Page last reviewed: 13/07/2011
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
Blood is pumped around the body by the heart. It supplies oxygen to the body's organs, muscles and tissues, and removes carbon dioxide.
The blood that circulates around the body contains many different substances. About 40% of the blood's volume is made up of blood cells. There are three types of blood cell:
- red blood cells carry oxygen from the lungs and transport it around the body
- white blood cells form part of the body's immune system and help defend the body against infection
- platelets are cells that help the blood to clot (thicken) when you cut yourself
The remaining 60% of blood is made up of plasma. Plasma mainly consists of water, but also contains proteins and chemicals, such as hormones, glucose and salt.
Uses of blood tests
Blood tests are very useful for a number of reasons, which are described below.
As blood circulates through your body's organs, it is very sensitive to any changes or damage to the organs. For example, if your liver becomes damaged, it will release certain enzymes into the blood, which can be detected using a blood test.
Checking for infection
As blood plays an important part in the immune system's defence against infection, changes in the make-up of blood can provide important clues about possible infections.
For example, certain viruses, such as HIV, will cause your immune system to produce special proteins called antibodies. By checking the blood for certain types of antibody, it is possible to confirm (or rule out) whether an infection has occurred.
Measuring oxygen and carbon dioxide
As blood is the body's oxygen supply system, testing can provide important information about possible respiratory conditions (conditions that affect your lungs).
For example, the volume of oxygen and carbon dioxide in your blood can be measured. This is one method of determining how well your lungs are working.
Blood tests are a convenient way to obtain a DNA sample for genetic testing and screening. For example, blood tests can be used to diagnose genetic conditions, such as cystic fibrosis (a genetic disorder that causes internal bodily secretions to become thick and sticky, interfering with the function of certain organs, such as the lungs).
During a blood test, a sample of blood is taken from a vein using a needle so that it can be examined in a laboratory.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Plasma is the liquid part of blood, which holds other blood cells together.
Glucose (or dextrose) is a type of sugar that is used by the body to produce energy.
Page last reviewed: 13/07/2011
Some widely used blood tests are described below.
Full blood count (FBC)
A full blood count (FBC) is probably the most widely used blood test. It is used to assess your general state of health and to screen for certain conditions, such as anaemia.
During an FBC, a small sample of blood will be taken from a vein in your arm. The amount of different types of blood cells in the sample will be measured.
On its own, an FBC cannot usually provide a definitive diagnosis of a condition, but it can provide important 'clues' about possible problems with your health.
- A low red blood cell count may be due to anaemia (iron deficiency), which has a number of possible causes, including internal bleeding or a poor diet.
- A high red blood cell count may be due to an underlying lung or kidney disease.
- A low white blood cell count may be due to problems with your bone marrow, such as a viral infection of your bone marrow or cancer of the bone marrow, such as leukaemia.
- A high white blood cell count usually suggests that you have an infection somewhere in your body.
- A low platelet count may be due to a viral infection, such as rubella (german measles), or an autoimmune condition (where the immune system attacks healthy tissue), such as lupus (inflammation in the body's tissues).
- A high platelet count may be due to inflammatory conditions, such as rheumatoid arthritis (pain and inflammation of the joints), or a problem with your spleen, such as a ruptured (split) spleen. The spleen is an organ that removes damaged blood cells from your blood.
An electrolyte test is used to measure the levels of electrolytes in your blood. This is sometimes known as your electrolyte balance. Electrolytes are minerals that are found in the body. They have a number of important functions, including:
- helping to move nutrients into cells (and waste products out of them)
- helping to maintain a healthy water balance in your body
- helping to stabilise levels of acid and alkali in your body
There are three main electrolytes that can be measured with an electrolyte test:
Raised or lowered levels of any of these electrolytes can have a number of possible causes.
- A raised sodium level (hypernatremia) could be the result of dehydration, kidney disease or persistent diarrhoea.
- A low sodium level (hyponatremia) could be the result of poorly controlled diabetes, liver disease, a lack of sodium in your diet or pneumonia. Some types of medication can also lower your sodium level, such as carbamazepine (used to treat epilepsy) and sertraline (sometimes used to treat depression).
- A raised potassium level (hyperkalemia) could be the results of kidney failure. A type of medication used to treat high blood pressure, known as an ACE inhibitor, can also raise potassium levels.
- A low potassium level (hypokalemia) could be the result of heavy sweating or persistent vomiting or diarrhoea.
- A raised chloride level (hyperchloremia) could be the result of some types of kidney disease, diarrhoea or overactive parathyroid glands (glands that are found in your neck and help to regulate the amount of calcium in your body).
- A decreased chloride level (hypochloremia) could be the result of heavy sweating, vomiting and some types of kidney disease.
Blood glucose test
A blood glucose test is used to help diagnose diabetes and to monitor the health of people who have had a diagnosis of diabetes confirmed.
