Key takeaways
- A full blood count (FBC) measures your red blood cells, white blood cells and platelets from a single sample.
- 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 reference range is common and often means little on its own.
- What a doctor reads is the pattern across several results, together with your symptoms.
- No fasting or special preparation is needed before the test.
You've felt tired for weeks, you've noticed more bruises than usual, or you'd simply like a general check. Whichever it is, a full blood count is very likely where a doctor would start.
A full blood count shows how many red cells, white cells and platelets are in your blood, with extra detail on their size and type. It's 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 the report looks at something different: how much oxygen your blood can carry, 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 your red cells vary in size. Check your own report for exactly which values are listed.
Here's what each group commonly shows when it's 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?
It 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 your red cells (MCV) helps a doctor narrow it 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. 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 the type of white cell that's raised gives a clue to the cause. A high neutrophil count is more typical of a bacterial infection or general inflammation. A high lymphocyte count is more typical of a viral infection.
A low white cell count can follow a viral illness or certain medicines, or less commonly a bone marrow problem. It usually needs a repeat test to see whether it settles. Smoking and recent exercise can also raise the count for a while, which is why a single result is read alongside how you were feeling at the time.
If you've 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. A high or low count is usually explained by something else going on in the body rather than a problem with the platelets themselves. A mildly high count often follows inflammation, iron deficiency or a recent infection.
A low count can follow a viral illness or certain medicines. It's also one of the results most often affected by clumping in the sample tube, which a repeat test can rule out. Very low 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.
Is one flagged value something to worry about?
One value sitting just outside the laboratory's reference range is common, and on its own it often isn't significant. Reference ranges are set to cover most healthy people, so a few healthy results will fall just outside them by chance. A doctor reads a flagged result alongside your symptoms, your history and the rest of the panel before deciding whether it needs following up.
What tests come after an abnormal FBC?
That depends on which part of the count is abnormal. Each of these is priced plus the £50 blood draw.
- A low haemoglobin usually leads to iron, ferritin, B12 and folate testing, which the Anaemia Profile covers together: £226.25, results in 2 days.
- For a general inflammation marker alongside the count, the Haematology Profile adds ESR to the FBC: £75, results in 1 day.
- Where iron status needs a closer look, the Iron Status Profile measures iron, total iron-binding capacity, ferritin and transferrin saturation: £126.25, results in 1 day.
When a result is only just outside the range, a repeat FBC after a few weeks is also a common next step.
How testing works at Blood London
There's no fasting or special preparation, and you don't 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. Results are usually back within 1 day, emailed to you as a laboratory report that shows each result next to its reference range. Start your booking.
Frequently asked questions
What is the difference between a full blood count and a haematology profile?
A full blood count measures red cells, white cells and platelets. The Haematology Profile measures the same things and adds ESR, a general inflammation marker.
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, which a doctor uses with your symptoms and history to decide what to check next.
Do I need to fast before a full blood count?
No. There's no fasting or other preparation, and you can eat and drink as normal beforehand.
How will I get my results?
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?
It's worth having a doctor look at any result well outside the reference range, or any result that comes with symptoms. 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. That's why a single mildly raised result isn't usually treated as abnormal by itself.
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