Everything you need to know about blood tests including why and how they are done, with links to other useful resources.
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
- screen for certain genetic conditions such as cystic fibrosis.
What happens during a blood test?
Most blood tests are carried out at your GP surgery or your local hospital under the supervision of a nurse, or in some cases, a doctor.
A test usually involves placing a needle attached to a syringe into one of the blood vessels in the inside of your elbow or wrist. You will feel a sharp prick as the needle goes in but this isn't particularly painful.
A sample of blood is then taken and the needle is removed. You will be given a cotton-wool pad to put pressure on the the site of the injection, which stops any bleeding and should prevent bruising.
Most blood tests only take a few minutes to complete.
Read more about how a blood test is performed.
Only a small amount of blood is taken during the test so you shouldn't feel any significant after-effects.
However, some people do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so 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. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.
Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.
A description of some widely used blood tests. A full blood count (FBC) is probably the most widely used blood test.
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.
- Low haemoglobin indicates anaemia, which has a number of possible causes, including internal bleeding or a poor diet.
- High haemoglobin may be due to an underlying lung disease or problems with the bone marrow.
- A low white blood cell count may be due to problems with your bone marrow, a viral infection or more rarely, cancer of the bone marrow. However, a low white blood count can also be genetic and of no significance.
- A high white blood cell count usually suggests that you have an infection somewhere in your body. Rarely, this could be a sign of leukaemia.
- A low platelet count may be due to a viral infection or an autoimmune condition (where the immune system attacks healthy tissue).
- A high platelet count may be due to inflammatory conditions, infection or a problem with the bone marrow.
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 several 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 various possible causes.
- A raised sodium level (hypernatremia) could be the result of dehydration, uncontrolled diabetes or persistent diarrhoea.
- A low sodium level (hyponatremia) is usually due to certain types of medication, such as diuretics. Rarely, it could be due to a condition such as diabetes insipidus.
- A raised potassium level (hyperkalemia) could be the results of kidney failure. Certain medications can raise potassium, for example ACE inhibitors, which are used to treat heart failure and high blood pressure.
- A low potassium level (hypokalemia) could be the result of heavy sweating or persistent vomiting or diarrhoea. It can also be caused by certain medications.
Erythrocyte sedimentation rate (ESR)
An erythrocyte sedimentation rate (ESR) test is a blood test that is used to check whether there is inflammation in the body.
The test works by measuring how long it takes for red blood cells to fall to the bottom of the test tube. The quicker they fall the more likely it is that there are high levels of inflammation.
An ESR is often used to aid diagnosis in conditions associated with inflammation such as arthritis, Crohn’s disease, temporal arteritis and polymyalgia rheumatica.
Along with other tests, an ESR can be useful in confirming whether you have an infection in your body.
Blood glucose (blood sugar) test
A blood glucose test is used to help diagnose diabetes and to monitor the health of people who have been diagnosed with diabetes.
Diabetes develops either because the body cannot produce enough insulin or because the insulin doesn't 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 sugar 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. Blood glucose test kits may be available to use at home. These only require a small "pin prick" of blood for testing.
People with type 2 diabetes usually don't need to check their sugar at home, it will be tested every three-to-six months at your GP surgery or hospital. The test shows the average blood sugar level over the past three months.
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 takes a small blood sample and checks to see if it contains the associated antibody.
Blood gas test
A blood gases sample is taken from an artery, usually at the wrist. It's likely to be painful and the test will alway be carried out in hospital.
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
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 this way 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 gene-related problems are suspected, but the healthcare professionals don't 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 found in every cell. By counting the chromosomes (each cell should have 23 pairs) 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.
Genetic screening may 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 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. Read more about blood groups.
Blood typing is used before a blood transfusion is given (or before you provide blood for donation). This is because it's 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.
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 their number of red blood cells.
Blood cholesterol test
Cholesterol is a fatty substance 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 too many 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 are at an increased risk of developing cardiovascular disease (CVD). A cardiovascular disease, such as a stroke or heart attack, affects the normal flow of blood through the body.
Things that increase your risk of CVD include:
- being over 40 years old
- being obese
- being a smoker
- being male
- having high blood pressure (hypertension)
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 when you're asleep). This will ensure that all food is completely digested and won't 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.
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 and levels of proteins that the liver produces begin to drop. By measuring the levels of these enzymes and proteins, it's possible to build up a 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 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 designed to encourage the growth of bacteria (a process known as culturing). If there are traces of bacteria in your blood, culturing should highlight this.
Two blood samples are needed in case one is accidentally contaminated by the bacteria that live on your skin.
A blood test usually involves taking a blood sample from a blood vessel in your arm.
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 to be taken from 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 will 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 clean the area with an antiseptic wipe.
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 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's 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.