Full Blood Count Results Explained

A full blood count produces more numbers than any other routine test, which makes it the one people most often stare at without knowing where to start. This guide explains the main markers, what they indicate, and which ones matter most.

It is part of our guide to understanding your blood test results. To book, see our private blood tests in Hampshire.

The three groups of cells an FBC measures

Red blood cells carry oxygen. The key marker is haemoglobin (Hb), the protein that does the carrying. Alongside it you will see MCV (average red cell size), MCH, and haematocrit. Typical adult haemoglobin sits roughly around 130–170 g/L for men and 120–150 g/L for women, varying by laboratory.

White blood cells fight infection. The total is the WBC count, broken down into neutrophils, lymphocytes, monocytes, eosinophils and basophils. Which type is raised offers a clue about what is going on.

Platelets help your blood clot. Too few can mean easy bruising or bleeding; too many can raise clotting risk.

What the common abnormal patterns suggest

• Low haemoglobin: anaemia. The MCV then helps point to the cause — small cells often suggest iron deficiency, large cells can suggest a B12 or folate deficiency.
• Raised white cells: commonly an infection or inflammation. Raised neutrophils often point to bacterial infection; raised lymphocytes more often viral.
• Low white cells: can follow a viral illness or relate to certain medications.
• Low platelets: can follow infection or relate to medication; significant drops need investigating.
• Raised platelets: often a reaction to inflammation, infection or iron deficiency.

Most abnormal FBC results have straightforward explanations, and many resolve on a repeat test a few weeks later. What matters is the pattern and the size of the change, not a single value slightly outside a range.

Why an FBC is such a useful general check

Because it covers oxygen transport, immune function and clotting in one sample, an FBC is often the first test used when someone feels persistently tired, run down, breathless, or is picking up infections more than usual. It is also a genuinely useful baseline to have on record when you are well, so future results have something to be compared against.

The link between an FBC and tiredness

Fatigue is the single most common reason people ask for a blood test, and anaemia is one of the most common findings behind it. That said, it is not the only candidate: thyroid problems and B12 deficiency produce very similar symptoms. If unexplained tiredness is your main concern, it is often worth looking at more than one thing.

• Thyroid test results explained — another very common cause of fatigue.
• Cholesterol levels explained — worth checking alongside general health markers.

Frequently asked questions

What does a full blood count show?
Your red cells and haemoglobin, your white cells, and your platelets — covering oxygen transport, immune function and clotting.

Can an FBC detect anaemia?
Yes. Low haemoglobin is the main indicator, and the red cell indices help point to the likely cause.

Do I need to fast for a full blood count?
Usually not. Any preparation is confirmed when you book.

What if one value is slightly out of range?
That is common and often not significant. Your pharmacist will explain whether it matters and whether a repeat test is sensible.

This guide is general information and not a diagnosis or a substitute for advice from your GP. Reference ranges vary between laboratories. If you have urgent or serious health concerns, please contact your GP or NHS 111.