Why do I need to fast before some blood tests

(Inside Aotea, April 2010)

You’ve just been told you need some blood tests and have been instructed to fast before the test. Apart from making you hungry, what are the real reasons for fasting before a blood test? Max Reed, Biochemistry Head of Department, answers some common questions

Which blood tests do I need to fast for?

Common tests that require fasting are:

  • fasting glucose
  • fasting lipids
  • insulin
  • glucose tolerance test
  • pregnancy glucose tolerance test

Your doctor may request other blood tests that require you to fast. If you’re unsure about what to do, call your doctor or our laboratory, or check our website.

Why do I need to fast?

After eating, changes occur in the blood, especially in levels of glucose, lipids and iron. A fasting period of 10-12 hours, typically overnight, reduces the variability in the levels of these substances (and others) in the blood. This ensures that your test results are not influenced by your latest morsel, and can be interpreted correctly by the laboratory and doctor.

Can I have anything to drink?

Yes — water! A drink of water is fine if you’re feeling thirsty. However, your other favourite beverages, including tea and coffee, will have to wait until after the test.

Can I take my normal medication?

In most cases you can continue to take your normal medication as scheduled, especially if this is what you’ve done for previous tests. If you are unsure check with your doctor. Certain medications can affect the results of fasting blood tests, so it’s up to your doctor to decide what is best for you.

What happens if I don’t fast?

This is a time when you really should do as you’re told! It’s necessary to follow the fasting instructions in order to get accurate test results.

Many tests are used to diagnose a medical condition, such as diabetes, so inaccurate results could potentially lead to unnecessary tests or a wrong diagnosis. If you do eat during the fasting period, it’s important to inform the person doing the blood test, so they can make a note on your record.