If you have been told to fast before a blood test, you are probably wondering exactly what that means. Can you drink water? What about black coffee? How long do you really need to fast? And what happens if you accidentally eat something?

These are common questions, and getting the answers right is important for accurate test results. This guide explains everything you need to know about fasting for blood tests, helping you prepare properly for your appointment.

Why Do Some Blood Tests Require Fasting?

Fasting ensures that food and drink do not interfere with your test results. When you eat or drink, nutrients enter your bloodstream and can temporarily alter various blood markers.

How food affects blood tests:

Blood glucose rises after eating carbohydrates

Triglycerides increase significantly after fatty meals

Certain vitamins and minerals fluctuate with food intake

Insulin levels spike in response to eating

For tests measuring these markers, eating before the test would give an inaccurate picture of your baseline health. Fasting provides a standardised state that allows for meaningful comparison of results over time and against reference ranges.

Which Tests Require Fasting?

### Tests That Typically Require Fasting

Fasting glucose:

Measures your blood sugar level after fasting. Eating would elevate glucose, masking potential diabetes or prediabetes.

Fasting lipid profile:

Includes total cholesterol, LDL, HDL, and triglycerides. Triglycerides in particular are significantly affected by recent food intake.

Fasting insulin:

Used to assess insulin resistance. Eating triggers insulin release, so fasting provides a baseline measurement. This is essential for HOMA-IR calculations. For more information, read our guide to HOMA-IR testing for insulin resistance.

Metabolic panels:

Comprehensive metabolic panels often include glucose and may require fasting for accurate results.

### Tests That Usually Do Not Require Fasting

HbA1c — Reflects average blood sugar over months, not affected by recent meals

Full blood count — Measures blood cells, not affected by food

Thyroid function tests — TSH and thyroid hormones are not significantly affected

Most hormone tests — Though some have specific timing requirements

Vitamin D — Not affected by recent food intake

Iron studies — Food can affect results slightly, but fasting is not always required

For comprehensive testing information, read our guide to what's included in a comprehensive health check.

Can I Drink Water While Fasting?

Yes! You should drink water while fasting for a blood test.

Water is not only permitted but actively encouraged. Staying hydrated:

Makes it easier for the phlebotomist to find your veins

Helps blood flow more freely

Reduces the risk of feeling faint during or after the blood draw

Does not affect the results of fasting blood tests

How much water should you drink?

Drink water normally in the hours leading up to your test. There is no need to drink excessive amounts, but do not restrict your fluid intake. A glass or two of water on the morning of your test is perfectly fine.

What About Other Drinks?

### Black Coffee and Tea

This is where it gets more complicated. Technically, black coffee and plain tea contain virtually no calories or nutrients that would affect most blood tests. However:

Caffeine can affect some hormones including cortisol and insulin

Some laboratories recommend avoiding all beverages except water

For the most accurate results, water only is the safest approach

If you must have coffee, stick to black coffee without sugar, milk, or cream. But when in doubt, water only is best.

### What to Definitely Avoid

Any food, including small snacks

Sugary drinks, including fruit juice

Milk or cream in tea or coffee

Alcohol (avoid for at least 24 hours before testing)

Chewing gum (even sugar-free varieties may affect results)

How Long Should You Fast?

The standard fasting period is 8 to 12 hours before your blood test. Most people find it easiest to:

Stop eating after dinner the night before

Schedule their blood test for early morning

Have breakfast after the test

Example timeline:

8:00 PM — Finish dinner

8:00 PM to 8:00 AM — Fast (water permitted)

8:00 AM — Blood test appointment

After test — Eat breakfast normally

If you have diabetes or take medications that require food, discuss timing with your healthcare provider.

Tips for Successful Fasting

### The Night Before

Eat a normal dinner (avoid unusually fatty or heavy meals)

Set a reminder for when to stop eating

Prepare your morning so you can get to your appointment efficiently

Avoid alcohol

### The Morning Of

Drink water to stay hydrated

Take your usual medications unless advised otherwise

Bring a snack to eat immediately after your test

Wear clothing with sleeves that roll up easily

### Managing Hunger

If you find fasting difficult:

Schedule the earliest appointment available

Keep busy to distract yourself from hunger

Remember it is only temporary

Focus on staying well-hydrated

What If You Accidentally Eat?

If you accidentally eat or drink something other than water before your fasting blood test:

Be honest with your healthcare provider. Tell them what you consumed and when. They can then decide whether to:

Proceed with the test and note that you were not fasting

Reschedule the test for another day

Perform only the tests that do not require fasting

Do not try to hide that you ate — inaccurate results could lead to misdiagnosis or inappropriate treatment.

Special Considerations

### Diabetes

If you have diabetes, particularly if you take insulin or medications that lower blood sugar, fasting requires careful management:

Discuss with your doctor before fasting

You may need to adjust medication timing

Monitor for hypoglycaemia symptoms

Bring glucose tablets or a snack for after the test

### Medications

Most medications can be taken as normal, but check with your provider about:

Medications that must be taken with food

Supplements that might affect results

Timing of diabetes medications

### Children

Fasting requirements may be modified for children. Discuss appropriate fasting duration with your paediatrician.

After Your Fasting Blood Test

Once your blood has been drawn:

Eat and drink normally — Have the breakfast or snack you brought

Stay seated briefly if you feel lightheaded

Keep the bandage on for a few hours to prevent bruising

Resume normal activities — Most people can continue their day immediately

Fasting Blood Tests in London

At [Private Blood Tests London](/), we offer early morning appointments at our CQC-registered clinic in South Kensington, making fasting as convenient as possible. Our experienced team ensures a comfortable experience, and we provide clear instructions on how to prepare for your specific tests.

Explore our [health screening packages](/health-screening), [diabetes testing](/diabetes-test), and [cholesterol testing](/cholesterol-test) services.

Proper preparation leads to accurate results. Book your blood test today and get the reliable information you need about your health.

Fasting for a Blood Test: Can I Still Have Water?