Full blood count (FBC): why your GP orders it and what it can show

Three families, not twenty verdicts

A full blood count (FBC) measures your red cells, white cells and platelets. In UK laboratories the white cell differential, which splits the white cells into neutrophils, lymphocytes, monocytes, eosinophils and basophils, is reported together with it. Everything else on the page describes those three families: haemoglobin, haematocrit, MCV, MCH, MCHC and RDW for the red cells, mean platelet volume for the platelets.

So results with twelve or twenty lines are not twelve or twenty findings. They are three questions with supporting detail, and the supporting detail is what makes each question answerable.

Two images help keep the families apart. Neutrophils are the front line, like the firefighters who arrive before anyone else. Platelets are the blood’s small patches, like the patches that seal a punctured tyre.

Units matter when you compare. UK laboratories give haemoglobin in grams per litre and cell counts in ×10⁹/L, so a haemoglobin of 135 g/L is the same as 13.5 g/dL on a result printed abroad, and each laboratory prints its own ranges.

Why would a GP ask for a full blood count?

It is one of the most commonly requested blood tests because it gives a broad picture in a single tube. A GP may ask for one if you have tiredness, weakness, breathlessness, bruising or unexplained bleeding, if an infection or inflammation is suspected, to monitor a condition such as anaemia or a treatment such as chemotherapy, or before surgery.

No preparation is needed: you can eat and drink normally. In NHS hospital laboratories the FBC is one of the fastest tests, with GP samples typically processed within 24 hours, though it may take a few days for the result to reach you. Privately, on 7 October 2026 one London clinic listed an FBC at £56 plus a £50 blood-draw fee, and one online provider at £59 plus £35 for a clinic sample.

How the lines are read together

Haemoglobin with MCV is the classic pair. A low haemoglobin says there is anaemia: the WHO definition is below 130 g/L in men and below 120 g/L in non-pregnant women. The MCV, the average size of the red cells, starts to say what kind. Small red cells point towards iron deficiency, long-standing inflammation or thalassaemia; large ones towards low B12 or folate, liver disease, an underactive thyroid or alcohol. A ferritin on the same request, if one was done, usually settles it.

White cells are read as a total and as a split. A raised total driven by neutrophils suggests one thing, a raised total driven by lymphocytes another, and neither is a diagnosis on its own.

Platelets are read with their own index, the mean platelet volume, which describes the average size of the platelets rather than their number. Younger platelets are larger, so a high MPV can mean the bone marrow is producing more of them than usual.

Would a full blood count show anything serious?

An FBC is a broad first look rather than a test for any one disease. Its red cell lines can show anaemia; its white cell lines can point towards an infection, an inflammatory or allergic reaction, or a problem in the bone marrow; its platelet line can help explain easy bruising or bleeding.

The list of possible causes is long in both directions. A high white count can come from an infection, inflammation, or leukaemia and other blood disorders. A low one can follow certain medicines, especially chemotherapy, a viral infection, alcohol excess, liver disease or an autoimmune condition. When results are abnormal, the laboratory often looks at a blood film, the cells under a microscope, before anyone draws a conclusion.

Cancer Research UK lists the FBC among the tests used when cancer is being looked for, but a normal count does not rule most cancers out, and an abnormal one far more often has an ordinary explanation.

Why healthy people get flagged here

An FBC has many lines, and each reference range is set so that about 1 in 20 healthy people fall outside it. On a panel this long, a flag somewhere is more likely than not in someone who is perfectly well.

Everyday life moves these lines too. Many medicines raise or lower the white count, heavy drinking affects all three families, a recent transfusion changes the picture, and not drinking enough water can push the red cell count and haemoglobin up.

Smoking is the explanation nobody mentions. Haemoglobin can run high in smokers, and studies comparing healthy smokers with non-smokers find higher white cell counts and higher haematocrit as well, growing with how much someone smokes. A flagged white count in a smoker often has a simpler cause than infection.

The sample itself can mislead. If the blood is hard to draw or not mixed quickly with the anticoagulant in the tube, tiny clots can form and give a falsely low platelet count, and an old or badly stored sample can shift several lines.

A low neutrophil count that is normal for you

One flag deserves its own section because it is so often misread. Lab Tests Online UK points out that normal neutrophil levels vary between ethnic groups. Many people of African or Middle Eastern ancestry carry the Duffy-null genotype, which comes with a lower neutrophil count.

In UK Biobank, 21% of people with the Duffy-null genotype had a neutrophil count below the threshold often used to decide access to some treatments, compared with 1% of everyone else, and they had no extra risk of infection, viral or bacterial. If your neutrophils are mildly low on every test and you feel well, that is a fair question to raise with your GP.

When to worry about FBC results

Ask how far outside, not whether outside. A platelet count of 145 against a lower limit of 150 and a platelet count of 40 can carry the same letter and mean entirely different things. Many FBC results slightly outside the range need no treatment, and the usual next step is a repeat after a short interval.

Then compare with your own previous count, which many surgeries share through the NHS App. A stable value slightly outside the range is a different matter from the same value appearing for the first time.

What deserves a prompt conversation rather than a routine one: a value far from its range, several families moving together, such as low red cells, white cells and platelets on the same results, or a flag that comes with fever, unusual bruising or bleeding. If you have heard nothing a few weeks after the test, contact your surgery.