Key takeaways
- A full blood count (FBC) measures your red blood cells, white blood cells and platelets from a single sample.
- At Blood London, an FBC costs £63.75 plus the £50 blood draw, with results back in 1 day.
- The report covers haemoglobin, red cell indices (MCV, MCH, MCHC), a white cell count with differential, and a platelet count.
- One value sitting just outside the laboratory's reference range is common and often not significant on its own.
- A pattern across several results, or a result alongside symptoms, is what a doctor actually reads.
- No fasting or special preparation is needed before the test.
A full blood count shows how many red cells, white cells and platelets are in your blood, and gives extra detail on their size and type. It is usually the first test ordered for tiredness, bruising, infection or a general check. You can book a full blood count at Blood London without a GP referral.
Book a full blood count at 117a Harley Street: £63.75 plus the £50 blood draw, results in 1 day.
What does each part of my full blood count measure?
Each line on an FBC report measures a different part of your blood: how much oxygen-carrying capacity you have, how your immune system is responding, and how well your blood can clot. Haemoglobin and haematocrit describe your red cells as a whole. The red cell indices (MCV, MCH, MCHC) describe the size and colour of individual red cells, which helps narrow down the cause of a low count. The white cell count and its differential (neutrophils, lymphocytes, monocytes, eosinophils and basophils) show how your immune system is behaving. The platelet count reflects clotting. Many UK laboratories also report red cell distribution width (RDW), a measure of how much red cells vary in size, alongside these; check your own report for exactly which values are listed.
Here is what each group commonly shows when it is high or low.
| Marker | What it measures | Common reasons high | Common reasons low |
|---|---|---|---|
| Haemoglobin / haematocrit | Oxygen-carrying red cell mass | Dehydration, smoking, some lung or heart conditions | Anaemia from iron, B12 or folate deficiency, blood loss |
| MCV, MCH, MCHC | Size and colour of red cells | Vitamin B12 or folate deficiency, some liver or thyroid conditions | Iron deficiency, thalassaemia trait |
| White cell count | Immune cell activity overall | Infection, inflammation, smoking, some medicines | Some viral infections, certain medicines, bone marrow conditions |
| Neutrophils | Response to bacterial infection or inflammation | Bacterial infection, inflammation, stress response | Some viral infections, certain medicines |
| Lymphocytes | Response to viral infection | Viral infection, some chronic conditions | Recent illness, some medicines |
| Platelets | Cells involved in clotting | Inflammation, iron deficiency, recent infection | Viral infection, some medicines, clumping in the sample |
What does a low haemoglobin or red cell count mean?
A low haemoglobin or red cell count usually points towards anaemia, meaning your blood is carrying less oxygen than usual. The most common cause worldwide is iron deficiency, often from diet, heavy periods or gradual blood loss elsewhere. The size of the red cells (MCV) helps a doctor narrow this down: small cells often point to iron deficiency, while larger cells can suggest B12 or folate deficiency. Iron studies, ferritin, B12 and folate are the usual next tests, and our guide to what MCHC means explains the red cell indices in more detail.
What does a raised or low white cell count mean?
A raised white cell count often reflects your immune system responding to infection or inflammation, and which type of white cell is raised gives a clue to the likely cause. A high neutrophil count is more typical of a bacterial infection or general inflammation, while a high lymphocyte count is more typical of a viral infection. A low white cell count can follow a viral illness, certain medicines, or less commonly a bone marrow problem, and usually needs a repeat test to see whether it settles. Smoking and recent exercise can also raise the count temporarily, which is why a single result is read alongside how you were feeling at the time. If you have been told your neutrophil count is very low and you develop a fever, seek medical advice the same day.
What does a low or high platelet count mean?
Platelets are the cells that help your blood clot, and both high and low counts are usually explained by something else going on in the body rather than a platelet problem itself. A mildly high platelet count often follows inflammation, iron deficiency or a recent infection. A low count can follow a viral illness or certain medicines, though it is also one of the more common results affected by clumping in the sample tube, which a repeat test can rule out. Very low platelet counts, or bleeding symptoms such as unusual bruising, blood in urine or stools, or bleeding gums, should be assessed by a doctor the same day.
Why might one flagged value not be significant?
One value sitting just outside the laboratory's reference range is common, and on its own it often is not significant. Reference ranges are set to cover most healthy people, so a small number of healthy results will naturally fall just outside them by chance. A doctor reads a flagged result alongside your symptoms, your history, and the rest of the panel, rather than in isolation, before deciding whether it needs following up.
What tests come after an abnormal FBC?
What happens next depends on which part of the count is abnormal. A low haemoglobin usually leads to iron, ferritin, B12 and folate testing, available together in the Anaemia Profile (£226.25 plus the £50 blood draw, results in 2 days). If you want a general inflammation marker alongside the count, the Haematology Profile adds ESR to the FBC (£75 plus the £50 blood draw, results in 1 day). Where iron status specifically needs a closer look, the Iron Status Profile measures iron, total iron-binding capacity, ferritin and transferrin saturation (£126.25 plus the £50 blood draw, results in 1 day). A repeat FBC after a few weeks is also a common next step when a result is only just outside the range.
How testing works at Blood London
A full blood count at Blood London costs £63.75 plus the £50 blood draw, and results are usually back within 1 day. No fasting or special preparation is needed, and you do not need a GP referral. Like all our private blood tests in London, you can walk in or book online at 117a Harley Street, seven days a week, and your results are sent to you by email as a laboratory report showing each result next to its reference range.
Book a full blood count at 117a Harley Street: £63.75 plus the £50 blood draw, results in 1 day. Start your booking.
Frequently asked questions
What is the difference between a full blood count and a haematology profile?
The full blood count on its own measures red cells, white cells and platelets. The Haematology Profile adds ESR, a general inflammation marker, to the same red cell, white cell and platelet count.
Can a full blood count diagnose what is wrong with me?
Not on its own. It shows a pattern, such as low haemoglobin or a raised white cell count, that a doctor uses alongside your symptoms and history to decide what to test or check next.
Do I need to fast before a full blood count?
No. There is no fasting or other preparation needed, and you can eat and drink as normal beforehand.
How will I get my results?
Results are sent to you by email, usually within 1 day, as a laboratory report showing each result against its reference range.
Should I see a doctor about an abnormal result?
If a result sits well outside the reference range, or you have symptoms alongside it, arrange for a doctor to look at it. You can add a doctor review of your report to your test for £75, or book a GP consultation at Medical Express Clinic for £150 to discuss what it means.
Why do my white blood cells go up when I am not ill?
Smoking, recent exercise, stress and even the time of day can raise a white cell count slightly. This is why a single mildly raised result is not usually treated as abnormal on its own.
References
- NHS: Blood tests
- Lab Tests Online UK: Full blood count (FBC)
- NHS: Anaemia
- NICE CKS: Anaemia, iron deficiency
- NICE CKS: Anaemia, B12 and folate deficiency