Key Takeaways
- An FBC measures red blood cells, white blood cells, platelets, and haemoglobin.
- It is the most commonly requested blood test in the NHS and does not require fasting.
- Abnormal results do not automatically mean something is wrong — many factors can cause temporary variations.
- Your GP will interpret results in the context of your overall health, symptoms, and medical history.
What Is a Full Blood Count?
A full blood count (FBC), sometimes called a complete blood count (CBC), is a blood test that measures the cellular components of your blood. It is one of the first tests a GP will order when investigating a wide range of conditions, from fatigue and recurrent infections to unexplained bruising.
The test requires a small sample of blood, usually taken from a vein in your arm. It does not require fasting and can be performed at any time of day. Results are typically available within 24 to 48 hours through the NHS, though private labs may return them sooner.
Components of an FBC
Red Blood Cells (RBCs)
Red blood cells carry oxygen from your lungs to every tissue in your body and bring carbon dioxide back for exhalation. The FBC measures several red cell parameters:
- RBC count — the total number of red cells per litre of blood. Normal range for men is approximately 4.5 to 5.5 x 10¹²/L; for women 3.8 to 5.0 x 10¹²/L.
- Haemoglobin (Hb) — the iron-containing protein inside red cells that binds oxygen. Normal is 130 to 170 g/L for men and 120 to 150 g/L for women. Low haemoglobin is the hallmark of anaemia.
- Haematocrit (Hct) — the proportion of blood volume occupied by red cells, expressed as a percentage or ratio.
- Mean Cell Volume (MCV) — the average size of each red cell. Low MCV (microcytic) may suggest iron deficiency; high MCV (macrocytic) may suggest B12 or folate deficiency.
- Mean Cell Haemoglobin (MCH) — the average amount of haemoglobin per red cell.
- Red Cell Distribution Width (RDW) — how much variation there is in the size of your red cells. A high RDW can indicate mixed nutritional deficiencies.
White Blood Cells (WBCs)
White blood cells are part of your immune system. An FBC provides a total white cell count and a differential — a breakdown of the five main types:
- Neutrophils — the most abundant type, first responders to bacterial infections. Normal range: 2.0 to 7.5 x 10⁹/L.
- Lymphocytes — key players in viral defence and long-term immunity. Normal range: 1.0 to 4.0 x 10⁹/L.
- Monocytes — help clean up damaged tissue and fight chronic infections. Normal range: 0.2 to 1.0 x 10⁹/L.
- Eosinophils — involved in allergic reactions and parasitic infections. Normal range: 0.0 to 0.5 x 10⁹/L.
- Basophils — the rarest white cells, involved in inflammatory responses. Normal range: 0.0 to 0.1 x 10⁹/L.
A raised total WBC count (leukocytosis) most commonly indicates infection but can also occur with stress, steroid use, or smoking. A low WBC count (leukopenia) may be caused by viral infections, bone marrow problems, or certain medications.
Platelets
Platelets are cell fragments that help your blood clot. The normal range is 150 to 400 x 10⁹/L. A low platelet count (thrombocytopenia) can cause easy bruising and bleeding. A high count (thrombocytosis) may increase the risk of blood clots. Both warrant further investigation.
When Might Your GP Order an FBC?
An FBC is requested in a huge range of clinical scenarios. Common reasons include:
- Persistent fatigue or unexplained tiredness
- Recurrent infections or slow wound healing
- Unexplained bruising or bleeding
- Monitoring during chemotherapy or immunosuppressive treatment
- Pre-operative assessment before surgery
- Investigation of pale skin, breathlessness, or dizziness
- Routine health screening
Understanding Your Results
When you receive your FBC results, each parameter will be shown alongside the laboratory's reference range. Values flagged as "high" or "low" do not necessarily indicate a serious problem. Many things can temporarily alter your blood count, including:
- Recent exercise (raises WBC temporarily)
- Menstruation (lowers haemoglobin and ferritin)
- Dehydration (artificially raises haematocrit)
- Pregnancy (increases blood volume, diluting some values)
- Altitude (raises RBC count as the body adapts to lower oxygen)
Your GP will always interpret results in context. If anything is significantly abnormal, they will usually repeat the test or order additional investigations. For a more detailed guide, see understanding your blood test results.
Do I Need to Fast for an FBC?
No. A full blood count does not require fasting. You can eat and drink normally before the test. However, if your GP has ordered other tests alongside the FBC (such as a lipid profile or fasting glucose), you may still need to fast for those. Always check when you book the appointment. For more detail, read our guide on whether you can eat before a blood test.
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