Key takeaways
- You can eat and drink normally before most routine blood tests, including a full blood count, ferritin, vitamin D, vitamin B12, folate and thyroid stimulating hormone (TSH).
- Blood London's Silver, Silver Plus, Gold and Platinum profiles need no fasting either.
- Fasting glucose needs at least 8 hours without food, and Blood London's insulin resistance panel, which includes the Fasting Insulin Resistance Index (FIRI), needs 10 to 12 hours.
- Water is always fine, including during a fast.
- Each test page at Blood London states its preparation, so you can check before you book.
Your blood test is first thing tomorrow, and you're wondering whether to skip breakfast. For most tests you can eat and drink as normal.
The ones that need fasting 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 below is plus the £50 blood draw, which is charged once per visit even if you have more than one test taken from the same sample.
Which blood tests need fasting?
Only a small number, 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, results in 1 day). 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, need no special preparation. You can have them at any time of day, with or without food beforehand.
| Test | Can you eat beforehand? | Why |
|---|---|---|
| Full blood count | Yes | Recent food makes no difference |
| Ferritin | Yes | Reflects iron stores, so a meal makes no difference |
| TSH | Yes | A meal makes no difference |
| Vitamin D, B12, folate | Yes | Recent food makes no difference |
| Silver, Silver Plus, Gold and Platinum profiles | Yes | These profiles need no fasting |
| Lipid profile | Usually yes | A recent meal can affect the triglyceride result, so fast only if your doctor asks you to |
| HbA1c | Yes | Measures average blood sugar over 2 to 3 months, not a single moment |
| Fasting glucose | No, fast for at least 8 hours | The result is read as a fasting baseline |
| Insulin resistance panel (FIRI) | No, fast for 10 to 12 hours | Insulin and glucose are read as a fasting baseline |
Does a lipid profile (cholesterol test) need fasting?
Most people can eat before 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. The HbA1c blood test (£71.25, results in 1 day) measures your average blood glucose over the previous two to three months. A meal beforehand won't shift it the way it can shift a fasting glucose test.
What counts as fasting, and can I drink water?
Fasting means nothing to eat or drink except water for the stated period, usually 8 to 12 hours depending on the test. Water is fine, and staying hydrated 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?
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. 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. That way the result reflects your usual level.
What if I'm not sure whether my test needs fasting?
Check the preparation section on your test page before you book, because it varies from test to test. If you're still unsure, call the clinic before your appointment and the team can confirm.
Booking your blood test
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 by email. You can add a doctor review to your test for £75 if you'd like your report explained.
Book one of our popular blood tests online, or walk in and we'll take it from there.
Frequently asked questions
Can I drink coffee before a fasting blood test?
Stick to water during the fasting period. Black coffee still counts as breaking a fast for most tests, since it can affect some results.
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. You can eat and drink normally beforehand.
Can I take my normal medication before a fasting blood test?
Most medicines can usually be taken as normal, with water, 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 can eat as normal before a lipid profile. If your doctor has specifically asked you to fast, 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 won't change it.
References
- NHS: Blood tests
- Lab Tests Online UK: Lipid profile
- NICE Clinical Knowledge Summaries: Lipid modification, CVD prevention
- Diabetes UK: HbA1c