Diabetes develops either because the body cannot produce enough insulin or because the insulin does not work in the right way. Insulin is a hormone that the body uses to convert glucose (sugar) into energy.
People with diabetes often have high levels of glucose in their blood. Reducing the glucose levels is an important part of the treatment of diabetes. This is because if the blood glucose levels become too high, a range of serious complications, such as kidney disease or nerve damage, may occur.
Therefore, most people with diabetes will need regular blood glucose tests. Small blood glucose test kits are available for use at home. These only require a small 'pin prick' of blood for testing.
Some types of blood glucose test require you not to eat anything for several hours before the test. Your GP or diabetes care team can tell you whether this is the case.
If you have a viral or a bacterial infection or you have developed an allergy, your immune system will produce specific antibodies in response to the infection or allergy.
The ELISA test involves taking a small blood sample and checking to see whether it contains the associated antibody.
Blood gas test
A blood gas test is used to check two things:
- the balance of oxygen and carbon dioxide in your blood
- the balance of acid and alkali in your blood (the pH balance)
An imbalance in either of these can be caused by:
- problems with your respiratory system
- problems with your metabolism (the chemical reactions that are used by the body to break down food into energy)
Respiratory causes of an imbalance could be:
Metabolic causes of an imbalance could be:
- kidney failure
- persistent vomiting
Some widely used types of genetic testing are described below.
A gene test is used when healthcare professionals suspect that a specific genetic mutation may be responsible for a person's symptoms. The test involves extracting a sample of DNA from your blood, then searching the sample for the suspected genetic mutation.
Genetic conditions that can be diagnosed using a gene test include:
- haemophilia : a condition that affects the blood's ability to clot (thicken)
- cystic fibrosis: a condition that causes a build-up of sticky mucus in the lungs
- sickle cell anaemia: a condition that causes a shortage of normal red blood cells
- polycystic kidney disease: a condition that causes cysts to develop inside the kidneys
Chromosome testing, also known as karyotyping, is a more general test than a gene test. It is used when healthcare professionals suspect that a person's symptoms may be caused by a gene-related problem but they do not know which gene is responsible.
Chromosome testing involves taking a blood sample and examining one of the blood cells under a powerful microscope. This allows the person who is carrying out the test to examine the chromosomes directly.
Chromosomes are coils of DNA that are found in every cell. By counting the chromosomes (each cell should have 23 pairs of chromosomes) and by checking their shape, it may be possible to detect genetic abnormalities.
Chromosome testing is often used:
- to test children who have physical or developmental problems that have no apparent cause
- for couples who have experienced repeated miscarriages (usually three or more in a row)
Genetic screening is similar to gene testing except that it is used in people who have no obvious symptoms. For example, genetic screening is carried out during pregnancy (antenatal screening) to check for some of the most common genetic conditions, such as:
- Down's syndrome
- sickle cell anaemia
Genetic screening may also be offered to people who are thought to be at risk of developing a genetic condition. For example, if your brother or sister developed a genetic condition in later life, such as Huntington's disease, you may want to find out whether there is a risk that you could also develop the condition.
A number of genetic conditions, such as sickle cell anaemia, can only be passed on to a child if both parents have a copy of the genetic mutation.
If there is a chance that you have one copy of the mutated gene, for example there is a history of sickle cell anaemia in your family, you and your partner may want to be screened to determine whether there is a risk of giving birth to a child with the condition.
A blood typing test is used to identify your blood group. Your blood group is determined by two specialised proteins, known as antigens, which are found on the surface of your red blood cells.
The first antigen, known as the ABO antigen, comes in four possible types:
The second antigen, known as the Rhesus antigen, comes in two possible types:
- Rhesus positive (which is usually shortened to Rhd+)
- Rhesus negative (which is usually shortened to Rhd-)
Your blood type is based on the combination of both antigens. For example, if you have a combination of the AB antigen and the Rhd + antigen, your blood type would be AB+.
Blood typing is used before a blood transfusion is given (or before you provide blood for donation). This is because it is important that anyone who receives blood is given blood that matches their blood group. If you were given blood that did not match your blood group, your immune system may attack the red blood cells, which could lead to potentially life-threatening complications.
Blood typing is also used during pregnancy as there is a small risk that the unborn child may have a different blood group from the mother. This could lead to the mother's immune system attacking the baby's red blood cells. This is known rhesus disease.
Rhesus disease can sometimes develop if the mother's blood group is Rhd- and the father's is Rh+. As a result, blood typing tests are used during the routine screening programme in pregnancy.
If testing reveals that there is a risk of rhesus disease developing, extra precautions can be taken to safeguard the health of your baby. For example, a blood transfusion can be given to the baby when it is still in the womb to increase the number of red blood cells.
