasting?
In most cases, yes — take your regular medications, unless your doctor tells you otherwise.
But double-check if you’re on:
- Diabetes meds
- Blood thinners
- Supplements like iron
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) advises consulting with your healthcare provider about medication use before fasting blood tests to ensure accurate results and avoid potential complications.
You can always contact our team via this form if you’re unsure.
Forgot and Ate Something?
It happens. Don’t try to wing it. Just let the phlebotomist or lab know before the draw. Some tests might still work, others might need to be rescheduled.
And if you’re working with mobile phlebotomy pros, they’ll guide you through what’s next. We’ve seen it all — no judgment.
Schedule Your Blood Draw Early
Want to make fasting less painful?
Schedule your blood draw early in the morning. That way, you’re sleeping through most of the fasting window.
Then it’s just a quick visit, a little poke, and boom — time for breakfast.
(And yes, bring a snack. You’ve earned it.)
Fasting Doesn’t Have to Be Hard
Fasting before blood work isn’t about making your life harder — it’s about making your results accurate. And the good news is, it’s usually just one night of skipping snacks and sipping water.
If you’re getting your labs done with myOnsite Healthcare, we’ll send reminders so you know what to avoid and when to stop eating.
Our team has mastered the art of blood collection — right at your door.
Frequently Asked Questions:
- Can I drink water during my fast?
Yes — drink lots! It keeps your veins plump and your blood draws smoother. - Can I brush my teeth before a fasting test?
Yes, but don’t swallow toothpaste or mouthwash. - Is black coffee really off-limits?
Yes. Even without sugar, caffeine can mess with your test. - Can I take my morning medications?
Usually yes — but ask your doctor if you’re not sure. - What if I eat something by mistake?
Tell the tech. Depending on the test, they might need to reschedule. - How strict is the 8–12 hour rule?
Pretty strict. Set a timer or stop eating after dinner the night before. - Do I have to fast for an A1C test?
Nope! Eat normally. - What about vitamins or supplements?
Some can affect results. Check with your provider. - I have diabetes — is fasting safe for me?
Talk to your doctor first. They may adjust meds or timing. - How soon can I eat afterward?
Right after the draw. Pack a snack if you’re going somewhere afterward.