Full Blood Count (FBC)
A full blood count is one of the most commonly used investigations in general practice. It provides detailed information about red blood cells, white blood cells and platelets, helping assess anaemia, infection, inflammation and overall blood health. FBC testing is used in routine health checks, symptom reviews and long term condition monitoring.
What an FBC Measures
An FBC includes multiple haematology markers reviewed together to identify patterns:
- Haemoglobin
- Haematocrit
- Red blood cell count
- Mean cell volume
- Mean cell haemoglobin
- White blood cell count
- Differential white cells
- Platelet count
These values help assess oxygen carrying capacity, immune activity and blood clotting.on diagnostic information based on current clinical understanding.
Why an FBC Is Used
An FBC helps clinicians identify:
- Anaemia linked to iron, B12 or folate deficiency
- Viral or bacterial infections
- Inflammatory or autoimmune conditions
- Bone marrow disorders
- Allergic responses
- Side effects from medicines
- Recovery patterns after illness
The test is a quick way to evaluate symptoms such as fatigue, breathlessness, dizziness or recurrent infections.
Common Reasons to Have an FBC
An FBC may be arranged if you have:
- Low energy or unexplained tiredness
- Shortness of breath
- Recurrent infections
- Bruising or bleeding concerns
- Ongoing inflammation
- Weight loss or night sweats
- Nutritional concerns
- Symptoms of deficiency
It is also used during general health assessments or before starting certain treatments.
Understanding FBC Patterns
Interpreting an FBC involves looking at how markers change together:
Low haemoglobin or red cell count
May indicate iron deficiency, B12 or folate deficiency, chronic disease or blood loss.
High haemoglobin or red cell count
Sometimes linked to dehydration, smoking, high altitude exposure or rare bone marrow conditions.
High white cell count
Often seen in infection or inflammation. Medicines, stress and smoking may also raise levels.
Low white cell count
May follow viral illness, some medicines or bone marrow suppression.
Platelet changes
Low platelets can increase bleeding risk. High platelets may follow inflammation, recovery after infection or iron deficiency.
How the Test Is Performed
A small venous blood sample is taken from the arm. No fasting or preparation is required and results are typically returned within 24 to 48 hours.
When to Discuss Results with a GP
Speak to a GP if:
- Any result falls outside the reference range
- You have persistent symptoms
- You take medicines that affect blood cell production
- You have a long term condition requiring regular monitoring
A GP can interpret your FBC in the context of lifestyle, medical history and symptoms.
Related Tests
These tests are often reviewed alongside an FBC:
- Iron, ferritin, B12 and folate
- Thyroid function tests
- Liver function tests
- ESR or CRP
- Kidney function tests
A private GP can review your concerns and help plan next steps.
For a wider overview of routine blood markers, you can visit our main Blood Tests section in the Knowledge Base.
If you would like to discuss blood tests, review existing results or plan a private health check, you can book an online GP appointment with AccessGP.
Last reviewed by Dr Zamiel Hussain, GMC registered GP
Updated: 7 December 2025