The Full Blood Count — commonly abbreviated as FBC, or CBC (Complete Blood Count) in the United States — is one of the most frequently requested blood tests in UK medicine. It gives your doctor a detailed snapshot of the cells circulating in your bloodstream and can reveal a wide range of conditions, from anaemia and infection to blood disorders and nutritional deficiencies.
Note: This article is for educational purposes only. Your GP or clinician is best placed to interpret your individual FBC results in the context of your symptoms and medical history.
What Does an FBC Test Measure?
An FBC analyses three main types of blood cells: red blood cells, white blood cells, and platelets. Each has a different function, and each component of the FBC tells a different story about your health.
Red Blood Cell Components
Haemoglobin (Hb)
Haemoglobin is the protein inside red blood cells that carries oxygen around your body. It is one of the most clinically important FBC measurements. Low haemoglobin indicates anaemia, which causes fatigue, breathlessness, and pallor.
| Group | Normal Range |
|---|---|
| Adult men | 130–170 g/L |
| Adult women | 120–150 g/L |
| Pregnant women | 110 g/L or above |
Red Blood Cell Count (RBC)
The total number of red blood cells per litre of blood. Low RBC confirms anaemia; elevated RBC may suggest polycythaemia (too many red cells) which can increase clot risk.
Haematocrit (HCT / PCV)
The proportion of your blood made up of red blood cells, expressed as a percentage. Low haematocrit accompanies anaemia; high haematocrit may indicate dehydration or polycythaemia.
Mean Cell Volume (MCV)
MCV measures the average size of your red blood cells. This is particularly helpful in diagnosing the type of anaemia:
- Low MCV (microcytic anaemia): Usually caused by iron deficiency or thalassaemia
- Normal MCV (normocytic anaemia): Often seen in chronic disease, kidney disease, or acute blood loss
- High MCV (macrocytic anaemia): Associated with B12 or folate deficiency, alcohol excess, or hypothyroidism
Mean Cell Haemoglobin (MCH) and MCHC
MCH measures the average amount of haemoglobin per red cell. MCHC measures the concentration of haemoglobin within red cells. These are used alongside MCV to characterise the type of anaemia.
Red Cell Distribution Width (RDW)
RDW measures the variation in size between your red blood cells. A high RDW (anisocytosis) suggests the bone marrow is producing cells of irregular size, which can be seen in iron deficiency, B12 deficiency, or mixed anaemia.
White Blood Cell Components
White blood cells (WBCs or leucocytes) are your immune system's soldiers. The FBC measures both the total WBC count and the differential — the breakdown of WBC types.
Total White Cell Count (WBC)
Normal range is approximately 4.0–11.0 × 10⁹/L. A raised WBC (leucocytosis) most commonly indicates infection or inflammation. A very low WBC (leucopenia) may result from viral infections, autoimmune conditions, or bone marrow problems.
Neutrophils
The most common white cells, making up 40–75% of all WBCs. Neutrophils are the first responders to bacterial infection. High neutrophils (neutrophilia) are most commonly caused by bacterial infection, steroids, or stress. Low neutrophils (neutropenia) can result from chemotherapy, viral illness, or autoimmune conditions, and can make you vulnerable to serious infections.
Lymphocytes
Lymphocytes (20–40% of WBCs) are key to your immune memory and viral defence. Raised lymphocytes are typical in viral infections such as glandular fever. Persistently elevated lymphocytes may prompt investigation for lymphoma or chronic lymphocytic leukaemia (CLL).
Monocytes, Eosinophils, and Basophils
These are less common WBC types. Raised eosinophils are associated with allergic conditions and parasitic infections. Raised monocytes may indicate chronic infection or inflammatory conditions.
Platelet Count
Platelets are small cell fragments essential for blood clotting. Normal range is approximately 150–400 × 10⁹/L.
- Low platelets (thrombocytopenia): May cause easy bruising or prolonged bleeding. Causes include viral infection, medications, autoimmune conditions, and rarely leukaemia.
- High platelets (thrombocytosis): Often a reactive response to infection or iron deficiency. Persistently very high platelets may increase clot risk.
Common Conditions Diagnosed with FBC
- Iron deficiency anaemia — low Hb, low MCV, low ferritin
- B12 or folate deficiency — low Hb, high MCV
- Bacterial infection — raised WBC and neutrophils
- Viral infection — raised lymphocytes, sometimes low WBC
- Leukaemia — abnormal WBC count or differential, often with anaemia and low platelets
- Polycythaemia — raised Hb, RBC, and haematocrit
⚠️ An abnormal FBC result does not automatically mean you have a serious condition. Many abnormalities have benign explanations. Your GP will interpret your results alongside your symptoms and medical history.
What Happens If My FBC Is Abnormal?
Depending on which values are abnormal, your GP may request further tests such as ferritin and iron studies (for low MCV anaemia), B12 and folate levels, a blood film (where a trained haematologist examines your cells under a microscope), or a referral to a haematologist if the pattern suggests a blood disorder.
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