Fasting before a blood test usually means no food or drink except plain water for 8 to 12 hours, and that includes black coffee. Only some tests need it. A fasting glucose test does, while HbA1c and most routine cholesterol tests do not. When your request form or laboratory gives its own instructions, follow those over any general rule.
Getting this wrong costs you either way. An unnecessary fast makes for a miserable morning, and a broken one can affect results or mean a second visit. Here is what guidance in the US, UK and Australia says about hours, drinks, medicines and the tests that genuinely need an empty stomach.
How many hours do you fast before blood work?
Most fasting blood tests need 8 to 12 hours without food, according to MedlinePlus, the patient information service of the US National Library of Medicine. It notes that an early morning appointment means most of the fast happens while you sleep.
Australia's healthdirect is more specific. It suggests eating nothing after your evening meal, which usually means an overnight fast of 10 to 12 hours, and says a fasting blood test should be done after no less than 8 hours and no more than 16 hours. Longer is not better.
The NHS gives no fixed number of hours. It says the GP, nurse or specialist who requests the test should tell you whether you need to prepare.
| Rule | US (MedlinePlus) | UK (NHS) | Australia (healthdirect) |
|---|---|---|---|
| Usual fast | 8 to 12 hours | As instructed by whoever requests the test | 10 to 12 hours overnight, between 8 and 16 hours |
| Drinks allowed | Plain water only | Water only | Still, plain water only |
| Regular medicines | Ask first, and do not stop unless told to | Some, such as anticoagulants, may need stopping | Keep taking unless your doctor says not to |
Can I drink coffee before a blood test?
No, not if the test requires fasting. Black coffee breaks a fast for blood test purposes. MedlinePlus says juice, coffee, soda and other drinks can get into your bloodstream and affect your results, and healthdirect rules out tea, coffee, cordial, soft drinks and milk.
The same applies to anything else you swallow. healthdirect also says not to chew gum or suck on candy while fasting, because swallowing anything other than water can make results inaccurate.
If your test does not require fasting, these restrictions do not apply. Check the whole request form first, though, because one fasting test on the list sets the rules for the whole sample.
Can I drink water before a blood test?
Yes. Plain water is allowed during a fast for blood tests by MedlinePlus, the NHS and healthdirect. healthdirect specifies still, plain water, so skip flavored and sparkling versions on the morning of a fasting test.
Do you need to fast for a cholesterol test?
Usually not. A lipid panel is the cholesterol test that measures total cholesterol, HDL cholesterol and triglycerides, with LDL cholesterol often calculated from them. Guideline bodies in the US, UK and Europe now accept a non-fasting sample for most people.
The NICE guideline NG238 describes a full lipid profile and states that a fasting sample is not mandated. It calls for a fasting repeat when triglycerides come back between 10 and 20 mmol/L. In the US, the 2018 AHA/ACC cholesterol guideline said screening could be performed with fasting or nonfasting measurement of lipids. A 2026 dyslipidemia guideline has since replaced it, so ask whether your clinician wants a fasting sample, particularly if your triglycerides have been high before or high cholesterol runs in your family.
A joint European Atherosclerosis Society and laboratory medicine consensus recommends non-fasting samples routinely. It found the largest average changes 1 to 6 hours after a normal meal were not clinically significant: triglycerides up 0.3 mmol/L (26 mg/dL), and total and LDL cholesterol down 0.2 mmol/L (8 mg/dL). It suggests a fasting sample may be considered when non-fasting triglycerides are above 5 mmol/L (440 mg/dL).
In Australia, healthdirect lists "some cholesterol tests" among those that might need fasting, so ask your GP which kind you are having. Our cholesterol panel guide explains each line of the result.
Which blood tests need fasting, and which do not?
The tests that most often need fasting are blood glucose tests, lipid tests in some settings and certain chemistry panels. Many others can be taken at any time.
| Test | Fasting usually needed? | What the guidance says |
|---|---|---|
| Fasting plasma glucose | Yes | No caloric intake for at least 8 hours (ADA) |
| HbA1c | No | Fasting is not required (ADA) |
| Lipid (cholesterol) panel | Usually not | Fasting sample not mandated (NICE) |
| Iron studies | Sometimes | Some need a 12-hour fast (healthdirect) |
| Basic metabolic panel, some liver and kidney panels | Sometimes | Listed among tests that may need fasting (MedlinePlus) |
The American Diabetes Association's 2026 Standards of Care define fasting as no caloric intake for at least 8 hours, and list convenience as an advantage of HbA1c because fasting is not required. Our HbA1c guide explains what that result means.
