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Blood tests

Find out about blood tests, how, why you have them and what happens afterwards.

Blood tests can:

  • check your general health, including your liver, kidneys and heart function
  • check numbers of blood cells
  • help diagnose cancer and other conditions

Before your blood test

You can eat and drink normally before most blood tests. Your doctor will tell you if you need to stop eating and drinking beforehand and for how long. This is called fasting. Tests that you might need to fast for include:

  • glucose tests
  • cholesterol tests

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

What happens

You sit or lie down to have the test.

A nurse or person specialised in taking blood (a phlebotomist) chooses the best vein to use. This is usually from your hand or arm. 

They put a tight band around your arm above the area where they take the sample. Then they put a small needle into your vein. Next, they attach a syringe or small bottle to the needle to draw out some blood. They might fill several bottles.

Once they have all the samples, they release the band around your arm. Then they take the needle out and put a cotton wool ball or small piece of gauze on the area. Pressing down on the cotton wool or gauze for a few minutes 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 your doctor about when you’ll get your results, and who will give them to you. Some results may be available quickly (for example, full blood count). Some tumour marker and genetic tests may can take several weeks.

Possible risks

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 have a tingling or shooting pain.

Swelling (oedema)

Don’t have 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.

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. 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.

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). And LFTs measure albumin. This is a protein 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

Tumour markers are substances that either the tumour or your body produces as a response to a cancer. They’re usually proteins.

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.

Tumour markers for stomach cancer include:

  • carcinoembryonic antigen (CEA)
  • carbohydrate antigen (CA 19-9)
  • tumour associated glycoprotein
Last reviewed: 
20 Jul 2016
  • Oxford handbook of clinical medicine anatomy and physiology in health and illness
    Ross and Wilson
    Churchill Livingstone, 2010

  • Prognostic significance of preoperative serum CA125, CA19-9 and CEA in gastric carcinom
    W Weng and others
    Oncotarget, 2016. Vol 10 

  • Differences and correlation of serum CEA, CA19-9 and CA72-4 in gastric cancer
    J Yu and others
    Molecular and clinical oncology, 2016. Vol 4

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