Full blood count (FBC): what it shows and what abnormal results mean
The full blood count (FBC), known in the US as a complete blood count (CBC), is the most frequently requested blood test. From one sample it gives information about red blood cells, white blood cells and platelets. It helps to detect anaemia, infection, inflammation and some blood disorders – but results are always interpreted alongside your symptoms and examination.
What is a full blood count?
A full blood count is a test on a sample of blood from a vein, analysed by an automated haematology analyser. It is part of almost every health check-up, pre-operative assessment and the monitoring of many conditions and treatments, such as chemotherapy.
It usually reports:
- red cells – red blood cell count (RBC), haemoglobin (Hb), haematocrit (HCT) and red cell indices (MCV, MCH, MCHC, RDW);
- white cells – total white cell count (WBC) and a differential showing neutrophils, lymphocytes, monocytes, eosinophils and basophils;
- platelets (PLT) and sometimes mean platelet volume (MPV).
The erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are not part of the FBC but are often requested with it as markers of inflammation.
How the test is done
Blood is taken from a vein in the arm into a tube containing an anticoagulant. Fasting is not needed for the FBC itself, but because it is often combined with tests such as glucose or cholesterol, you may be asked to come in the morning without breakfast. Strenuous exercise, dehydration, smoking and some medicines can slightly affect the results. Results are usually available within hours.
Parameters and typical ranges
The values below are approximate for adults. Always use the reference range printed on your own report, as it depends on the laboratory, equipment, age and sex.
| Parameter | What it measures | Approximate adult range |
|---|---|---|
| Haemoglobin (Hb) | The oxygen-carrying protein | women about 120–155 g/L, men about 130–175 g/L |
| Red cell count (RBC) | Number of red blood cells | women about 3.8–5.2, men about 4.3–5.8 ×10^12/L |
| Haematocrit (HCT) | Proportion of blood made up of cells | women about 0.36–0.46, men about 0.40–0.50 |
| MCV | Average size of red cells | about 80–100 fL |
| White cell count (WBC) | White blood cells | about 4–11 ×10^9/L |
| Platelets (PLT) | Cells involved in clotting | about 150–400 ×10^9/L |
Red cells: anaemia and polycythaemia
Low haemoglobin (anaemia) can cause tiredness, pale skin, breathlessness on exertion, palpitations and dizziness. The WHO describes anaemia as a widespread public health problem, particularly among women of reproductive age and young children. Red cell indices help to find the cause:
- low MCV (microcytic anaemia) – most often iron deficiency, for example from heavy periods (see uterine fibroids), bleeding from the gut or low dietary iron; less often thalassaemia trait, which is more common in Mediterranean and Balkan populations;
- high MCV (macrocytic anaemia) – vitamin B12 or folate deficiency, alcohol, liver or thyroid disease and some medicines;
- normal MCV – anaemia of chronic disease, chronic kidney disease or acute blood loss.
In adults with iron-deficiency anaemia, particularly men and women after the menopause, doctors look for a source of bleeding, often with gastroscopy and colonoscopy.
High haemoglobin and haematocrit may be due to dehydration, smoking, living at high altitude, chronic lung disease, sleep apnoea or, less commonly, a bone marrow condition (polycythaemia vera).
White cells
A raised white cell count (leucocytosis) is most often a response to bacterial infection or inflammation, but it can also be caused by stress, smoking, pregnancy or steroid medicines. The differential gives clues:
- raised neutrophils – usually bacterial infection or inflammation;
- raised lymphocytes – often a viral infection;
- raised eosinophils – allergies, asthma or parasites.
A low white cell count (leucopenia) may follow viral infections or be caused by some medicines, autoimmune conditions, chemotherapy or bone marrow disorders. A very low neutrophil count increases the risk of serious infection.
A very high white cell count, immature cells (blasts) on the film, or abnormalities in more than one cell line need prompt assessment by a haematologist to rule out leukaemia, lymphoma or other blood disorders.
Platelets
Low platelets (thrombocytopenia) can show up as easy bruising, bleeding gums or nosebleeds and tiny red spots on the skin. Causes include viral infections, immune thrombocytopenia, some medicines, liver disease, pregnancy and bone marrow disorders. Occasionally a low result is a laboratory artefact caused by platelets clumping in the tube, and the test is repeated.
High platelets (thrombocytosis) are most often reactive – due to inflammation, infection, iron deficiency or recent surgery. Less commonly they reflect a bone marrow condition that increases the risk of blood clots.
What happens after an abnormal result
A single, slightly abnormal value without symptoms is often of no significance – your doctor may simply repeat the test after a few weeks. Significant or persistent changes lead to further tests: ferritin and iron studies, vitamin B12 and folate, reticulocyte count, a blood film (examined under a microscope), kidney and liver tests, inflammatory markers and, where needed, referral to a haematologist and a bone marrow test. An FBC alone cannot prove or exclude cancer – read more about the limits of blood tests in our articles on tumour markers and medical diagnostics.
When to see a doctor
Discuss any result outside the reference range with your GP, even if you feel well. See a doctor promptly if you have:
- ongoing tiredness, pallor, breathlessness or palpitations;
- unexplained bruising, bleeding gums, frequent nosebleeds or blood in your urine or stools;
- frequent or severe infections or a persistent fever;
- night sweats, unexplained weight loss or swollen lymph nodes.
Seek urgent care for heavy bleeding, black tarry stools, a high fever when you know your white cell count is low (for example during chemotherapy) or fainting.
Blood tests as part of a check-up abroad
An FBC is inexpensive and widely available, so there is no need to travel for it alone. Some patients, however, combine blood tests with a comprehensive health check abroad. Large accredited hospitals in Istanbul offer check-up packages that include laboratory, imaging and cardiology tests over one or two days (see check-up in Turkey). Organisers such as New City Clinic coordinate the hospital, appointments, hotel and an interpreter. Take previous results with you for comparison and ask for a written report in English.
Frequently asked questions
Do I need to fast before a full blood count?
Fasting is not necessary for the FBC itself. Because it is often taken together with glucose or cholesterol tests, you may be asked to come in the morning before eating.
Can a full blood count detect cancer?
An FBC cannot prove or rule out cancer. Blood cancers such as leukaemia often cause abnormal results, but with most solid tumours the FBC may be normal or show only anaemia.
How often should I have a blood count?
Healthy adults without symptoms usually have an FBC as part of a check-up or when their doctor considers it useful. People with chronic conditions or on certain treatments are tested more often.
What does a slightly high or low value mean?
A value just outside the reference range is often not clinically significant, especially if the other results are normal. Your doctor will interpret it alongside your symptoms and may repeat the test.
Sources
This article is for education and does not replace consultation, diagnosis or treatment by a doctor. In an emergency call 112.
