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 the patient fasts before their test.
Vitamins, minerals, fats, carbohydrates, and proteins make up all food and drinks. If you eat before your blood test, these can cloud your test results and affect your blood-level readings.
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. We have listed examples of fasting times for some blood tests 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.
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?
Do not change your medication, unless your doctor tells you otherwise.
If you need to take your medicine with food and this will interfere with the timing of your fasting blood test, please speak to 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.