All you need to Know about Preparing for a Fasting Blood Test
The purpose of a blood test is to test for the level of certain elements in your blood. However, consuming food and some drinks can cause the chemicals in your blood to change quite significantly.
Eating and drinking can change the outcome of blood tests and so render the results invalid or unreliable. This does not apply to all blood tests but, for those where it does, your doctor will ask you to fast before the test takes place. The length of the fast will vary depending on the test but your doctor should keep you informed.
Blood Tests that Require Fasting
The tests that normally require fasting in the hours leading up to them include:
- Glucose tests that check the amount of sugar in your blood, where high levels indicate you are diabetic or in a pre-diabetes state.
- B12 tests that measure the level of the vitamin in your blood, possibly helping the diagnosis of anaemia or other problems.
- Testing if iron levels are too high or too low.
- Lipid profile tests to check cholesterol and other blood fat levels that may indicate a risk of heart disease or a stroke. Younger patients may not need to fast, and a non-fasting result may also be sought.
- GGT tests show the level of the gamma-glutamyl transferase enzyme that may indicate alcohol abuse, liver disease or bile duct problems.
Depending on the blood test you’re being given, you will be told if fasting is needed and how long before the test you need to start. Most fasting blood tests take place early morning so you can fast overnight, which means it’s easier to do. Since changes to chemicals in the blood can occur after extended periods of fasting, you should never be asked to fast for longer than sixteen hours.
What You Need to do while Fasting
Start your fast at least the number of hours advised before the blood test is due to take place. During the hours that follow, you should not eat anything at all, which includes not chewing gum. You should also not smoke and try to avoid unnecessary exercise since this can increase your feeling of hunger.
You can drink water, but it must be pure water without additives so do not drink tea, coffee, cordials or any similar drinks. You should also not consume alcohol for a longer period, generally a full 24 hours before your blood test is due.
You will usually continue to take any prescribed medication as normal unless advised otherwise by your doctor. However, seek advice before taking any other drugs. Anyone who is a diabetic should generally not fast without medical advice and children under twelve years old should fast for no longer than four to six hours.
Always seek the advice of your doctor well before your blood test is due. And, if you do inadvertently take something you shouldn’t during the fasting period, let your doctor know without delay. This will affect the results of your test and may mean you have to reschedule.