A complete blood count, or CBC (also called a full blood count, FBC), is one of the most common blood tests. It counts the three main kinds of cells in your blood and measures a few of their features.
It is often part of a routine check-up. Doctors also use it to look for anaemia, infection, inflammation and some blood conditions, or to follow a treatment.
Every lab sets its own normal range, based on its equipment and the people it tests. Always compare your result with the range printed on your own report.
Why this test is done
- Routine health check or before surgery
- Tiredness, weakness, bruising easily or frequent infections
- Following a known condition or a medicine that can affect blood cells
What each result means
Haemoglobin(Hb, HGB)
The protein in red blood cells that carries oxygen.
Typical adult range: About 13.2 to 16.6 g/dL in men and 11.6 to 15.0 g/dL in women (132 to 166 and 116 to 150 g/L).
If high: High values can come with dehydration, smoking, living at high altitude, lung or heart conditions, and rarely a bone marrow condition.
If low: Low haemoglobin is called anaemia. Common reasons include low iron, blood loss, low vitamin B12 or folate, and long-term illness.
Haematocrit(Hct, PCV)
The share of your blood made up of red cells.
Typical adult range: About 38 to 49% in men and 35 to 45% in women.
If high: Often a sign of dehydration, or the same causes as high haemoglobin.
If low: Usually moves with haemoglobin; low values point the same way as anaemia.
Red blood cell count(RBC)
How many red cells are in a set amount of blood.
Typical adult range: About 4.35 to 5.65 million per microlitre in men and 3.92 to 5.13 in women (x10^12/L).
If high: Seen with dehydration and the same causes as high haemoglobin.
If low: Seen with anaemia from many causes.
Mean cell volume(MCV)
The average size of your red cells.
Typical adult range: About 80 to 100 fL.
If high: Large red cells can be linked to low vitamin B12 or folate, alcohol, liver disease, thyroid problems or some medicines.
If low: Small red cells are often linked to low iron or a family trait such as thalassaemia.
White blood cell count(WBC, leukocytes)
The cells that fight infection. Many reports also split them into types (neutrophils, lymphocytes and others).
Typical adult range: About 3.4 to 9.6 thousand per microlitre (x10^9/L).
If high: Often a response to infection, inflammation, stress, smoking or steroids. Very high values need prompt follow-up.
If low: Can follow a viral infection or some medicines. A very low count raises the risk of infection and needs a doctor's advice soon.
Platelets(PLT, thrombocytes)
Small cell pieces that help blood clot.
Typical adult range: About 150 to 400 thousand per microlitre (x10^9/L).
If high: Often a reaction to infection, inflammation, low iron or bleeding.
If low: Low platelets can cause easy bruising or bleeding. Causes include viral infections, some medicines and liver disease.
Before the test
- No fasting is usually needed for a CBC on its own. Follow your clinic's advice if other tests are taken at the same time.
Questions to ask your doctor
- Which of my results are outside the range, and does that matter for me?
- Could low iron, B12 or folate explain this?
- Do I need a repeat test, and when?
- Could any of my medicines affect these numbers?
When to get help fast: Get medical help quickly if you have a very low or very high result together with fever, bleeding that will not stop, chest pain, fainting or shortness of breath. In an emergency, call your local emergency number.
Sources
- MedlinePlus: Complete Blood Count (CBC)
- NHS: Blood tests
- MedlinePlus: How to understand your lab results
This guide is general information written in plain words. It is not medical advice and has not been reviewed by a doctor for your situation. Read our medical disclaimer.