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Why you have blood tests

Blood tests can

  • Check your general health, including how well your liver, kidneys, heart and other organs are working
  • Check the numbers of different types of blood cells
  • Help to diagnose cancer and other conditions
  • Check for infections
  • Check levels of certain drugs in your body
  • Help to find out if a cancer has come back

How you have the test

A doctor, nurse or phlebotomist may do the test. They can take the blood from any vein but usually use a vein in the inner part of your elbow. They put a small needle into your vein and attach a syringe or small bottle to draw out some blood. Then they take the needle out and put a cotton wool ball or small piece of gauze on the area. If you have a central line, your doctor or nurse will usually use this to take the blood sample instead.

Before the blood test

You can eat and drink normally before most blood tests. But for some tests you need to stop eating and drinking some time beforehand. Your doctor or nurse will tell you if you need to do this and for how long. You may also need to stop taking certain tablets for some tests, so check with your doctor or nurse.

Getting the results

You may get the results of some tests within a couple of hours – for example, a full blood count. Other types of test may take a few days or a few weeks.

 

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Why you have blood tests

Blood tests can

  • Check your general health, including how well your liver, kidneys, heart and other organs are working
  • Check numbers of blood cells
  • Help diagnose cancer and other conditions
  • Check for infections
  • Check levels of certain drugs in your body
  • Help to find out if a cancer has come back
 

Types of blood tests

There are a number of different blood tests. These include

A full blood count (FBC)

This is one of the most common blood tests. A full blood count measures the number of red cells, white cells and platelets in your blood.

Red cells carry oxygen around our bodies. Haemoglobin is the part of the red cell that carries the oxygen.  If you have a low red cell count, your doctor may say you are anaemic (pronounced a-nee-mic). This can cause tiredness, shortness of breath and dizziness.

White cells fight infections. There are several different types of white cells, including neutrophils and lymphocytes. You may hear your doctor talk about your neutrophil count when you are recovering from chemotherapy.

Platelets help clot the blood. If you have low platelets, you may have abnormal bleeding, such as bleeding gums and nosebleeds.

There isn’t an exact range of normal for blood counts. The range of figures quoted as normal varies slightly and also differs between men and women.

  Adult woman Adult man
Red blood cells 3.8 to 5 x 10 12/l (3.8 to 5 million per cubic millimeter of blood) 4.5 to 6.5 x 10 12/l (4.5 to 6.5 million per cubic millimeter of blood)
Haemoglobin (Hb)

11.5 to 16.5 grams per 100ml of blood 

(115 to 165 grams per litre of blood)

13 to 18 grams per 100ml of blood

(130 to 180 grams per litre of blood)

White blood cells 4 to 11 x 10 9/l (4,000 to 11,000 per cubic millimeter of blood) As for women
Neutrophils 2.0 to 7.5 x 10 9/l (2,000 to 7,500 per cubic millimeter of blood) As for women
Lymphocytes 1.3 to 4.0 x 10 9/l (1,300 to 4,000 per cubic millimeter of blood) As for women
Platelets (thrombocytes) 150 to 440 x 10 9/l (150,000 to 440,000 per cubic millimeter of blood) As for women

Our figures are from the Oxford Handbook of Clinical Medicine and Anatomy and Physiology in Health and Illness (Ross and Wilson).

We have more information about the different blood cells.

Urea and electrolytes (U&Es)

These blood tests show how well your kidneys are working. Urea is a waste chemical produced from digesting protein. Our kidneys remove urea from the blood and get rid of it in the urine. Electrolytes are substances such as sodium, potassium, chloride and bicarbonate. Some treatments may cause changes in the level of electrolytes in your blood, or if you are dehydrated due to diarrhoea or being sick (vomiting) for example. The balance of urea and electrolytes is important for our bodies to work normally.

Liver function tests (LFTs)

Liver function tests are another group of commonly used blood tests. Doctors do them to find out how well your liver is working. Sometimes this is purely routine. At other times the doctor use them to find out why you have particular symptoms.

LFTs look for levels of enzymes and proteins made by the liver. We've listed some of these below.

  • Alanine aminotransferase (ALT) 
  • Aspartate aminotransferase (AST) 

are both enzymes that help to process proteins. They may be raised if your liver is inflamed or injured.

  • Alkaline Phosphatase (ALP) 
  • Gamma-Glutamyl transferase (Gamma GT) 

are also both enzymes. They may be raised if you have a blockage in your liver or bile duct, or if you drink a lot of alcohol.

  • Bilirubin 

is a chemical in bile. High levels in the blood cause jaundice. Bilirubin may be raised if you have a problem with your liver or gallbladder.

  • Albumin

is a protein that may be low in some types of cancer. It can also be low if you haven't been eating properly and are malnourished.

You may also have blood tests called clotting studies. These show if your blood is clotting normally to stop bleeding. Because the liver makes the proteins needed for clotting, these tests can also help to show up liver problems.

Blood cultures

These are samples of blood that a doctor or nurse takes to check for infection.

If you have signs of infection, such as a high temperature, your doctor needs to find out what type of infection you have so they can make sure you are on the right type of treatment. They take samples from different areas of your body to send to the lab. This may include a urine sample, swabs from a wound or central line, and blood cultures.

Tumour marker tests

Tumour markers are substances produced by a tumour or by the body as a response to cancer. They are usually proteins. Some tumour markers are only produced by one type of cancer, while others can be made by several cancer types. Some markers are found in non cancerous conditions as well as cancer. Doctors may use tumour markers to help diagnose a cancer, or to see how well the cancer is responding to treatment. Examples of tumour markers include

Genetic test

Some people may have a blood test to look for a change in a gene (mutation) that increases the risk of cancer. We have more information about having a cancer gene test.

 

Before your blood test

You can eat and drink normally before most blood tests. But for some tests you need to stop eating and drinking for a while beforehand. This is called fasting. Tests that you might need to fast for include

  • Glucose tests
  • Cholesterol tests

Your doctor will tell you how long you need to fast for.

For tests that check the levels of particular substances in the blood you may need to stop taking certain tablets, such as iron or thyroxine. Check with your doctor beforehand whether you need to stop taking tablets.

 

Having a blood test

You sit or lie down to have the test. Your doctor, nurse, or someone who specialises in taking blood (a phlebotomist) takes the sample. They can take it from any vein but usually use one in the inner part of your elbow. First you have a tight band put around your upper arm. Then you have a small needle put into your vein. The person taking the blood attaches a syringe or small bottle to the needle and draws out some blood. They then release the band around your arm, take the needle out and put a cotton wool ball or small piece of gauze on the area.

You need to press down on the cotton wool or gauze for a few minutes to help stop bleeding and bruising. You may then have a plaster put over the area to keep it clean. If you have a central line, your doctor or nurse will usually use this to take the sample rather than your arm. The blood sample goes to the laboratory for testing.

Many people don’t like needles and don’t like the sight of blood. Some feel faint at the sight of it. Tell the person taking the blood if you are worried or you start to feel faint. You may want to look away while they take the blood.

After the test you may have a small bruise where the blood was taken from. But most people can only see a very small mark.

 

Getting your results

Ask your doctor when you are likely to get the results and who will give them to you. You can get the results of some tests within a couple of hours - for example, a full blood count. Others take a few days.

Some tumour marker tests take a few weeks. Genetic tests can take from a couple of weeks to a few months.

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Updated: 21 April 2015