Blood tests

A blood test is when a sample of blood is taken for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test. For example, a blood test can be used to:

  • assess your general state of health
  • confirm the presence of a bacterial or viral infection
  • see how well certain organs, such as the liver and kidneys, are functioning

A blood test usually involves the phlebotomist taking a blood sample from a blood vessel in your arm. and the usual place for a sample is the inside of the elbow or wrist, where the veins are relatively close to the surface. Blood samples from children are most commonly taken from the back of the hand. The child’s hand will be anaesthetised (numbed) with a special cream before the sample is taken.

Non-fasting blood tests

What is the difference between a fasting and non-fasting blood test?

A fasting test gives you a ‘bottom line’ result – this is the lowest you can expect the sugar and cholesterol figures to be.

If you have eaten just before the test, then the levels of sugar and cholesterol are determined by what you have just eaten – not by your body’s ability to handle these substances.

  • If a non-fasting level is high, it might not mean anything at all is wrong.
  • If a fasting level is high, then something is definitely wrong.

Why is fasting necessary and if you do eat what does it do to the results of the tests?

A fasting glucose test is conducted to determine the amount of sugar or glucose that is in your blood, after refraining from eating or drinking for a set time prior to the test. This test is also known as a “blood sugar” or “plasma glucose” test. This test is usually the first one conducted when diagnosing diabetes and may also be used periodically to monitor the progress of diabetic individuals. The test is simple, relatively painless and takes very little time to perform, although it does require advance preparation in order not to disrupt the results.


For a fasting “blood sugar” test, the doctor will have asked you to not eat or drink anything except water for at least eight hours, prior to having your blood test. If you have any specific nutritional needs that may impact on your ability to fast for the required length of time, please notify the practice as soon as possible.

You can find out more about blood tests, their purpose and the way they are performed on the NHS website.