For iron, healthdirect's iron studies page says some tests need a 12-hour fast with water only, and that you should not take iron supplements before the test. If ferritin is the result that brought you here, read high ferritin in men.
What about medicines, supplements and exercise?
Keep taking regular medicines unless you are told otherwise. healthdirect advises carrying on with medicines while fasting unless your doctor says not to, and MedlinePlus says not to stop them without your provider's instruction. The NHS notes that some people are asked to stop certain medicines first, such as anticoagulants (blood thinners), so ask rather than guess.
Tell the person requesting the test about vitamins and supplements too, as MedlinePlus advises. MedlinePlus also lists smoking, chewing gum and exercise among things to avoid while fasting.
If you have diabetes, do not fast without a plan. healthdirect warns that fasting makes hypoglycemia (low blood glucose) more likely. It advises asking your doctor for written instructions covering blood sugar checks, medicine or insulin changes and how to treat a low, and calling your doctor if you have to break the fast. If your blood sugar goes low, treat it right away under your agreed plan and tell the collection center; the test can be rebooked. If someone becomes unconscious or is not responding normally, call 911, 999 or 000.
What about a testosterone test?
Testosterone has timing rules of its own, on top of any fasting instruction. Rather than repeat the detail here, our guide to testosterone levels by age sets out the morning timing and fasting advice from US, UK and Australian sources. Book that test with the clock in mind, not just the fast.
Practical checks for fasting before a blood test
1. Read the request form before the evening before.
Check which tests are listed and whether any are marked as fasting. If the form is unclear, call the practice or collection center while there is still time to plan your evening meal.
2. Book the earliest appointment you can.
An early slot means most of the fast happens overnight. It also keeps you well inside healthdirect's 16-hour upper limit.
3. Drink plain water, and nothing else.
Water is allowed and makes the morning easier. Coffee, tea, milk, juice, gum and candy are not, whatever the label says about calories.
4. Write down your last meal and this morning's medicines.
Note the time you last ate and what you took. If you broke the fast, say so at collection rather than hoping it will not matter; the result can then be read correctly.
5. Get a plan first if you have diabetes.
Ask your doctor in advance how to handle medicines, insulin and a low blood sugar during the fast. Do not improvise on the morning of the test.
When should you call your doctor before the test?
Call before fasting if you have diabetes, take anticoagulants or other regular medicines you are unsure about, or are ill on the day. In the US, start with the primary care physician who ordered the test; in the UK, the GP practice; in Australia, your GP or the pathology collection center.
Ask three questions: which tests on the form need fasting, which medicines to take that morning, and what to do if the appointment runs late. Our annual blood work guide covers which tests are worth having in the first place.
Frequently asked questions
How many hours do you fast before blood work?
Most fasting blood tests need 8 to 12 hours without food or any drink other than plain water. Australia's healthdirect suggests an overnight fast of 10 to 12 hours, with a minimum of 8 and a maximum of 16. In the UK, the NHS says the person requesting the test should tell you what preparation is needed.
Does black coffee break a fast for a blood test?
Yes. MedlinePlus and healthdirect both exclude coffee during a fast for blood tests, and MedlinePlus explains that drinks other than water can get into the bloodstream and affect results. That includes black coffee and tea. If your test does not require fasting, the restriction does not apply, so check the request form first.
Do I need to fast for a lipid panel?
Usually not. NICE states that a fasting sample is not mandated for a full lipid profile, US cholesterol guidance accepts fasting or non-fasting screening, and a European consensus recommends non-fasting samples routinely. Your doctor may still ask for a fasting repeat, for example when triglycerides come back high.
Can I drink water before a fasting blood test?
Yes. MedlinePlus, the NHS and healthdirect all allow plain water during a fast for blood tests. healthdirect specifies still, plain water, so skip flavored or sparkling drinks. Tea, coffee, juice, soft drinks and milk are not allowed, and healthdirect also rules out chewing gum and candy.
Can I take my medication before a fasting blood test?
Usually, yes. healthdirect advises keeping up regular medicines unless your doctor says otherwise, and MedlinePlus says not to stop them without instructions. The NHS notes that some medicines, such as anticoagulants, may need to be stopped before certain tests. If you have diabetes, ask for a written plan before fasting.
The short version
Fasting for a blood test means plain water only, usually for 8 to 12 hours, and black coffee breaks it. Many common tests, including HbA1c and most cholesterol checks, do not need fasting at all. Keep taking regular medicines unless told otherwise, and get a plan first if you have diabetes. The instructions on your own request form come first.
This article is general education, not medical advice. It does not replace your own physician.