Blood cholesterol test
Cholesterol is a fatty substance that is known as a lipid. It is mostly created by the liver from the fatty foods in your diet and is vital for the normal functioning of the body.
Having an excessively high level of lipids in your blood (hyperlipidemia) can have a serious effect on your health because it increases your risk of having a heart attack or stroke.
Blood cholesterol testing is usually recommended if you have one or more risk factors that increase your chances of developing a cardiovascular disease (CVD). A cardiovascular disease is a disease that affects the normal flow of blood through the body, such as a stroke or heart attack.
The risk factors for developing a CVD include:
- being diagnosed with coronary heart disease, stroke or mini-stroke, or leg artery disease
- being over 40 years of age
- being obese
- having high blood pressure (hypertension)
If you are unsure whether you would benefit from having a blood cholesterol test, ask your GP for advice.
Blood cholesterol levels are measured with a simple blood test. Before having the test, you may be asked not to eat for 12 hours (which usually includes the period when you are asleep at night) but you may drink water and take your regular medication. This will ensure that all food is completely digested and will not affect the outcome of the test.
Your GP or practice nurse can carry out the blood test and will take a blood sample, either using a needle and a syringe, or by pricking your finger. If the test identifies high cholesterol levels, treatment options include:
- making lifestyle changes, such as reducing the amount of fat in your diet
- taking medication, such as statins, to reduce your cholesterol level
Liver function test
A liver function test is a type of blood test that is used to help diagnose certain liver conditions, such as:
- hepatitis (infection of the liver)
- cirrhosis (scarring of the liver)
- alcoholic liver disease (liver damage and related loss of function which is caused by excessive alcohol consumption)
When the liver is damaged, it releases enzymes into the blood. At the same time, levels of proteins that the liver produces to keep the body healthy begin to drop.
By measuring the levels of these enzymes and proteins, it is possible to build up a reasonably accurate picture of how well the liver is functioning.
A blood culture is a test to check whether there is a bacterial infection of the blood (septicaemia). Septicaemia is potentially very dangerous because, if the infection spreads throughout the blood, it can trigger a massive drop in blood pressure. This is known as septic shock and it can be fatal.
A blood culture involves taking a small sample of blood from a vein in your arm and from another part of your body. Both samples are introduced to nutrients that are designed to encourage the growth of bacteria (a process known as culturing). If there are any traces of bacteria in your blood, culturing should allow the person carrying out the test to identify them.
Page last reviewed: 13/07/2011
Taking a blood sample
A blood test usually involves taking a blood sample from a blood vessel in your arm. The arm is a convenient part of the body to use because it can be easily uncovered. 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 usually taken from the back of the hand. The child's hand may be anaesthetised (numbed) with a special cream before the sample is taken.
A tight band (tourniquet) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood out of the arm, and causing the vein to swell with blood. This makes it easier for a blood sample to be taken.
Before taking the sample, the doctor or nurse may need to wipe the area with an antiseptic wipe, although this is not always necessary.
A needle attached to a syringe or to a special blood collecting container is pushed into the vein. The syringe is used to draw out a sample of your blood. You may feel a slight pricking sensation as the needle goes in, but it should not be painful. If you do not like needles and injections, tell the person who is taking the sample so they can make you more comfortable. If you feel you might faint, lie down.
When the sample has been taken, the needle will be removed. Pressure is applied to the tiny break in the skin for a few minutes using a cotton wool pad to stop the bleeding and to prevent bruising. A plaster may then be put on the small wound to keep it clean and prevent infection.
After the test
After the blood sample has been taken, it will be put into a bottle and labelled with your name. It will then be sent to a laboratory where it will be examined under a microscope or tested with chemicals, depending on what is being checked. The results are sent back to the hospital or to your GP, and you will be told when and how you will be given them.
Sometimes, receiving results can be stressful and upsetting. If you are worried about the outcome of a test, you may choose to take a trusted friend or relative with you. For some tests, such as HIV, you will be offered specialist counselling to help you deal with your results.
Page last reviewed: 13/07/2011
During a blood test, only a small amount of blood is usually taken, so you should not feel ill from the loss of blood. However, occasionally some people feel faint while having their blood taken.
If you feel weak or dizzy while having a blood test, tell the doctor or nurse who is taking the sample as they can help you feel more comfortable.
After a blood test, you may have a small bruised area on your skin where the needle went in. If you develop redness or swelling (inflammation), you may have an infection. See your GP for advice.
Occasionally, a larger area of bruising may appear after a blood test. Bruises are the result of bleeding under the skin.
The most common causes of bruising after a blood test are:
- lack of pressure at the site of the jab until the bleeding has stopped
- a vein or other blood vessel being damaged by the needle while it was being inserted
Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.
- Veins are blood vessels that carry blood to the heart from the rest of the body.
- Blood vessels
- Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
- Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
- Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.