Fasting blood tests: what it means and how long to fast

"Fasting required" on your pathology request form — does that mean no breakfast, no coffee, no water? And how long is long enough?

The short version: for most tests that need it, you'll fast for 8–12 hours (usually just overnight), water is not only allowed but encouraged, and black coffee and tea are out. Here's the full rundown — which tests need fasting, what's allowed, and what to do if you slip up.

Why Some Blood Tests Need Fasting

After you eat, your body absorbs nutrients into the bloodstream — and several of them (glucose, triglycerides) sit at higher levels for hours afterwards. Fasting gives the lab a baseline reading unaffected by your last meal, which matters for:

• Diagnosis — a fasting glucose above the diagnostic threshold is a meaningful result; a random one after breakfast tells you much less
• Comparison over time — your results are only comparable across years if the conditions were the same
• Reference ranges — lab "normal" ranges for some tests assume a fasted state

Which Tests Typically Need Fasting

Plenty of common tests need no fasting at all — full blood count, kidney function, thyroid tests, most vitamin D and many others. Check your request form or instructions; when in doubt, ask the collection centre.

How Long to Fast: The Practical Rule

• Standard answer: 8–12 hours. For most fasting tests, an overnight fast does the job — dinner at 7pm, blood draw at 8am is a textbook example
• Don't fast longer than instructed. Starving yourself for 24 hours doesn't improve accuracy — it can distort results (and make you dizzy during the draw)
• Morning slots are easiest. Book the first appointments of the day, fast overnight while you sleep, and eat breakfast straight after
• Diabetics take note: if you take insulin or diabetes medication, talk to your doctor before fasting — hypoglycaemia risk is a real consideration, and your medication timing may need adjusting

What You CAN Have While Fasting

• ✅ Water — plain, still water is always fine, and being hydrated actually makes the blood draw easier
• ✅ Your regular medications — unless your doctor has specifically said otherwise (they'll usually tell you if a medication must wait)
• ✅ Prescribed morning medications with a small sip of water — missing essential medication does far more harm than a tiny sip

What You CAN'T Have

• ❌ Food — including lollies, chewing gum and mints, which stimulate digestion
• ❌ Coffee and tea — even black. Coffee affects glucose and lipid metabolism; the milk and sugar in your usual cup are worse. Herbal tea is also out
• ❌ Alcohol — avoid for at least 24 hours before the test, as it alters triglycerides and liver enzymes
• ❌ Juice, soft drink and sports drinks — all contain sugars that hit your glucose result
• ❌ Smoking or vaping before the test — nicotine affects some results

What If You Accidentally Ate?

Don't cancel in a panic — but be honest. Options:

1. Ring the collection centre first. They'll tell you whether your specific tests are affected, and whether they can still proceed
2. You may still be able to test — some panels are relatively unaffected by a small amount of food, and the lab can note "non-fasting" on the result so your doctor interprets it correctly
3. Rebook if needed. For strict fasting tests like OGTT, an eaten breakfast means rebooking another day
4. Tell the collector what and when you ate — never let a non-fasting sample masquerade as a fasting one; mislabelled results can lead to wrong clinical decisions

Where to Get a Fasting Blood Test Request

If your GP has asked you to organise a fasting blood test, the request is the easy part these days. A telehealth GP can assess you and issue a pathology referral online — at NewTeleDoc Medical, pathology referrals start from $25, sent digitally, and the request form will note fasting requirements so the collection centre knows what's needed. See our guide to getting a blood test referral online in Australia.