Fasting for your blood test
Information for patients from the Clinical Biochemistry Department
What is fasting?
Fasting means that from the the night before your blood test, you must not eat anything.
Why do I need to fast before my blood test?
Some blood tests give more useful results if you fast before the sample is taken. The vitamins, minerals, fats, carbohydrates, and proteins that make up all food and drinks can affect blood-level readings, clouding the results of your test.
Your doctor will tell you if you need to fast before your blood test.
How long do I need to fast before my blood test?
Your doctor will tell you how long to fast, as it is different for each test. Examples of fasting times for some blood tests are listed in the table below.
Table listing how long patients should fast before certain blood tests
| Test | Time |
|---|---|
| Fasting glucose | 8 hours |
| Gut hormones (including chromogranin A and B) | 8 hours |
| Homocysteine | 8 hours |
| Lipids / triglycerides / cholesterol | 12 to 14 hours |
| Oral glucose tolerance test | 8 to 14 hours |
| Plasma free metapephrines | 8 to 14 hours |
| Testosterone | 8 hours |
How do I fast for my blood test?
You can eat your dinner the night before your test.
Do not eat breakfast on the morning of your blood test.
You can only drink water on the morning of your test.
Do not drink tea, coffee, or fizzy drinks (for example cola) on the morning of your test. This includes black coffee and sugar-free (diet) drinks.
Do not chew gum or smoke on the morning of your test. This includes vaping and using nicotine patches.
Can I take my usual medicine on the morning of my blood test?
You should not change your medication unless you are told to do so by your doctor. If your medicine needs to be taken with food and this will interfere with the timing of your fasting blood test, please discuss this with your doctor.
I’m diabetic, what should I do?
If you are diabetic, please speak to your doctor before you begin fasting for any test.