How Long To Fast Before A Blood Test? A Guide
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Overview
We’ve all been there…holding a pathology referral form, staring at the words “Fasting Required,” and wondering if that morning coffee is truly off-limits.
Getting a blood test is a standard part of looking after your health in Australia, but the rules around fasting can be confusing. Does "fasting" mean strictly nothing? Can you drink water? And what happens if you accidentally eat a biscuit?
Here is a simple guide to fasting for blood tests, so you can get it right the first time and avoid a repeat visit to the pathology collection centre.
Why fasting matters before a blood test
When you eat or drink, your blood composition changes. Nutrients like sugars, proteins, and fats enter your bloodstream, which can temporarily skew your test results.
Fasting ensures your doctor gets a "baseline" reading—a snapshot of your health without the interference of your last meal. This is crucial for diagnosing conditions like diabetes or high cholesterol accurately.
What Does ‘Fasting’ Mean Before a Blood Test?
Definition of fasting
In the context of blood tests, fasting means you must not eat or drink anything containing calories, sugars, or caffeine for a set period before your sample is taken.
What counts as fasting?
- Water: Yes, plain tap water is absolutely fine (and encouraged) to keep you hydrated.
What breaks a fast?
The following will break your fast and may force you to reschedule:
- Coffee & Tea: Even black coffee or tea (without sugar or milk) is generally not allowed.
- Alcohol: Definitely not allowed.
- Chewing gum: Even sugar-free gum stimulates digestion and can alter results.
- Smoking: Can affect white blood cell counts and glucose levels.
- Juices & Soft Drinks: These contain sugar and will spike glucose levels immediately.
How Long Should You Fast?
Different tests require different fasting windows. Below are the typical durations recommended by Australian pathology providers. Always check your specific referral form.
| Blood Test | Fasting Required? | How long to fast | Notes |
| Glucose (Blood Sugar) | Yes | 8–10 hours | Used to check for diabetes or pre-diabetes. |
| OGTT (Glucose Tolerance) | Yes | 8–10 hours | Common in pregnancy. You must stay at the centre for 2+ hours. |
| Lipid Profile (Cholesterol) | Usually Yes | 10–12 hours | Fasting is often preferred for accuracy. |
| Iron Studies | Sometimes | 10–12 hours | Helps measure stored iron accurately. |
| Renal (Kidney) Function | No | N/A | Drink water as normal. |
| Vitamin B12 & Folate | Sometimes | 6–8 hours | Rarely needed for B12; may be required for Folate. |
| Full Blood Count (FBC) | No | N/A | You can eat and drink normally. |
Why Is Fasting Needed for Some Blood Tests?
Glucose Tests:
Carbohydrates break down into glucose quickly. Eating before testing can spike levels and cause false diabetes readings.
Lipid (Cholesterol) Tests:
Triglycerides remain elevated for hours after meals. Fasting provides a true baseline cholesterol level.
Iron Studies:
Iron from recent meals can temporarily raise blood levels. Fasting measures stored iron instead.
What You Can & Can’t Have While Fasting
Allowed:
- Plain water (tap or bottled)
- Prescribed medications (unless your doctor told you to pause them)
Not Allowed:
- Coffee or tea (even black)
- Milk or milk alternatives
- Fruit juice, cordial, or soft drinks
- Alcohol
- Chewing gum or mints
- Cigarettes or vapes
Tips to Make Fasting Easier
- Book an early appointment: Schedule your test for 8:00 AM or as early as the collection centre opens. This way, you sleep through most of the fasting period.
- Stay hydrated: Drink plenty of water. It keeps your veins plump, making it easier for the phlebotomist to take blood (meaning fewer needle pricks).
- Bring a snack: Pack a muesli bar or piece of fruit to eat immediately after your test is done.
- Keep warm: Cold veins can be harder to find. Wear a jumper if it’s a chilly morning.
What Happens If You Accidentally Break Your Fast?
It happens—you wake up groggy and autopilot a piece of toast into your mouth.
If you eat or drink something you shouldn't have:
- Don't hide it. Tell the pathology collector.
- Ask if you can proceed. For some tests, it might not matter too much.
- Reschedule if needed. It’s better to come back another day than to get inaccurate results that lead to the wrong treatment.
When to See a GP
Once your results are in, your GP will review them against standard Australian reference ranges. You should book a follow-up if:
- Your results show "abnormal" flags.
- You are monitoring a chronic condition like diabetes or high cholesterol.
- You need to adjust your medication based on the results.
How Hola Health Can Help
Managing your health shouldn't be a hassle. Hola Health connects you with Australian-registered GPs from the comfort of your home.
- Need a referral? If you need a blood test form, you can request a blood test referral online quickly without sitting in a waiting room.
- Follow-up results: Once your pathology results are ready, our online GPs can discuss them with you via a telehealth call and explain what they mean in plain English.
- Scripts: If your results show you need medication (like for cholesterol or iron), we can send a prescription instantly to your phone.
FAQs
Do I need to fast for a full blood count?
No. A Full Blood Count (FBC) checks your red and white cells and platelets. Food does not affect these counts immediately.
Can I drink water before a blood test?
Yes, and you should – being hydrated makes the blood draw easier and faster. Just avoid sparkling water or flavoured waters.
Can I take medications while fasting?
Generally, yes. Continue your heart, blood pressure, or thyroid medications as usual with water. However, if the medication needs to be taken with food (like some anti-inflammatories) or is for diabetes, ask your doctor for specific advice to avoid side effects.
What if I feel unwell while fasting?
If you feel faint, dizzy, or shaky, safety comes first. Eat something and drink water immediately. You can always reschedule the test for another day.
How long fasting affects cholesterol tests?
Ideally, 10–12 hours. Fasting less than 8 hours may show falsely high triglyceride levels, while fasting too long (over 16 hours) can cause your body to start breaking down stored fats, also skewing the results.
Fasting vs non-fasting lipid test differences?
Current Australian guidelines increasingly allow for non-fasting cholesterol checks for general screening. However, if your initial results are high, or if you have a family history of heart disease, your doctor will likely request a fasting test for a more precise reading.
Can Pregnant Women Fast Before Blood Tests?
Yes, but extra care is needed. Pregnant women often need the Glucose Tolerance Test (OGTT). Because fasting can make morning sickness or dizziness worse, it is vital to drink water and bring a snack to take immediately after the test. Always follow your midwife or GP's instructions.
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Disclaimer
This blog is for general informational purposes only and does not indicate that Hola Health provides all treatments or preventive measures mentioned. It is not intended to be a substitute for professional medical advice. Always seek the guidance of your doctor or other qualified health professional with any questions you may have regarding your health or a medical condition. For emergencies please immediately contact 000. Any medical topics discussed are intended to educate, not to imply availability through Hola Health.