Medical test · Blood & haematology · full clinical detail
Full blood count
Also known as: full blood count, blood count
The most commonly ordered blood test: an automated count of the red cells, white cells and platelets in a small sample, with the size and haemoglobin content of the red cells.
- Abbreviations
- CBC, FBC
- Kind
- Test panel
- Categories
- Blood & haematology
- Specimen
- Whole blood (EDTA)
- Method
- Automated cell counter
- Terminology
- LOINC 58410-2, 57021-8
- Type
- Blood test
- Sample
- Blood from a vein
- Used for
- General health, anaemia, infection, blood disorders, monitoring
- Measures
- Blood cells and related indices: red cells, white cells and platelets
- Result
- A panel of numeric results, each with its own laboratory reference interval, plus flags for the differential
Why is it used?
- Tiredness, pallor or breathlessness (anaemia)
- Suspected infection or inflammation
- Unexplained bruising or bleeding
- Monitoring chemotherapy and many long-term drugs
- Routine pre-operative check
What does it measure?
| Component | What it measures | A higher value may be seen in | A lower value may be seen in |
|---|---|---|---|
| Haemoglobin Hb | Oxygen-carrying protein concentration | Polycythaemia, dehydration, chronic hypoxia, smoking | Anaemia of any cause: iron, B12 or folate deficiency, blood loss, chronic disease, marrow failure |
| Haematocrit Hct | Fraction of blood volume occupied by red cells | Dehydration, polycythaemia | Anaemia, over-hydration |
| Red cell count RBC | Number of red cells per litre | Polycythaemia, thalassaemia trait (high count with small cells) | Anaemia |
| Mean corpuscular volume (MCV) MCV Mean corpuscular volume | Average red-cell size | B12 or folate deficiency, alcohol, liver disease, hypothyroidism, some drugs | Iron deficiency, thalassaemia trait |
| Mean corpuscular haemoglobin MCH | Average haemoglobin per red cell | Macrocytosis | Iron deficiency, thalassaemia |
| Mean corpuscular haemoglobin concentration MCHC | Haemoglobin concentration within red cells | Hereditary spherocytosis, dehydration | Iron deficiency |
| Red cell distribution width RDW | Variation in red-cell size | Mixed deficiencies, early iron deficiency, recent transfusion | Rarely meaningful |
| White blood cell count WBC White cell count | Total leucocytes | Infection, inflammation, steroids, leukaemia, smoking | Viral infection, chemotherapy, marrow failure, some drugs |
| Neutrophils neut | Cells that fight bacteria (from the differential) | Bacterial infection, inflammation, steroids, tissue injury | Chemotherapy, some drugs, viral infection, sepsis, benign ethnic neutropenia |
| Lymphocytes lymph | Cells of adaptive immunity | Viral infections, chronic lymphocytic leukaemia | Steroids, HIV, acute illness, immunosuppressants |
| Monocytes, eosinophils, basophils | Other white-cell types in the differential | Chronic infection (monocytes); allergy, asthma, parasites, drug reactions (eosinophils); myeloproliferative disease (basophils) | Steroids (eosinophils) |
| Platelets PLT | Clotting cell fragments | Reactive (infection, inflammation, iron deficiency, bleeding), essential thrombocythaemia | Immune destruction, drugs, liver disease, marrow failure, sepsis, clumping in the tube |
| Mean platelet volume MPV | Average platelet size | Increased platelet turnover | Marrow suppression |
A high or low value can be associated with several different situations; the result is interpreted with other results, symptoms, history and clinical findings.
How is it performed?
A few millilitres of blood from an arm vein into a tube with an anticoagulant; results within hours.
Specimen. Venous blood in an EDTA (anticoagulant) tube
Do I need to prepare?
None.
What do the results generally mean?
| Finding | What it may indicate |
|---|---|
| Low haemoglobin with small red cells | Iron deficiency, usually from blood loss (gut, periods) or poor intake |
| High neutrophils | Bacterial infection, inflammation, steroids or stress |
| Low platelets | Immune destruction, marrow problems, liver disease, some drugs |
| Everything low (pancytopenia) | Bone marrow failure or infiltration, needs urgent specialist review |
Read against the laboratory's own reference interval. Every component has a reference interval that depends on the analyser, age, sex, pregnancy and ethnic background; children's values differ markedly from adults'. The report flags values outside the laboratory's own interval, and a flagged value is a prompt for interpretation, not a diagnosis.
What can affect the result?
- Pregnancy, dehydration and altitude change red-cell values
- Recent infection, exercise, smoking, stress and steroids change white cells
- Clumping, clotting or delay in the tube gives spurious platelet and cell counts
- Ethnic background (benign ethnic neutropenia)
- Recent transfusion, iron infusion or bleeding
Medicines. Chemotherapy, methotrexate, carbimazole, clozapine and many other medicines lower counts; corticosteroids raise neutrophils and lower lymphocytes and eosinophils; heparin can lower platelets.
What can the test not tell you?
- The cause of an abnormal count: it shows that something has changed, and further tests (iron studies, B12 and folate, a blood film, sometimes marrow examination) show why
- Whether platelets are working properly (only their number)
- Whether a normal count excludes disease that has not yet affected the blood
Tells you that something is wrong, not what: an abnormal count is a prompt for further tests such as iron studies, a blood film or marrow examination.
Other tests commonly used with it
Medicines monitored with this test
Each medicine page cites the official document that names this test in its monitoring plan.
Educational content. Reference ranges differ between laboratories, methods, ages and sexes, and results are always read against the person, the question asked and other findings. This page cannot interpret an individual result.