Do You Need to Fast Before Bloodwork? Rules for Every Common Blood Test

The Quick Answer: Most Blood Tests Don't Require Fasting

Most routine bloodwork doesn't require fasting at all. A complete blood count (CBC), thyroid testing (TSH), kidney and liver function, electrolytes, hemoglobin A1C, and most STI blood tests can be done after a normal breakfast. You'll get accurate results either way.

Only a few tests truly need an empty stomach. The main ones are a fasting blood glucose test, a glucose tolerance test, and a cholesterol panel when your provider asks for a fasting sample. For these, you'll usually need to skip food and drinks other than water for 8 to 12 hours. Your requisition will normally say "fasting" if it matters. If it doesn't say so and you're unsure, call the clinic that ordered the test before you skip a meal you don't need to skip.

You can almost always drink plain water before a blood test, including a fasting one. It's actually encouraged. Good hydration makes your veins easier to find and can make the draw quicker. So let's go through the tests where fasting really does change your results.

Which Blood Tests Require Fasting?

Fasting Blood Glucose

This test measures your blood sugar after your body has gone without food for a while. It's one of the ways providers screen for prediabetes and type 2 diabetes. Food raises your blood sugar for hours, so eating beforehand can make your numbers look higher than they really are. Diabetes Canada defines fasting as no caloric intake for at least 8 hours. Most people find it easiest to book an early morning draw and fast overnight.

Fasting Cholesterol Test (Lipid Panel)

This one surprises people. For years, everyone fasted before a cholesterol test. Current Canadian cardiovascular guidelines say a non-fasting lipid panel is fine for most adults. Your total cholesterol, LDL, and HDL barely change after a meal. Triglycerides can rise after eating, but usually not enough to change your risk assessment.

Your provider may still ask for a fasting cholesterol test in some cases. That can happen if your triglycerides were high on a previous test, if you're being monitored for a lipid disorder, or if a non-fasting result came back unclear. If your requisition asks for a fasting sample, fast for 10 to 12 hours. Also skip alcohol for about 24 hours beforehand, since alcohol can push triglycerides up. If you'd like help reading your numbers afterward, our guide to blood test results explained, including your CBC, cholesterol, and glucose numbers covers what each value means.

Glucose Tolerance Test

A standard oral glucose tolerance test requires fasting. You'll have a baseline blood draw, drink a sugary solution, and have your blood drawn again after a set time. Plan to be at the lab for about two hours. The one-hour screening test often used in pregnancy is usually done without fasting. Follow the instructions on your specific requisition, because the two tests are easy to confuse.

Tests Where Timing Matters More Than Food

Some tests don't strictly require fasting but work best at a certain time of day. Testosterone is usually drawn in the morning, when levels are highest. Some providers prefer a morning sample for iron studies because serum iron naturally changes over the day. If you're on a medication that needs level monitoring, your provider may ask you to time the draw around your dose. These details are easy to miss, so ask when your test is ordered.

Can You Drink Water, Coffee, or Take Medications Before a Blood Test?

Water: Yes. Drink plain water before any blood test, fasting or not. Sparkling water without flavour or sweetener is generally fine too. Anything with calories, sugar, or flavouring counts as breaking your fast.

Coffee and tea: For non-fasting tests, go ahead. For fasting tests, skip them, even black. Caffeine can affect blood sugar readings slightly, and most people don't drink their coffee truly plain. The same applies to gum, mints, and juice. Save your Tim Hortons run for after the draw.

Medications: In most cases, you should take your regular medications as usual, with water, even before a fasting test. There are some important exceptions. If you manage diabetes with insulin or certain oral medications, fasting can drop your blood sugar too low, so talk to your provider about how to adjust before the day of your test. Biotin, a common hair and nail supplement, can interfere with some thyroid and hormone tests, so let your provider know if you take it. If you take other medications, your provider can help you plan timing so your results stay accurate and your treatment stays safe.

If you're pregnant, breastfeeding, or planning a pregnancy, check with your provider before starting or changing any medication, including around a blood test. When you book bloodwork through Care&, your NP will tell you ahead of time whether you need to fast and how to handle your medications, so you're not guessing the night before.

Once you know whether to fast, a few simple habits can make the draw itself easier.

How to Prepare for Bloodwork So It Goes Smoothly

A little preparation goes a long way, especially if you've ever felt woozy in the chair or needed more than one poke. These tips help whether or not you're fasting:

• Drink a couple of glasses of water in the hours before your draw.
• Book an early morning slot for fasting tests so most of your fast happens while you sleep.
• Wear a short-sleeved shirt or layers that roll up easily. In a Toronto winter, that means a layer under your parka.
• Avoid a hard workout the day before, since intense exercise can temporarily change some results, including muscle and kidney markers.
• Pack a snack to eat right after a fasting test.
• Tell the person drawing your blood if you tend to faint so you can lie down.
• Bring your requisition and health information, and note any supplements you take.

Don't fast longer than you're told. Going past 12 to 14 hours can actually skew some results and leave you lightheaded. If your fasting test gets delayed, have a sip of water and let the staff know how long it's been since you ate.

Children's bloodwork follows many of the same principles, but fasting rules and preparation can differ for kids. Check with your child's healthcare provider for specific instructions before a pediatric blood test.

When to See Your Nurse Practitioner

Book a visit if you're due for routine screening, if you've noticed symptoms like unusual fatigue, increased thirst, or unexplained weight changes, or if you're managing an ongoing condition that needs regular monitoring. Care& offers chronic disease management for conditions like diabetes and high cholesterol, where regular bloodwork and follow-up matter. For STI testing, our sexual health services include bloodwork with confidential follow-up.

OHIP-covered care is a good option for many Ontarians, and community labs across Toronto process OHIP-insured tests every day. If you don't have a family provider to order your tests, or you want unhurried time to go over your results, Care& offers Family Practice with a dedicated NP. It's a membership model that isn't covered by OHIP, with unlimited visits and on-site lab work. If you're planning ahead, our list of blood tests and screenings to book before December is a helpful place to start.

Frequently Asked Questions