Step-by-step guide for the night before and morning of your fasting blood test.
Avoid a very high-fat meal the night before a cholesterol test if you can, but otherwise eat normally. Finish eating by around 10 pm for a morning appointment.
A 7–9 am appointment keeps your fast to 9–11 hours — long enough, but not so long that you’re lightheaded by the time you arrive. Avoid midday fasting appointments if you can.
Plain water does not break a fast for any blood test. Staying hydrated makes your veins easier to find, reduces the chance of a difficult draw, and prevents lightheadedness.
Black coffee raises triglycerides and can affect fasting glucose and some hormone levels. For most other tests it makes little difference, but since you don’t always know exactly which tests are being run, skipping it is safest.
Take them with a small amount of water. Unless your GP has specifically said to hold a medication, continue as usual. This includes blood pressure tablets, thyroid medication, and most others.
Pack something to eat for immediately after — a muesli bar, banana, or whatever you like. You will be hungry and possibly lightheaded after a 12-hour fast. Don’t drive until you have eaten.
Mention any difficult veins, needle anxiety, or recent blood draws on the same arm. If you accidentally ate or drank something, tell the nurse now — they can advise whether to proceed.
If you walked briskly or rushed to get there, sit and rest for 5–10 minutes before the draw. Brief exercise temporarily elevates glucose and some other markers.
The nurse fills several small tubes. If you feel faint at any point, say so immediately — lying down while blood is drawn is completely normal and the nurses are used to it.
Eat the moment you leave the chair. Low blood sugar after a 12-hour fast causes dizziness, headaches, and shakiness. Do not skip eating and do not drive until you feel completely well.
It depends on the test. For cholesterol / triglycerides: ideally rebook — black coffee raises triglycerides. For fasting glucose: black coffee (no milk or sugar) has minimal effect, but it’s best to tell the nurse and let them decide. For most other tests: it’s fine to proceed. When in doubt, call the lab before going in.
8–12 hours for most fasting tests. Try not to fast longer than 12–14 hours — prolonged fasting can actually distort triglyceride and glucose results in ways that make them harder to interpret. A morning appointment after an overnight fast is ideal.
Tell the nurse immediately. Vasovagal reactions (feeling faint from needles) and low blood sugar from fasting are both very common. Nurses are trained to manage both. They can recline your chair, give you something to eat or drink, and make sure you are safe before you leave.
Usually need fasting: fasting lipids (cholesterol panel), fasting glucose, iron studies, some hormone tests.
Do not need fasting: FBC, HbA1c, TSH, eGFR, liver function tests, most other blood tests.
See our full fasting lookup tool for 26 individual tests.