Fasting Blood Test Guide
Feb 8, 2026
Fasting Blood Test Guide
A fasting blood test requires you to avoid eating for 8-14 hours before your blood draw to ensure accurate results—particularly for glucose and cholesterol tests where recent food intake directly affects measurements. However, not all blood tests require fasting, and knowing which do helps you prepare correctly.
At Blood Test London, we advise on fasting requirements when you book.
Which Tests Require Fasting?
Fasting Required (8-14 hours)
Test | Why Fasting Needed |
|---|---|
Fasting glucose | Food raises blood sugar |
Fasting lipid panel | Triglycerides affected by recent food |
Fasting insulin | Food triggers insulin release |
Metabolic panel | Glucose component affected |
Fasting Preferred but Not Essential
Test | Note |
|---|---|
Iron studies | Morning, fasting gives most consistent results |
Vitamin B12 | Fasting preferred for consistency |
No Fasting Required
Test | Note |
|---|---|
HbA1c | 3-month average, not affected by recent food |
Thyroid function | Not affected by food |
Full blood count | Not affected by food |
Liver function | Not significantly affected |
Kidney function | Not significantly affected |
Vitamin D | Not affected by food |
Hormone tests | Most not affected |
How to Fast Properly
The Night Before
Eat your normal evening meal
Stop eating after 10pm (for morning test)
No late-night snacks
During the Fast
You CAN have:
Water (and plenty of it—helps blood draw)
Black tea or coffee (no milk or sugar)
Prescribed medications (usually)
You CANNOT have:
Food of any kind
Sugary drinks
Milk in tea/coffee
Alcohol (avoid 24 hours before)
Chewing gum (contains sugar)
Morning of Test
Take medications as normal (unless advised otherwise)
Drink water
Book early appointment if possible
Fasting Timeline
Test Time | Stop Eating By |
|---|---|
8am | 10pm night before (10 hours) |
9am | 9pm night before (12 hours) |
10am | 8pm night before (14 hours) |
Afternoon | More difficult—morning preferred |
What If I Accidentally Ate?
Tell the phlebotomist before your blood draw
Some tests can still be done (non-fasting options)
May need to reschedule for glucose/lipids
Better to be honest than get inaccurate results
Tips for Successful Fasting
Book early morning appointment
Drink plenty of water (easier blood draw)
Get good sleep the night before
Bring a snack for after the test
Set a reminder to stop eating
Book Your Blood Test
We'll advise whether fasting is needed for your specific tests.
Phone: 020 3951 3429
FAQ
Can I drink water during a fast?
Yes—water is encouraged. It makes the blood draw easier.
Can I take my medications?
Usually yes, but check with us when booking. Some medications may need adjustment.
What if I can only test in the afternoon?
Fasting for afternoon tests is harder. Morning tests are preferred for fasting panels.
Does black coffee break the fast?
Black coffee (no milk, no sugar) is usually acceptable, but water is better.
Blood Test London by The Wellness. Expert guidance on fasting blood tests.