Preparing for a Blood Test – Information for patients, relatives and carers

Patient Experience

  • Reference Number: HEY952-2024
  • Departments: Pathology
  • Last Updated: 3 June 2024

Why is a blood sample needed?

Blood tests have a wide range of purposes 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, check if you have an infection or see how well certain organs such as the liver and kidneys are functioning. Some tests are used to help in the diagnosis of a specific health condition or to check how your treatment is progressing.

Your doctor will order a specific range of tests for you, your symptoms and medical history.

How do I prepare for the test/procedure?

In the vast majority of cases you do not need to prepare for a blood test. If you do need to make preparations, the healthcare professional who is requesting your blood tests will provide you with special instructions that you will need to follow before your test. For instance, you may be asked to avoid eating or drinking (except water) for up to 12 hours before your blood test. This is known as a fasting blood test. In some cases, you may be asked to stop taking a certain medication. You may also be told not to drink alcohol or not to smoke before your test.

If you are taking a medication that could affect the results of a blood test and are unsure about continuing, continue your medicine as prescribed until you have checked with your doctor. You may not necessarily need to stop taking as your doctor may be able to interpret the results of the blood test taking the effects of the medication into account. Herbal remedies, vitamins or supplements can also affect results, so if you take any of these then please tell your doctor.

It is important to follow the instructions you are given as this may affect the result of the test and mean that it needs to be repeated.

What happens during a blood test?

Most blood tests only take a few minutes to complete and are carried out at your GP surgery or local hospital by a doctor, nurse or phlebotomist (a specialist in taking blood samples).

It usually involves taking a blood sample from a blood vessel in your inner elbow or wrist where the veins are relatively close to the surface. In children, blood may be taken from the back of the hand. The person taking the blood may clean the area with an antiseptic wipe beforehand. For children, the skin may be numbed with a special spray or cream before the sample is taken.

The person taking the sample should ask you to confirm your full name and date of birth to ensure they are collecting the sample from the correct person. A tight band (tourniquet) is placed around your upper arm. This temporarily slows the flow of blood and causes the vein to swell making it easier to collect the sample. A needle attached to a syringe or special container is then inserted into the vein and a sample of your blood is collected. You may feel a slight pricking or scratching sensation as the needle enters the skin. If you don’t like needles or blood, tell the person who is taking the sample so they can make you more comfortable.

The number of syringes or containers required depends on the types of tests your doctor has ordered. They are different colours because they contain different preservatives to keep the sample fresh or stop the blood from clotting.

When the sample has been taken, the needle will be removed. Pressure is then applied to the skin for a few minutes using a cotton wool pad. A plaster may also be applied to the area.

Can there be any complications or risks?

Only a small amount of blood is taken during the test so you should not feel any significant side effects. However, some people feel dizzy and faint during and after the test. If this has happened in the past, tell the person who is taking the sample so they can help you feel more comfortable. Following the test you may develop a small bruise. Whilst bruises can be painful, they are usually harmless and fade after a few days.

What will happen to my sample?

The blood samples will be labelled with your name and date of birth and the date and time of collection. They will then be sent to the laboratory with a request form completed by your doctor, which tells the laboratory which tests are required. All laboratories have strict quality control procedures in place to ensure the correct result is given for the correct test and the correct patient.

What will happen afterwards?

When the test results are ready, they will be sent back to your GP or consultant (whoever requested the test) and they will discuss these with you. In some cases the result will be ready the same day or a few days afterwards. However, in other cases results may not be available for a few weeks. This is because some specialist testing is performed in a laboratory in another part of the country. Please do not contact the laboratory directly for your test results. Your results will be sent to the doctor or nurse who requested the test.

Occasionally, you may be asked to provide a second blood sample. This is not a cause for alarm. It may be because there was not quite enough blood in the first sample or the laboratory would like to confirm the results.

Sometimes, receiving results can be stressful and upsetting. If you are worried about the outcome of a test, you may wish to take a trusted friend or relative with you for support. For some tests (e.g. HIV or genetic testing) you will be offered specialist counselling to help you deal with your results.

What can I find out more information?

Your doctor should be able to discuss any further questions or queries with you.

Lab Tests Online-UK ( is written by practising laboratory doctors and scientists to help you understand the many clinical laboratory tests that are used in diagnosis, monitoring and treatment of disease.

Further information

For more information, please contact:

Duty Biochemist – Monday – Friday 09:00 to 17:00

York and Scarborough: tel: 01904 726366

Hull and East Riding: tel: 01482 607755


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