Fasting and Blood Tests: What You Need to Know
Blood tests are an indispensable part of your healthcare. Human bodies are complicated things – and doctors cannot always understand any health problems you may have just by looking at or talking to you. Rather, they’ll sometimes need to see what is going on at a chemical level inside you. And blood tests are the best way to do this.
To check your blood sugar levels, to monitor your kidney function, or to keep an eye on your electrolytes, blood tests do a lot of things. However, you do want to ensure that you get accurate results. And to do this you may have to fast.
Why? Because the things you eat affect your blood test results. Simple.
Here we’ll give you the full picture on fasting before blood tests. We’ll cover why you need to fast in more detail – along with the specific tests that will ask you to go hungry before you take them.
Can You Eat Before a Blood Test?
It’s one of the most common questions that people have about their blood work. So, do you need to fast before a blood test?
It depends. Some tests take a snapshot of your blood’s chemical composition at that precise moment – and what you have recently eaten will affect those results. Others, such as the HbA1c test, take a longer view, meaning that fasting won’t have an impact.
While we’ll cover the general guidelines, it is best to follow the advice of the healthcare professional who is administering the test. Rules on fasting can change from country to country and region to region – depending on the technologies used locally.
Why Do You Need to Fast?
Fasting helps doctors get a true sense of the normal chemical composition of your blood. Say that you eat lots of sugary foods, or foods high in fat, prior to a glucose test. These will have a significant impact on your blood sugar readings. You are what you eat, and all that.
Yet, it is not always as simple as that example suggests. You personally probably won’t be able to predict what will or will not have an effect on your blood test results. So, when you are told to fast, it is probably best if you do it properly.
Which Tests Do You Need to Fast For?
The most common tests that you will need to fast for are those referred to as ‘fasting blood tests’. Surprise surprise. These include the following:
- The fasting blood glucose test. These are often taken by people with diabetes, or by those at risk of diabetes. It tests for the amount of glucose in your blood (blood sugar) – and you’ll usually be required to fast for 12 hours beforehand.
- The cholesterol test. This one tests for lipids (including HDL and LDL, the ‘good’ and ‘bad’ cholesterol levels) or triglycerides – a type of fat in your blood. The fats that are found in your food will affect these test results. Again, 12 hours of fasting is usually recommended.
- The iron blood test looks at the iron levels in your blood. It’s important, because it can show if you are at risk of anaemia (or anemia), a harmful deficiency. Any iron from your recent diet will affect these test results.
These are the most common tests for which you will need to fast. Alternatively, however, non-fasting tests include:
- Liver function tests
- Kidney function tests
- Thyroid function tests
- HbA1c or haemoglobin A1c tests.
If you are taking any of these tests, you don’t need to worry about fasting. What you eat won’t affect their results.
How to Fast for a Blood Test
So, by now you know when and when not to fast. But what does that actually mean? What can you eat and drink and what shouldn’t you? And how do you survive the hunger?
Well, firstly, let’s be clear. If asked to fast, you should drink only water for the time prior to your test that you have been told to fast. This means no food or supplements, no coffee or tea, no alcohol, and no cigarettes. These will all affect the readings of your test. Your doctor will know if you have done this – and you may be asked to take the test again.
However, do still take any medications that you have been prescribed. It’s important that you don’t actually endanger your health by fasting. So, if you are diabetic, or if you are on medication for high blood pressure for example, let your doctor know and take your medication as normal.
The best strategy for fasting for blood tests is to sleep through it. That’s why the majority of tests will happen in the morning. This means you can have your evening meal as normal, go to bed, and skip breakfast prior to the test.
But be sensible. If you feel weak or unable to concentrate, don’t drive or use heavy machinery. Ask someone to take you to your appointment.
Key Takeaways
So, can you eat before a blood test? Again, it depends. While tests on your glucose content, lipid profile, or triglyceride levels will require fasting, others blood tests don’t. The best thing you can do is follow your doctor’s advice.
Remember, though, fasting means fasting. During the 12 hours prior to the test, you should only drink water. Sleeping through the hunger is the best way to manage it.
While we've ensured that everything you read on the Health Centre is medically reviewed and approved, information presented here is not intended to be a substitute for professional medical advice, diagnosis, or treatment. It should never be relied upon for specific medical advice. If you have any questions or concerns, please talk to your doctor.
