Most routine NHS blood tests do not need fasting. If your GP or nurse asks you to fast, it usually means nothing to eat from your evening meal the night before and only water on the morning of the test: about 8 hours for a fasting glucose, and longer for some cholesterol (lipid) tests if fasting is requested.
| Category | Reference range |
|---|---|
| Fasting glucose | 8 hours without food (water allowed) |
| Oral glucose tolerance test (OGTT) | 8–14 hours without food (water allowed) |
| Testosterone | 8 hours without food (water allowed) |
| Homocysteine | 8 hours without food (water allowed) |
| Lipids (cholesterol, triglycerides) — only if fasting is requested | 12–14 hours without food (water allowed) |
| Full blood count, liver tests, CRP, HbA1c, vitamin B12 | no fasting needed hours without food (water allowed) |
Example times from an NHS hospital biochemistry leaflet (East Kent). Your own GP surgery or hospital may give different instructions — theirs come first.
Usually not. The NHS says your GP, nurse or specialist should tell you if you need to do anything to prepare, and fasting is only needed for some tests. If nobody mentioned it, you can normally eat and drink as usual.
Tests where fasting is commonly requested are the ones that measure things food changes within hours: fasting glucose, the oral glucose tolerance test and a few specialist tests. A cholesterol test no longer routinely needs fasting: UK practice moved to non-fasting lipid profiles in 2014, although a doctor may still ask for a fasting sample, for example if a previous triglyceride result was very high.
Many common tests need no preparation at all, including the full blood count, liver function tests, CRP, HbA1c and a standard vitamin B12 test.
Water is the one drink every NHS fasting leaflet allows. Plain tap or bottled water is fine; flavoured or sweetened waters do not count as plain water.
Brushing your teeth is not mentioned in the UK fasting leaflets we reviewed. If you brush, rinse with water and avoid swallowing toothpaste or mouthwash; if you are unsure, ask the surgery or blood clinic.
Smoking and vaping should wait until after the test. NHS trust leaflets ask you not to smoke on the morning of a fasting test because it can affect the results.
Do not stop or change your usual medicines unless your doctor has told you to. The NHS notes that some tests need a medicine paused — for example, anticoagulants (blood thinners) — but that instruction should come from the clinician who requested the test.
If a medicine has to be taken with food and that clashes with the fast, speak to your GP or pharmacist beforehand rather than skipping it.
If you have diabetes, talk to your GP or diabetes team before fasting for any test, especially if you use insulin or tablets that can cause low blood sugar.
Once the blood has been taken you can eat and drink normally — it helps to bring a snack. Results usually take a few days, sometimes a few weeks. Some GP surgeries only contact you if they need to discuss a result, and you may be able to see results in the NHS App or your NHS account online. If you have heard nothing after a few weeks, contact the surgery.
Checked against:
No need to decode the numbers yourself: take a photo of the report and the app explains “Fasting before a blood test” and the rest in plain words — what is normal and what deserves attention.
App StoreGoogle PlayWeb versionYes. Plain water is allowed and is the only drink NHS fasting leaflets permit. Avoid tea, coffee (even black), squash and diet drinks.
Only as long as you are told. Typical examples are 8 hours for fasting glucose and 12–14 hours for a lipid test when fasting has been requested. Most tests need no fasting.
Usually not. Since 2014 UK labs routinely use non-fasting lipid profiles. Fast only if your GP specifically asks, for example to recheck high triglycerides.
No. NHS trust leaflets ask you not to smoke, vape or chew gum on the morning of a fasting test, as it can affect the results.
UK fasting leaflets do not address it. If you brush, do not swallow toothpaste or mouthwash, and ask the clinic if you are unsure.
Keep taking them as usual unless your doctor told you otherwise. If you have diabetes, ask your GP or diabetes team before you fast.
This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.
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