How to Prepare for a Glucose Test

How long to fast, and what counts as fasting

Before a glucose test you generally need to avoid all food and any drink except plain still water for 8–12 hours before the sample is taken. Your particular laboratory may specify a different interval, so it's worth checking your referral form. In practice this usually means having your last meal in the evening and having the test done the next morning. Don't exercise that morning while you wait for the test — MedlinePlus lists exercise among the things to avoid while fasting before a blood test.

Why morning is the best time

It's best to have a glucose test done in the morning, before you've eaten anything. Glucose results depend on when you last ate rather than on the time of day itself, so a morning appointment lets part of the fasting period overlap with your overnight sleep — making the required 8–12 hours much easier to keep.

Step by step: the day before, the morning of, and afterward

• The day before: eat normally, but avoid a heavy, fatty meal and alcohol — both can affect your blood values the next day.
• The day before: avoid intense physical exercise, since it can affect the concentration of certain substances in the blood.
• The morning of the test: don't eat or drink anything except plain still water. Coffee, tea and juice are off-limits — caffeine can affect the glucose result.
• The morning of the test: don't smoke or chew gum before the sample is taken.
• Right before the draw: sit calmly for a few minutes beforehand — staying relaxed helps avoid stress-related swings in the result.
• After the draw: press on the puncture site for about 5 minutes, don't remove the plaster for at least an hour, and avoid strenuous exercise so you don't end up with a bruise.
• After the draw: the test is done — you can eat and drink normally again. A short spell of light-headedness can follow a longer fast, so it's worth having a snack on hand.

Medication and supplements

Tell the laboratory or your doctor about any medication and supplements you take. Don't stop a doctor-prescribed medication on your own — discuss whether to take it before the test with your doctor or the lab staff instead.

What happens if you don't fast properly

Eating, or drinking coffee, tea or juice, raises blood glucose, so the result won't reflect your true fasting state and may come back falsely high. If you weren't able to keep the fast, it's worth telling the laboratory staff — the test may need to be repeated at another time.