Blood tests can:
- check your general health, including how well your liver and kidneys are working
- check numbers of blood cells
- help diagnose cancer and other conditions
Your blood sample is sent to the laboratory. A specialist doctor called a pathologist looks at your sample under a microscope.
They can see the different types of cells and can count the different blood cells. They can also test for different kinds of chemicals and proteins in the blood.
Preparing for your blood tests
You can eat and drink normally before most blood tests. For fasting blood tests you need to stop eating and drinking beforehand. Your doctor will tell you for how long.
You sit or lie down to have the test.
A doctor, nurse or person specialised in taking blood (a phlebotomist) chooses the best vein to use. This is usually from your arm or hand. Let them know if you are afraid of needles, get unwell with the sight of blood or are allergic to plasters or latex.
They put a tight band (tourniquet) around your arm above the area where they take the sample. You may be asked to clench your fist to make it easier to find a vein.
They clean your skin and then put a small needle into your vein. Next, they attach a small bottle or syringe to the needle to draw out some blood. They might fill several small bottles.
Once they have all the samples, they release the band around your arm. They then take the needle out and put pressure on the area with a cotton wool ball or small piece of gauze for a few minutes. This helps to stop bleeding and bruising.
Look away when they’re taking the blood if you prefer. Tell your nurse or phlebotomist if you feel unwell.
Getting your results
Ask the phlebotomist, doctor or nurse when and how you will get your results. Some results might be available quickly, such as a full blood count. But some other tests might take several weeks.
Blood sampling (phlebotomy) is a safe test. There is a possibility of:
You can bleed if you’re taking medicines to thin your blood (anticoagulants) such as aspirin. Pressing hard when the needle is removed helps to stop it.
Sometimes blood leaks out of the vein and collects under your skin. This can look like a small dark swelling under the skin (haematoma). Pressing hard once the needle is removed can help.
The site of the test can be tender for a few minutes. Tell the person taking the blood if you feel any tingling, shooting pain, or a burning sensation.
You should avoid whenever possible having blood taken from an arm that is swollen or has a risk of swelling: for example, after surgery or radiotherapy to the lymph nodes on that side. Ask your nurse to use the opposite arm to take the sample.
Feeling faint or fainting
This can happen whilst you are having the blood test or just after. Tell the person doing your blood test if you are feeling lightheaded or dizzy at any time.
Having a blood test is a safe test. But because your skin has been broken there is a very small risk of infection. The person taking the blood will make sure they do everything to reduce this rare side effect.
Types of blood tests
Full blood count
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 cell that carries oxygen. If you have a low red cell count, your doctor might say you’re anaemic (pronounced a-nee-mic). This can make you feel tired, short of breath and dizzy.
- White cells fight infections. There are several different types of white cells, including neutrophils and lymphocytes.
- Platelets help clot the blood. Symptoms of a low platelet count include 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.
Urea and electrolytes
These blood tests show how well your kidneys are working. Waste chemicals called urea and creatinine are produced as proteins are broken down. Our kidneys remove them from our blood and get rid of it in our urine.
Electrolytes are substances such as sodium, potassium, chloride and bicarbonate.
Liver function tests (LFTs)
Liver function tests (LFTs) check how well your liver is working. LFTs look for levels of enzymes and proteins made by the liver. They include:
- alanine aminotransferase (ALT)
- aspartate aminotransferase (AST)
- alkaline phosphatase (ALP)
- gamma-glutamyl transferase (Gamma GT)
They might be raised if you have a blockage in your liver or bile duct, or if you drink a lot of alcohol.
LFTs also look at the amount of bilirubin in the blood. This is a chemical in bile.
Bilirubin can be raised if you have a problem with your liver or gallbladder. Bilirubin can cause yellowing of your skin and eyes (jaundice).
LFTs also measure albumin. This is a protein in the blood that can be low in some types of cancer. You can also have low albumin if you’ve been eating small amounts and are malnourished.
Tumour markers blood test
Tumour markers are substances that either the tumour or your body produces as a response to a cancer. They’re usually proteins. They can be found in the blood, urine or the bodies tissues.
Some tumour markers are only produced by one type of cancer. Others can be made by several types. Some markers are found in non cancerous conditions as well as cancer.
Doctors might use tumour markers to help diagnose a cancer. And if you have cancer they can also help to monitor how well your cancer treatment is working or check if the cancer has come back.
We have more information on tests, treatment and support if you have been diagnosed with cancer.