What it is
A complete blood count (CBC), also called a full blood count or haemogram, counts the cells in a small sample of your blood. It counts red blood cells, which carry oxygen, white blood cells, which fight infection, and , which help the blood to clot. It also measures , the iron-rich protein in red cells that carries oxygen, and haematocrit, the share of your blood that is made up of red cells. A standard count gives the total number of white cells. A CBC with differential also counts each main type of white cell separately.
Why it is done
A CBC is one of the most common blood tests and is often part of a routine check-up. A doctor may also ask for it to find out whether you have anaemia, to look into why you have certain symptoms, or to follow a condition or treatment that affects the blood. It helps to find blood disorders such as sickle cell disease. In dengue, doctors look at the white cell and platelet counts, which can fall during the illness and rise again as you recover.
What it helps with
How to prepare
A CBC usually needs no special preparation. If other tests are done on the same sample, you may be asked not to eat or drink for several hours beforehand. The lab or your doctor will tell you.
Do
- Ask the lab whether you need to fast
- Tell the nurse about every medicine you take
Do not
- Do not stop any medicine unless your doctor tells you to
What happens
The blood sample is taken like this:
- You sit down and roll up your sleeve. A soft strap may be tied around your upper arm.
- A small needle goes into a vein, usually on the inside of the elbow. You may feel a brief sting.
- Blood runs into a small tube. This usually takes under five minutes.
- The needle is taken out, and cotton wool or a plaster is put over the spot.
- The tube is sent to the lab.
Afterwards
Bruising or swelling around the spot can happen and should fade within a few days. Results usually take a few days, and most come within a few weeks. Ask the lab when and how you will get yours. If you have heard nothing after a few weeks, contact the doctor who sent you.
Risks and side effects
A CBC carries very little risk. You may have slight pain or bruising where the needle went in, and this usually fades quickly. If you are worried about needles, tell the person taking your blood. You can bring someone with you for support.
Results and what comes next
Your report lists each count next to the lab's own normal range, which depends on age, sex and the method the lab uses. Compare your result only with the range on your own report. A result outside the range does not always mean you have a condition that needs treatment. Diet, activity, medicines, periods and not drinking enough water can all change it, and healthy people sometimes fall outside it.
A low can point to anaemia, and a high white cell count can point to infection or a reaction to a medicine. A CBC points to a problem but does not name the cause, so a doctor reads it with your symptoms and an examination. If anaemia is found, the doctor may check for other kinds, such as low vitamin B12 or folate, may look for a cause if it is not clear, for example with a stool test for blood, and may repeat blood tests over the following months to see whether iron levels are returning to normal.
Questions to ask your doctor
Tick the ones you want answered and take them with you.
Common questions
Do I need to fast for a CBC?
Usually not. A CBC on its own needs no special preparation. You may be asked to fast only if other tests are done on the same sample, so ask the lab when you book.
What is the difference between a CBC and a CBC with differential?
A standard CBC gives the total number of white cells. A CBC with differential also counts each main type of white cell separately. Your doctor decides which one you need.
Does a result outside the normal range mean I am ill?
Not always. Diet, activity, medicines, periods and not drinking enough water can change the counts, and healthy people sometimes fall outside the range. Your doctor reads the result with your symptoms and may repeat the test.
Why is a CBC done in dengue?
Low white cell and platelet counts are commonly seen in dengue. As a person recovers, the white cell count usually starts to rise first, followed by the platelets. Doctors read these counts together with your symptoms to follow the illness.