Key takeaways
- Most routine blood tests, including a full blood count, ferritin, vitamin D, vitamin B12, folate and thyroid stimulating hormone (TSH), do not need fasting.
- Some tests do need fasting: fasting glucose needs at least 8 hours, and Blood London's insulin resistance panel, which includes the Fasting Insulin Resistance Index (FIRI), needs 10 to 12 hours.
- HbA1c does not need fasting, because it measures your average blood sugar over the previous two to three months rather than a single reading.
- Water is fine during a fast; it's food and drink other than water that you need to avoid.
- Each test page at Blood London states whether fasting is needed, so check before you book if you're unsure.
- A lipid profile at Blood London costs £72.50 plus the £50 blood draw, and most people do not need to fast for it unless a doctor has specifically asked them to.
Most blood tests don't require fasting at all, and the ones that do are usually glucose, insulin or some lipid-related tests, so check the preparation notes on your test page before you come in. Our popular blood tests page lists the most requested tests, each with its own preparation notes.
Walk in or book a blood test at 117a Harley Street, seven days a week. Each test price is plus the £50 blood draw.
Which blood tests need fasting?
Fasting is only required for a small number of tests, mainly those that measure blood sugar or insulin directly. A fasting glucose test needs at least 8 hours with nothing but water, so the reading reflects your baseline rather than a recent meal. The Fasting Insulin Resistance Index (FIRI) is, as its name says, a fasting test (£111.88 plus the £50 blood draw, results in 1 day), and Blood London's insulin resistance panel asks for a 10 to 12 hour fast. Most other tests, including a full blood count, ferritin, vitamin D, vitamin B12, folate and TSH, do not need any special preparation and can be taken at any time of day, with or without food beforehand.
| Test | Fasting needed? | Why |
|---|---|---|
| Full blood count | No | Not affected by recent food |
| Ferritin | No | Reflects iron stores, not affected by a meal |
| TSH | No | Not affected by a meal |
| Vitamin D, B12, folate | No | Not affected by recent food |
| Lipid profile | Usually no | A recent meal can affect the triglyceride result, so fast only if your doctor asks you to |
| HbA1c | No | Measures average blood sugar over 2 to 3 months, not a single moment |
| Fasting glucose | Yes, at least 8 hours | The result is read as a fasting baseline |
| Insulin resistance panel (FIRI) | Yes, 10 to 12 hours | Insulin and glucose are read as a fasting baseline |
Does a lipid profile (cholesterol test) need fasting?
Most people do not need to fast for a lipid profile, which measures cholesterol and triglycerides (£72.50 plus the £50 blood draw, results in 1 day). A recent meal mainly affects the triglyceride result, so fasting is only needed if your doctor has specifically asked for it. If that applies to you, follow their instructions and let the phlebotomist know your fasting status when you arrive.
Does an HbA1c test need fasting?
No, an HbA1c test does not need fasting. The HbA1c blood test (£71.25 plus the £50 blood draw, results in 1 day) measures your average blood glucose over the previous two to three months, so a single meal beforehand does not change the result in the way it can with a fasting glucose test.
What counts as fasting, and can I drink water?
Fasting means having nothing to eat or drink except water for the stated period beforehand, usually 8 to 12 hours depending on the test. Water is fine, and staying hydrated also makes it easier for the phlebotomist to find a vein. Tea, coffee, juice, milk and anything with sugar or calories count as breaking the fast. If you have diabetes and take insulin or other medicines that lower blood sugar, ask your doctor before fasting, because fasting can cause low blood sugar.
What about tests timed to the morning, not fasting?
A few tests are best taken at a specific time of day rather than fasted. Testosterone peaks in the morning, so a sample taken between 7am and 10am gives the most reliable reading, and a low result is usually repeated on a second morning before any diagnosis is made. If you take levothyroxine for an underactive thyroid, many clinicians advise taking your dose after the blood sample on the day of a TSH test, so the result reflects your usual level.
What if I'm not sure whether my test needs fasting?
Check the preparation section on your specific test page before you book, since this varies by test and is not a single rule for the whole panel. If you're still unsure, call the clinic before your appointment and the team can confirm.
How blood testing works at our Harley Street clinic
You can walk in or book private blood tests in London online at 117a Harley Street, seven days a week, with no GP referral needed. Most routine results, including a lipid profile and HbA1c, are back within 1 day. Every test price on the site is shown plus the £50 blood draw, which is charged once per visit even if you have more than one test taken from the same sample. Results are sent by email, and you can add a doctor review to your test for £75 if you want your report explained.
Book one of our popular blood tests online or walk in seven days a week at 117a Harley Street. Check your test page first if you're not sure whether to fast.
Frequently asked questions
Can I drink coffee before a fasting blood test?
No. Black coffee still counts as breaking a fast for most tests, since it can affect some results. Stick to water during the fasting period.
What happens if I eat before a test that needs fasting?
Tell the phlebotomist before your sample is taken. They can note that the sample is non-fasting, though the result may need to be repeated depending on the test.
Do I need to fast before a full blood count?
No, a full blood count does not need fasting and you can eat and drink normally beforehand.
Can I take my normal medication before a fasting blood test?
Water is fine during a fast, and most medicines can usually be taken as normal unless a clinician has told you otherwise. If you take insulin or other diabetes medicines, ask your doctor before fasting, as the dose may need adjusting.
How long should I fast before a lipid profile?
Most people do not need to fast for a lipid profile at all. If your doctor has specifically asked you to, follow their instructions on timing.
Does fasting affect HbA1c results?
No, HbA1c reflects your average blood sugar over the previous two to three months, so a recent meal does not change the result.
References
- NHS: Blood tests
- Lab Tests Online UK: Lipid profile
- NICE Clinical Knowledge Summaries: Lipid modification, CVD prevention
- Diabetes UK: HbA1c